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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.lang.Thread.UncaughtExceptionHandler;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.security.AccessControlContext;
import java.security.ProtectionDomain;
import java.security.Permissions;

An ExecutorService for running ForkJoinTasks. A ForkJoinPool provides the entry point for submissions from non-ForkJoinTask clients, as well as management and monitoring operations. 
A ForkJoinPool differs from other kinds of ExecutorService mainly by virtue of employing work-stealing: all threads in the pool attempt to find and execute tasks submitted to the pool and/or created by other active tasks (eventually blocking waiting for work if none exist). This enables efficient processing when most tasks spawn other subtasks (as do most ForkJoinTasks), as well as when many small tasks are submitted to the pool from external clients. Especially when setting asyncMode to true in constructors, 
ForkJoinPools may also be appropriate for use with event-style tasks that are never joined. 
A static commonPool() is available and appropriate for most applications. The common pool is used by any ForkJoinTask that is not explicitly submitted to a specified pool. Using the common pool normally reduces resource usage (its threads are slowly reclaimed during periods of non-use, and reinstated upon subsequent use). 
For applications that require separate or custom pools, a 
ForkJoinPool may be constructed with a given target parallelism level; by default, equal to the number of available processors. The pool attempts to maintain enough active (or available) threads by dynamically adding, suspending, or resuming internal worker threads, even if some tasks are stalled waiting to join others. However, no such adjustments are guaranteed in the face of blocked I/O or other unmanaged synchronization. The nested ManagedBlocker interface enables extension of the kinds of synchronization accommodated. 
In addition to execution and lifecycle control methods, this class provides status check methods (for example getStealCount) that are intended to aid in developing, tuning, and monitoring fork/join applications. Also, method toString returns indications of pool state in a convenient form for informal monitoring. 
As is the case with other ExecutorServices, there are three main task execution methods summarized in the following table. These are designed to be used primarily by clients not already engaged in fork/join computations in the current pool. The main forms of these methods accept instances of ForkJoinTask, but overloaded forms also allow mixed execution of plain 
Runnable- or Callable- based activities as well. However, tasks that are already executing in a pool should normally instead use the within-computation forms listed in the table unless using async event-style tasks that are not usually joined, in which case there is little difference among choice of methods. 

Summary of task execution methods
 

   

   
 Call from non-fork/join clients
   
 Call from within fork/join computations
 
 

   
 Arrange async execution
   
 execute(ForkJoinTask<?>)
   
 ForkJoinTask.fork
 
 

   
 Await and obtain result
   
 invoke(ForkJoinTask<Object>)
   
 ForkJoinTask.invoke
 
 

   
 Arrange exec and obtain Future
   
 submit(ForkJoinTask<Object>)
   
 ForkJoinTask.fork (ForkJoinTasks are Futures)
 

The common pool is by default constructed with default parameters, but these may be controlled by setting three system properties: 

java.util.concurrent.ForkJoinPool.common.parallelism - the parallelism level, a non-negative integer 
java.util.concurrent.ForkJoinPool.common.threadFactory - the class name of a ForkJoinWorkerThreadFactory 
java.util.concurrent.ForkJoinPool.common.exceptionHandler - the class name of a UncaughtExceptionHandler 
 If a SecurityManager is present and no factory is specified, then the default pool uses a factory supplying threads that have no Permissions enabled. The system class loader is used to load these classes. Upon any error in establishing these settings, default parameters are used. It is possible to disable or limit the use of threads in the common pool by setting the parallelism property to zero, and/or using a factory that may return null. However doing so may cause unjoined tasks to never be executed. 
Implementation notes: This implementation restricts the maximum number of running threads to 32767. Attempts to create pools with greater than the maximum number result in IllegalArgumentException. 
This implementation rejects submitted tasks (that is, by throwing RejectedExecutionException) only when the pool is shut down or internal resources have been exhausted. 
Author:
Doug Lea
Since:
1.7/**
 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
 * A {@code ForkJoinPool} provides the entry point for submissions
 * from non-{@code ForkJoinTask} clients, as well as management and
 * monitoring operations.
 *
 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
 * ExecutorService} mainly by virtue of employing
 * <em>work-stealing</em>: all threads in the pool attempt to find and
 * execute tasks submitted to the pool and/or created by other active
 * tasks (eventually blocking waiting for work if none exist). This
 * enables efficient processing when most tasks spawn other subtasks
 * (as do most {@code ForkJoinTask}s), as well as when many small
 * tasks are submitted to the pool from external clients.  Especially
 * when setting <em>asyncMode</em> to true in constructors, {@code
 * ForkJoinPool}s may also be appropriate for use with event-style
 * tasks that are never joined.
 *
 * <p>A static {@link #commonPool()} is available and appropriate for
 * most applications. The common pool is used by any ForkJoinTask that
 * is not explicitly submitted to a specified pool. Using the common
 * pool normally reduces resource usage (its threads are slowly
 * reclaimed during periods of non-use, and reinstated upon subsequent
 * use).
 *
 * <p>For applications that require separate or custom pools, a {@code
 * ForkJoinPool} may be constructed with a given target parallelism
 * level; by default, equal to the number of available processors.
 * The pool attempts to maintain enough active (or available) threads
 * by dynamically adding, suspending, or resuming internal worker
 * threads, even if some tasks are stalled waiting to join others.
 * However, no such adjustments are guaranteed in the face of blocked
 * I/O or other unmanaged synchronization. The nested {@link
 * ManagedBlocker} interface enables extension of the kinds of
 * synchronization accommodated.
 *
 * <p>In addition to execution and lifecycle control methods, this
 * class provides status check methods (for example
 * {@link #getStealCount}) that are intended to aid in developing,
 * tuning, and monitoring fork/join applications. Also, method
 * {@link #toString} returns indications of pool state in a
 * convenient form for informal monitoring.
 *
 * <p>As is the case with other ExecutorServices, there are three
 * main task execution methods summarized in the following table.
 * These are designed to be used primarily by clients not already
 * engaged in fork/join computations in the current pool.  The main
 * forms of these methods accept instances of {@code ForkJoinTask},
 * but overloaded forms also allow mixed execution of plain {@code
 * Runnable}- or {@code Callable}- based activities as well.  However,
 * tasks that are already executing in a pool should normally instead
 * use the within-computation forms listed in the table unless using
 * async event-style tasks that are not usually joined, in which case
 * there is little difference among choice of methods.
 *
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Summary of task execution methods</caption>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
 *    <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange async execution</b></td>
 *    <td> {@link #execute(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Await and obtain result</b></td>
 *    <td> {@link #invoke(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#invoke}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange exec and obtain Future</b></td>
 *    <td> {@link #submit(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
 *  </tr>
 * </table>
 *
 * <p>The common pool is by default constructed with default
 * parameters, but these may be controlled by setting three
 * {@linkplain System#getProperty system properties}:
 * <ul>
 * <li>{@code java.util.concurrent.ForkJoinPool.common.parallelism}
 * - the parallelism level, a non-negative integer
 * <li>{@code java.util.concurrent.ForkJoinPool.common.threadFactory}
 * - the class name of a {@link ForkJoinWorkerThreadFactory}
 * <li>{@code java.util.concurrent.ForkJoinPool.common.exceptionHandler}
 * - the class name of a {@link UncaughtExceptionHandler}
 * </ul>
 * If a {@link SecurityManager} is present and no factory is
 * specified, then the default pool uses a factory supplying
 * threads that have no {@link Permissions} enabled.
 * The system class loader is used to load these classes.
 * Upon any error in establishing these settings, default parameters
 * are used. It is possible to disable or limit the use of threads in
 * the common pool by setting the parallelism property to zero, and/or
 * using a factory that may return {@code null}. However doing so may
 * cause unjoined tasks to never be executed.
 *
 * <p><b>Implementation notes</b>: This implementation restricts the
 * maximum number of running threads to 32767. Attempts to create
 * pools with greater than the maximum number result in
 * {@code IllegalArgumentException}.
 *
 * <p>This implementation rejects submitted tasks (that is, by throwing
 * {@link RejectedExecutionException}) only when the pool is shut down
 * or internal resources have been exhausted.
 *
 * @since 1.7
 * @author Doug Lea
 */
@sun.misc.Contended
public class ForkJoinPool extends AbstractExecutorService {

    /*
     * Implementation Overview
     *
     * This class and its nested classes provide the main
     * functionality and control for a set of worker threads:
     * Submissions from non-FJ threads enter into submission queues.
     * Workers take these tasks and typically split them into subtasks
     * that may be stolen by other workers.  Preference rules give
     * first priority to processing tasks from their own queues (LIFO
     * or FIFO, depending on mode), then to randomized FIFO steals of
     * tasks in other queues.  This framework began as vehicle for
     * supporting tree-structured parallelism using work-stealing.
     * Over time, its scalability advantages led to extensions and
     * changes to better support more diverse usage contexts.  Because
     * most internal methods and nested classes are interrelated,
     * their main rationale and descriptions are presented here;
     * individual methods and nested classes contain only brief
     * comments about details.
     *
     * WorkQueues
     * ==========
     *
     * Most operations occur within work-stealing queues (in nested
     * class WorkQueue).  These are special forms of Deques that
     * support only three of the four possible end-operations -- push,
     * pop, and poll (aka steal), under the further constraints that
     * push and pop are called only from the owning thread (or, as
     * extended here, under a lock), while poll may be called from
     * other threads.  (If you are unfamiliar with them, you probably
     * want to read Herlihy and Shavit's book "The Art of
     * Multiprocessor programming", chapter 16 describing these in
     * more detail before proceeding.)  The main work-stealing queue
     * design is roughly similar to those in the papers "Dynamic
     * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
     * (http://research.sun.com/scalable/pubs/index.html) and
     * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
     * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
     * The main differences ultimately stem from GC requirements that
     * we null out taken slots as soon as we can, to maintain as small
     * a footprint as possible even in programs generating huge
     * numbers of tasks. To accomplish this, we shift the CAS
     * arbitrating pop vs poll (steal) from being on the indices
     * ("base" and "top") to the slots themselves.
     *
     * Adding tasks then takes the form of a classic array push(task):
     *    q.array[q.top] = task; ++q.top;
     *
     * (The actual code needs to null-check and size-check the array,
     * properly fence the accesses, and possibly signal waiting
     * workers to start scanning -- see below.)  Both a successful pop
     * and poll mainly entail a CAS of a slot from non-null to null.
     *
     * The pop operation (always performed by owner) is:
     *   if ((base != top) and
     *        (the task at top slot is not null) and
     *        (CAS slot to null))
     *           decrement top and return task;
     *
     * And the poll operation (usually by a stealer) is
     *    if ((base != top) and
     *        (the task at base slot is not null) and
     *        (base has not changed) and
     *        (CAS slot to null))
     *           increment base and return task;
     *
     * Because we rely on CASes of references, we do not need tag bits
     * on base or top.  They are simple ints as used in any circular
     * array-based queue (see for example ArrayDeque).  Updates to the
     * indices guarantee that top == base means the queue is empty,
     * but otherwise may err on the side of possibly making the queue
     * appear nonempty when a push, pop, or poll have not fully
     * committed. (Method isEmpty() checks the case of a partially
     * completed removal of the last element.)  Because of this, the
     * poll operation, considered individually, is not wait-free. One
     * thief cannot successfully continue until another in-progress
     * one (or, if previously empty, a push) completes.  However, in
     * the aggregate, we ensure at least probabilistic
     * non-blockingness.  If an attempted steal fails, a thief always
     * chooses a different random victim target to try next. So, in
     * order for one thief to progress, it suffices for any
     * in-progress poll or new push on any empty queue to
     * complete. (This is why we normally use method pollAt and its
     * variants that try once at the apparent base index, else
     * consider alternative actions, rather than method poll, which
     * retries.)
     *
     * This approach also enables support of a user mode in which
     * local task processing is in FIFO, not LIFO order, simply by
     * using poll rather than pop.  This can be useful in
     * message-passing frameworks in which tasks are never joined.
     * However neither mode considers affinities, loads, cache
     * localities, etc, so rarely provide the best possible
     * performance on a given machine, but portably provide good
     * throughput by averaging over these factors.  Further, even if
     * we did try to use such information, we do not usually have a
     * basis for exploiting it.  For example, some sets of tasks
     * profit from cache affinities, but others are harmed by cache
     * pollution effects. Additionally, even though it requires
     * scanning, long-term throughput is often best using random
     * selection rather than directed selection policies, so cheap
     * randomization of sufficient quality is used whenever
     * applicable.  Various Marsaglia XorShifts (some with different
     * shift constants) are inlined at use points.
     *
     * WorkQueues are also used in a similar way for tasks submitted
     * to the pool. We cannot mix these tasks in the same queues used
     * by workers. Instead, we randomly associate submission queues
     * with submitting threads, using a form of hashing.  The
     * ThreadLocalRandom probe value serves as a hash code for
     * choosing existing queues, and may be randomly repositioned upon
     * contention with other submitters.  In essence, submitters act
     * like workers except that they are restricted to executing local
     * tasks that they submitted (or in the case of CountedCompleters,
     * others with the same root task).  Insertion of tasks in shared
     * mode requires a lock (mainly to protect in the case of
     * resizing) but we use only a simple spinlock (using field
     * qlock), because submitters encountering a busy queue move on to
     * try or create other queues -- they block only when creating and
     * registering new queues. Additionally, "qlock" saturates to an
     * unlockable value (-1) at shutdown. Unlocking still can be and
     * is performed by cheaper ordered writes of "qlock" in successful
     * cases, but uses CAS in unsuccessful cases.
     *
     * Management
     * ==========
     *
     * The main throughput advantages of work-stealing stem from
     * decentralized control -- workers mostly take tasks from
     * themselves or each other, at rates that can exceed a billion
     * per second.  The pool itself creates, activates (enables
     * scanning for and running tasks), deactivates, blocks, and
     * terminates threads, all with minimal central information.
     * There are only a few properties that we can globally track or
     * maintain, so we pack them into a small number of variables,
     * often maintaining atomicity without blocking or locking.
     * Nearly all essentially atomic control state is held in two
     * volatile variables that are by far most often read (not
     * written) as status and consistency checks. (Also, field
     * "config" holds unchanging configuration state.)
     *
     * Field "ctl" contains 64 bits holding information needed to
     * atomically decide to add, inactivate, enqueue (on an event
     * queue), dequeue, and/or re-activate workers.  To enable this
     * packing, we restrict maximum parallelism to (1<<15)-1 (which is
     * far in excess of normal operating range) to allow ids, counts,
     * and their negations (used for thresholding) to fit into 16bit
     * subfields.
     *
     * Field "runState" holds lockable state bits (STARTED, STOP, etc)
     * also protecting updates to the workQueues array.  When used as
     * a lock, it is normally held only for a few instructions (the
     * only exceptions are one-time array initialization and uncommon
     * resizing), so is nearly always available after at most a brief
     * spin. But to be extra-cautious, after spinning, method
     * awaitRunStateLock (called only if an initial CAS fails), uses a
     * wait/notify mechanics on a builtin monitor to block when
     * (rarely) needed. This would be a terrible idea for a highly
     * contended lock, but most pools run without the lock ever
     * contending after the spin limit, so this works fine as a more
     * conservative alternative. Because we don't otherwise have an
     * internal Object to use as a monitor, the "stealCounter" (an
     * AtomicLong) is used when available (it too must be lazily
     * initialized; see externalSubmit).
     *
     * Usages of "runState" vs "ctl" interact in only one case:
     * deciding to add a worker thread (see tryAddWorker), in which
     * case the ctl CAS is performed while the lock is held.
     *
     * Recording WorkQueues.  WorkQueues are recorded in the
     * "workQueues" array. The array is created upon first use (see
     * externalSubmit) and expanded if necessary.  Updates to the
     * array while recording new workers and unrecording terminated
     * ones are protected from each other by the runState lock, but
     * the array is otherwise concurrently readable, and accessed
     * directly. We also ensure that reads of the array reference
     * itself never become too stale. To simplify index-based
     * operations, the array size is always a power of two, and all
     * readers must tolerate null slots. Worker queues are at odd
     * indices. Shared (submission) queues are at even indices, up to
     * a maximum of 64 slots, to limit growth even if array needs to
     * expand to add more workers. Grouping them together in this way
     * simplifies and speeds up task scanning.
     *
     * All worker thread creation is on-demand, triggered by task
     * submissions, replacement of terminated workers, and/or
     * compensation for blocked workers. However, all other support
     * code is set up to work with other policies.  To ensure that we
     * do not hold on to worker references that would prevent GC, All
     * accesses to workQueues are via indices into the workQueues
     * array (which is one source of some of the messy code
     * constructions here). In essence, the workQueues array serves as
     * a weak reference mechanism. Thus for example the stack top
     * subfield of ctl stores indices, not references.
     *
     * Queuing Idle Workers. Unlike HPC work-stealing frameworks, we
     * cannot let workers spin indefinitely scanning for tasks when
     * none can be found immediately, and we cannot start/resume
     * workers unless there appear to be tasks available.  On the
     * other hand, we must quickly prod them into action when new
     * tasks are submitted or generated. In many usages, ramp-up time
     * to activate workers is the main limiting factor in overall
     * performance, which is compounded at program start-up by JIT
     * compilation and allocation. So we streamline this as much as
     * possible.
     *
     * The "ctl" field atomically maintains active and total worker
     * counts as well as a queue to place waiting threads so they can
     * be located for signalling. Active counts also play the role of
     * quiescence indicators, so are decremented when workers believe
     * that there are no more tasks to execute. The "queue" is
     * actually a form of Treiber stack.  A stack is ideal for
     * activating threads in most-recently used order. This improves
     * performance and locality, outweighing the disadvantages of
     * being prone to contention and inability to release a worker
     * unless it is topmost on stack.  We park/unpark workers after
     * pushing on the idle worker stack (represented by the lower
     * 32bit subfield of ctl) when they cannot find work.  The top
     * stack state holds the value of the "scanState" field of the
     * worker: its index and status, plus a version counter that, in
     * addition to the count subfields (also serving as version
     * stamps) provide protection against Treiber stack ABA effects.
     *
     * Field scanState is used by both workers and the pool to manage
     * and track whether a worker is INACTIVE (possibly blocked
     * waiting for a signal), or SCANNING for tasks (when neither hold
     * it is busy running tasks).  When a worker is inactivated, its
     * scanState field is set, and is prevented from executing tasks,
     * even though it must scan once for them to avoid queuing
     * races. Note that scanState updates lag queue CAS releases so
     * usage requires care. When queued, the lower 16 bits of
     * scanState must hold its pool index. So we place the index there
     * upon initialization (see registerWorker) and otherwise keep it
     * there or restore it when necessary.
     *
     * Memory ordering.  See "Correct and Efficient Work-Stealing for
     * Weak Memory Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013
     * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
     * analysis of memory ordering requirements in work-stealing
     * algorithms similar to the one used here.  We usually need
     * stronger than minimal ordering because we must sometimes signal
     * workers, requiring Dekker-like full-fences to avoid lost
     * signals.  Arranging for enough ordering without expensive
     * over-fencing requires tradeoffs among the supported means of
     * expressing access constraints. The most central operations,
     * taking from queues and updating ctl state, require full-fence
     * CAS.  Array slots are read using the emulation of volatiles
     * provided by Unsafe.  Access from other threads to WorkQueue
     * base, top, and array requires a volatile load of the first of
     * any of these read.  We use the convention of declaring the
     * "base" index volatile, and always read it before other fields.
     * The owner thread must ensure ordered updates, so writes use
     * ordered intrinsics unless they can piggyback on those for other
     * writes.  Similar conventions and rationales hold for other
     * WorkQueue fields (such as "currentSteal") that are only written
     * by owners but observed by others.
     *
     * Creating workers. To create a worker, we pre-increment total
     * count (serving as a reservation), and attempt to construct a
     * ForkJoinWorkerThread via its factory. Upon construction, the
     * new thread invokes registerWorker, where it constructs a
     * WorkQueue and is assigned an index in the workQueues array
     * (expanding the array if necessary). The thread is then
     * started. Upon any exception across these steps, or null return
     * from factory, deregisterWorker adjusts counts and records
     * accordingly.  If a null return, the pool continues running with
     * fewer than the target number workers. If exceptional, the
     * exception is propagated, generally to some external caller.
     * Worker index assignment avoids the bias in scanning that would
     * occur if entries were sequentially packed starting at the front
     * of the workQueues array. We treat the array as a simple
     * power-of-two hash table, expanding as needed. The seedIndex
     * increment ensures no collisions until a resize is needed or a
     * worker is deregistered and replaced, and thereafter keeps
     * probability of collision low. We cannot use
     * ThreadLocalRandom.getProbe() for similar purposes here because
     * the thread has not started yet, but do so for creating
     * submission queues for existing external threads.
     *
     * Deactivation and waiting. Queuing encounters several intrinsic
     * races; most notably that a task-producing thread can miss
     * seeing (and signalling) another thread that gave up looking for
     * work but has not yet entered the wait queue.  When a worker
     * cannot find a task to steal, it deactivates and enqueues. Very
     * often, the lack of tasks is transient due to GC or OS
     * scheduling. To reduce false-alarm deactivation, scanners
     * compute checksums of queue states during sweeps.  (The
     * stability checks used here and elsewhere are probabilistic
     * variants of snapshot techniques -- see Herlihy & Shavit.)
     * Workers give up and try to deactivate only after the sum is
     * stable across scans. Further, to avoid missed signals, they
     * repeat this scanning process after successful enqueuing until
     * again stable.  In this state, the worker cannot take/run a task
     * it sees until it is released from the queue, so the worker
     * itself eventually tries to release itself or any successor (see
     * tryRelease).  Otherwise, upon an empty scan, a deactivated
     * worker uses an adaptive local spin construction (see awaitWork)
     * before blocking (via park). Note the unusual conventions about
     * Thread.interrupts surrounding parking and other blocking:
     * Because interrupts are used solely to alert threads to check
     * termination, which is checked anyway upon blocking, we clear
     * status (using Thread.interrupted) before any call to park, so
     * that park does not immediately return due to status being set
     * via some other unrelated call to interrupt in user code.
     *
     * Signalling and activation.  Workers are created or activated
     * only when there appears to be at least one task they might be
     * able to find and execute.  Upon push (either by a worker or an
     * external submission) to a previously (possibly) empty queue,
     * workers are signalled if idle, or created if fewer exist than
     * the given parallelism level.  These primary signals are
     * buttressed by others whenever other threads remove a task from
     * a queue and notice that there are other tasks there as well.
     * On most platforms, signalling (unpark) overhead time is
     * noticeably long, and the time between signalling a thread and
     * it actually making progress can be very noticeably long, so it
     * is worth offloading these delays from critical paths as much as
     * possible. Also, because inactive workers are often rescanning
     * or spinning rather than blocking, we set and clear the "parker"
     * field of WorkQueues to reduce unnecessary calls to unpark.
     * (This requires a secondary recheck to avoid missed signals.)
     *
     * Trimming workers. To release resources after periods of lack of
     * use, a worker starting to wait when the pool is quiescent will
     * time out and terminate (see awaitWork) if the pool has remained
     * quiescent for period IDLE_TIMEOUT, increasing the period as the
     * number of threads decreases, eventually removing all workers.
     * Also, when more than two spare threads exist, excess threads
     * are immediately terminated at the next quiescent point.
     * (Padding by two avoids hysteresis.)
     *
     * Shutdown and Termination. A call to shutdownNow invokes
     * tryTerminate to atomically set a runState bit. The calling
     * thread, as well as every other worker thereafter terminating,
     * helps terminate others by setting their (qlock) status,
     * cancelling their unprocessed tasks, and waking them up, doing
     * so repeatedly until stable (but with a loop bounded by the
     * number of workers).  Calls to non-abrupt shutdown() preface
     * this by checking whether termination should commence. This
     * relies primarily on the active count bits of "ctl" maintaining
     * consensus -- tryTerminate is called from awaitWork whenever
     * quiescent. However, external submitters do not take part in
     * this consensus.  So, tryTerminate sweeps through queues (until
     * stable) to ensure lack of in-flight submissions and workers
     * about to process them before triggering the "STOP" phase of
     * termination. (Note: there is an intrinsic conflict if
     * helpQuiescePool is called when shutdown is enabled. Both wait
     * for quiescence, but tryTerminate is biased to not trigger until
     * helpQuiescePool completes.)
     *
     *
     * Joining Tasks
     * =============
     *
     * Any of several actions may be taken when one worker is waiting
     * to join a task stolen (or always held) by another.  Because we
     * are multiplexing many tasks on to a pool of workers, we can't
     * just let them block (as in Thread.join).  We also cannot just
     * reassign the joiner's run-time stack with another and replace
     * it later, which would be a form of "continuation", that even if
     * possible is not necessarily a good idea since we may need both
     * an unblocked task and its continuation to progress.  Instead we
     * combine two tactics:
     *
     *   Helping: Arranging for the joiner to execute some task that it
     *      would be running if the steal had not occurred.
     *
     *   Compensating: Unless there are already enough live threads,
     *      method tryCompensate() may create or re-activate a spare
     *      thread to compensate for blocked joiners until they unblock.
     *
     * A third form (implemented in tryRemoveAndExec) amounts to
     * helping a hypothetical compensator: If we can readily tell that
     * a possible action of a compensator is to steal and execute the
     * task being joined, the joining thread can do so directly,
     * without the need for a compensation thread (although at the
     * expense of larger run-time stacks, but the tradeoff is
     * typically worthwhile).
     *
     * The ManagedBlocker extension API can't use helping so relies
     * only on compensation in method awaitBlocker.
     *
     * The algorithm in helpStealer entails a form of "linear
     * helping".  Each worker records (in field currentSteal) the most
     * recent task it stole from some other worker (or a submission).
     * It also records (in field currentJoin) the task it is currently
     * actively joining. Method helpStealer uses these markers to try
     * to find a worker to help (i.e., steal back a task from and
     * execute it) that could hasten completion of the actively joined
     * task.  Thus, the joiner executes a task that would be on its
     * own local deque had the to-be-joined task not been stolen. This
     * is a conservative variant of the approach described in Wagner &
     * Calder "Leapfrogging: a portable technique for implementing
     * efficient futures" SIGPLAN Notices, 1993
     * (http://portal.acm.org/citation.cfm?id=155354). It differs in
     * that: (1) We only maintain dependency links across workers upon
     * steals, rather than use per-task bookkeeping.  This sometimes
     * requires a linear scan of workQueues array to locate stealers,
     * but often doesn't because stealers leave hints (that may become
     * stale/wrong) of where to locate them.  It is only a hint
     * because a worker might have had multiple steals and the hint
     * records only one of them (usually the most current).  Hinting
     * isolates cost to when it is needed, rather than adding to
     * per-task overhead.  (2) It is "shallow", ignoring nesting and
     * potentially cyclic mutual steals.  (3) It is intentionally
     * racy: field currentJoin is updated only while actively joining,
     * which means that we miss links in the chain during long-lived
     * tasks, GC stalls etc (which is OK since blocking in such cases
     * is usually a good idea).  (4) We bound the number of attempts
     * to find work using checksums and fall back to suspending the
     * worker and if necessary replacing it with another.
     *
     * Helping actions for CountedCompleters do not require tracking
     * currentJoins: Method helpComplete takes and executes any task
     * with the same root as the task being waited on (preferring
     * local pops to non-local polls). However, this still entails
     * some traversal of completer chains, so is less efficient than
     * using CountedCompleters without explicit joins.
     *
     * Compensation does not aim to keep exactly the target
     * parallelism number of unblocked threads running at any given
     * time. Some previous versions of this class employed immediate
     * compensations for any blocked join. However, in practice, the
     * vast majority of blockages are transient byproducts of GC and
     * other JVM or OS activities that are made worse by replacement.
     * Currently, compensation is attempted only after validating that
     * all purportedly active threads are processing tasks by checking
     * field WorkQueue.scanState, which eliminates most false
     * positives.  Also, compensation is bypassed (tolerating fewer
     * threads) in the most common case in which it is rarely
     * beneficial: when a worker with an empty queue (thus no
     * continuation tasks) blocks on a join and there still remain
     * enough threads to ensure liveness.
     *
     * The compensation mechanism may be bounded.  Bounds for the
     * commonPool (see commonMaxSpares) better enable JVMs to cope
     * with programming errors and abuse before running out of
     * resources to do so. In other cases, users may supply factories
     * that limit thread construction. The effects of bounding in this
     * pool (like all others) is imprecise.  Total worker counts are
     * decremented when threads deregister, not when they exit and
     * resources are reclaimed by the JVM and OS. So the number of
     * simultaneously live threads may transiently exceed bounds.
     *
     * Common Pool
     * ===========
     *
     * The static common pool always exists after static
     * initialization.  Since it (or any other created pool) need
     * never be used, we minimize initial construction overhead and
     * footprint to the setup of about a dozen fields, with no nested
     * allocation. Most bootstrapping occurs within method
     * externalSubmit during the first submission to the pool.
     *
     * When external threads submit to the common pool, they can
     * perform subtask processing (see externalHelpComplete and
     * related methods) upon joins.  This caller-helps policy makes it
     * sensible to set common pool parallelism level to one (or more)
     * less than the total number of available cores, or even zero for
     * pure caller-runs.  We do not need to record whether external
     * submissions are to the common pool -- if not, external help
     * methods return quickly. These submitters would otherwise be
     * blocked waiting for completion, so the extra effort (with
     * liberally sprinkled task status checks) in inapplicable cases
     * amounts to an odd form of limited spin-wait before blocking in
     * ForkJoinTask.join.
     *
     * As a more appropriate default in managed environments, unless
     * overridden by system properties, we use workers of subclass
     * InnocuousForkJoinWorkerThread when there is a SecurityManager
     * present. These workers have no permissions set, do not belong
     * to any user-defined ThreadGroup, and erase all ThreadLocals
     * after executing any top-level task (see WorkQueue.runTask).
     * The associated mechanics (mainly in ForkJoinWorkerThread) may
     * be JVM-dependent and must access particular Thread class fields
     * to achieve this effect.
     *
     * Style notes
     * ===========
     *
     * Memory ordering relies mainly on Unsafe intrinsics that carry
     * the further responsibility of explicitly performing null- and
     * bounds- checks otherwise carried out implicitly by JVMs.  This
     * can be awkward and ugly, but also reflects the need to control
     * outcomes across the unusual cases that arise in very racy code
     * with very few invariants. So these explicit checks would exist
     * in some form anyway.  All fields are read into locals before
     * use, and null-checked if they are references.  This is usually
     * done in a "C"-like style of listing declarations at the heads
     * of methods or blocks, and using inline assignments on first
     * encounter.  Array bounds-checks are usually performed by
     * masking with array.length-1, which relies on the invariant that
     * these arrays are created with positive lengths, which is itself
     * paranoically checked. Nearly all explicit checks lead to
     * bypass/return, not exception throws, because they may
     * legitimately arise due to cancellation/revocation during
     * shutdown.
     *
     * There is a lot of representation-level coupling among classes
     * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask.  The
     * fields of WorkQueue maintain data structures managed by
     * ForkJoinPool, so are directly accessed.  There is little point
     * trying to reduce this, since any associated future changes in
     * representations will need to be accompanied by algorithmic
     * changes anyway. Several methods intrinsically sprawl because
     * they must accumulate sets of consistent reads of fields held in
     * local variables.  There are also other coding oddities
     * (including several unnecessary-looking hoisted null checks)
     * that help some methods perform reasonably even when interpreted
     * (not compiled).
     *
     * The order of declarations in this file is (with a few exceptions):
     * (1) Static utility functions
     * (2) Nested (static) classes
     * (3) Static fields
     * (4) Fields, along with constants used when unpacking some of them
     * (5) Internal control methods
     * (6) Callbacks and other support for ForkJoinTask methods
     * (7) Exported methods
     * (8) Static block initializing statics in minimally dependent order
     */

    // Static utilities

    
If there is a security manager, makes sure caller has
permission to modify threads.
/**
     * If there is a security manager, makes sure caller has
     * permission to modify threads.
     */
    private static void checkPermission() {
        SecurityManager security = System.getSecurityManager();
        if (security != null)
            security.checkPermission(modifyThreadPermission);
    }

    // Nested classes

    
Factory for creating new ForkJoinWorkerThreads. A ForkJoinWorkerThreadFactory must be defined and used for ForkJoinWorkerThread subclasses that extend base functionality or initialize threads with different contexts. /**
     * Factory for creating new {@link ForkJoinWorkerThread}s.
     * A {@code ForkJoinWorkerThreadFactory} must be defined and used
     * for {@code ForkJoinWorkerThread} subclasses that extend base
     * functionality or initialize threads with different contexts.
     */
    public static interface ForkJoinWorkerThreadFactory {
        
Returns a new worker thread operating in the given pool.
Params:
pool – the pool this thread works in
Throws:
NullPointerException – if the pool is null
Returns:
the new worker thread/**
         * Returns a new worker thread operating in the given pool.
         *
         * @param pool the pool this thread works in
         * @return the new worker thread
         * @throws NullPointerException if the pool is null
         */
        public ForkJoinWorkerThread newThread(ForkJoinPool pool);
    }

    
Default ForkJoinWorkerThreadFactory implementation; creates a
new ForkJoinWorkerThread.
/**
     * Default ForkJoinWorkerThreadFactory implementation; creates a
     * new ForkJoinWorkerThread.
     */
    static final class DefaultForkJoinWorkerThreadFactory
        implements ForkJoinWorkerThreadFactory {
        public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
            return new ForkJoinWorkerThread(pool, true);
        }
    }

    
Class for artificial tasks that are used to replace the target
of local joins if they are removed from an interior queue slot
in WorkQueue.tryRemoveAndExec. We don't need the proxy to
actually do anything beyond having a unique identity.
/**
     * Class for artificial tasks that are used to replace the target
     * of local joins if they are removed from an interior queue slot
     * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
     * actually do anything beyond having a unique identity.
     */
    static final class EmptyTask extends ForkJoinTask<Void> {
        private static final long serialVersionUID = -7721805057305804111L;
        EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
        public final Void getRawResult() { return null; }
        public final void setRawResult(Void x) {}
        public final boolean exec() { return true; }
    }

    // Constants shared across ForkJoinPool and WorkQueue

    // Bounds
    static final int SMASK        = 0xffff;        // short bits == max index
    static final int MAX_CAP      = 0x7fff;        // max #workers - 1
    static final int EVENMASK     = 0xfffe;        // even short bits
    static final int SQMASK       = 0x007e;        // max 64 (even) slots

    // Masks and units for WorkQueue.scanState and ctl sp subfield
    static final int SCANNING     = 1;             // false when running tasks
    static final int INACTIVE     = 1 << 31;       // must be negative
    static final int SS_SEQ       = 1 << 16;       // version count

    // Mode bits for ForkJoinPool.config and WorkQueue.config
    static final int MODE_MASK    = 0xffff << 16;  // top half of int
    static final int LIFO_QUEUE   = 0;
    static final int FIFO_QUEUE   = 1 << 16;
    static final int SHARED_QUEUE = 1 << 31;       // must be negative

    
Queues supporting work-stealing as well as external task
submission. See above for descriptions and algorithms.
Performance on most platforms is very sensitive to placement of
instances of both WorkQueues and their arrays -- we absolutely
do not want multiple WorkQueue instances or multiple queue
arrays sharing cache lines. The @Contended annotation alerts
JVMs to try to keep instances apart.
/**
     * Queues supporting work-stealing as well as external task
     * submission. See above for descriptions and algorithms.
     * Performance on most platforms is very sensitive to placement of
     * instances of both WorkQueues and their arrays -- we absolutely
     * do not want multiple WorkQueue instances or multiple queue
     * arrays sharing cache lines. The @Contended annotation alerts
     * JVMs to try to keep instances apart.
     */
    @sun.misc.Contended
    static final class WorkQueue {

        
Capacity of work-stealing queue array upon initialization.
Must be a power of two; at least 4, but should be larger to
reduce or eliminate cacheline sharing among queues.
Currently, it is much larger, as a partial workaround for
the fact that JVMs often place arrays in locations that
share GC bookkeeping (especially cardmarks) such that
per-write accesses encounter serious memory contention.
/**
         * Capacity of work-stealing queue array upon initialization.
         * Must be a power of two; at least 4, but should be larger to
         * reduce or eliminate cacheline sharing among queues.
         * Currently, it is much larger, as a partial workaround for
         * the fact that JVMs often place arrays in locations that
         * share GC bookkeeping (especially cardmarks) such that
         * per-write accesses encounter serious memory contention.
         */
        static final int INITIAL_QUEUE_CAPACITY = 1 << 13;

        
Maximum size for queue arrays. Must be a power of two less
than or equal to 1 << (31 - width of array entry) to ensure
lack of wraparound of index calculations, but defined to a
value a bit less than this to help users trap runaway
programs before saturating systems.
/**
         * Maximum size for queue arrays. Must be a power of two less
         * than or equal to 1 << (31 - width of array entry) to ensure
         * lack of wraparound of index calculations, but defined to a
         * value a bit less than this to help users trap runaway
         * programs before saturating systems.
         */
        static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M

        // Instance fields
        volatile int scanState;    // versioned, <0: inactive; odd:scanning
        int stackPred;             // pool stack (ctl) predecessor
        int nsteals;               // number of steals
        int hint;                  // randomization and stealer index hint
        int config;                // pool index and mode
        volatile int qlock;        // 1: locked, < 0: terminate; else 0
        volatile int base;         // index of next slot for poll
        int top;                   // index of next slot for push
        ForkJoinTask<?>[] array;   // the elements (initially unallocated)
        final ForkJoinPool pool;   // the containing pool (may be null)
        final ForkJoinWorkerThread owner; // owning thread or null if shared
        volatile Thread parker;    // == owner during call to park; else null
        volatile ForkJoinTask<?> currentJoin;  // task being joined in awaitJoin
        volatile ForkJoinTask<?> currentSteal; // mainly used by helpStealer

        WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
            this.pool = pool;
            this.owner = owner;
            // Place indices in the center of array (that is not yet allocated)
            base = top = INITIAL_QUEUE_CAPACITY >>> 1;
        }

        
Returns an exportable index (used by ForkJoinWorkerThread).
/**
         * Returns an exportable index (used by ForkJoinWorkerThread).
         */
        final int getPoolIndex() {
            return (config & 0xffff) >>> 1; // ignore odd/even tag bit
        }

        
Returns the approximate number of tasks in the queue.
/**
         * Returns the approximate number of tasks in the queue.
         */
        final int queueSize() {
            int n = base - top;       // non-owner callers must read base first
            return (n >= 0) ? 0 : -n; // ignore transient negative
        }

        
Provides a more accurate estimate of whether this queue has
any tasks than does queueSize, by checking whether a
near-empty queue has at least one unclaimed task.
/**
         * Provides a more accurate estimate of whether this queue has
         * any tasks than does queueSize, by checking whether a
         * near-empty queue has at least one unclaimed task.
         */
        final boolean isEmpty() {
            ForkJoinTask<?>[] a; int n, m, s;
            return ((n = base - (s = top)) >= 0 ||
                    (n == -1 &&           // possibly one task
                     ((a = array) == null || (m = a.length - 1) < 0 ||
                      U.getObject
                      (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null)));
        }

        
Pushes a task. Call only by owner in unshared queues.  (The
shared-queue version is embedded in method externalPush.)
Params:
task – the task. Caller must ensure non-null.
Throws:
RejectedExecutionException – if array cannot be resized/**
         * Pushes a task. Call only by owner in unshared queues.  (The
         * shared-queue version is embedded in method externalPush.)
         *
         * @param task the task. Caller must ensure non-null.
         * @throws RejectedExecutionException if array cannot be resized
         */
        final void push(ForkJoinTask<?> task) {
            ForkJoinTask<?>[] a; ForkJoinPool p;
            int b = base, s = top, n;
            if ((a = array) != null) {    // ignore if queue removed
                int m = a.length - 1;     // fenced write for task visibility
                U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task);
                U.putOrderedInt(this, QTOP, s + 1);
                if ((n = s - b) <= 1) {
                    if ((p = pool) != null)
                        p.signalWork(p.workQueues, this);
                }
                else if (n >= m)
                    growArray();
            }
        }

        
Initializes or doubles the capacity of array. Call either
by owner or with lock held -- it is OK for base, but not
top, to move while resizings are in progress.
/**
         * Initializes or doubles the capacity of array. Call either
         * by owner or with lock held -- it is OK for base, but not
         * top, to move while resizings are in progress.
         */
        final ForkJoinTask<?>[] growArray() {
            ForkJoinTask<?>[] oldA = array;
            int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
            if (size > MAXIMUM_QUEUE_CAPACITY)
                throw new RejectedExecutionException("Queue capacity exceeded");
            int oldMask, t, b;
            ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
            if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
                (t = top) - (b = base) > 0) {
                int mask = size - 1;
                do { // emulate poll from old array, push to new array
                    ForkJoinTask<?> x;
                    int oldj = ((b & oldMask) << ASHIFT) + ABASE;
                    int j    = ((b &    mask) << ASHIFT) + ABASE;
                    x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj);
                    if (x != null &&
                        U.compareAndSwapObject(oldA, oldj, x, null))
                        U.putObjectVolatile(a, j, x);
                } while (++b != t);
            }
            return a;
        }

        
Takes next task, if one exists, in LIFO order.  Call only
by owner in unshared queues.
/**
         * Takes next task, if one exists, in LIFO order.  Call only
         * by owner in unshared queues.
         */
        final ForkJoinTask<?> pop() {
            ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m;
            if ((a = array) != null && (m = a.length - 1) >= 0) {
                for (int s; (s = top - 1) - base >= 0;) {
                    long j = ((m & s) << ASHIFT) + ABASE;
                    if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
                        break;
                    if (U.compareAndSwapObject(a, j, t, null)) {
                        U.putOrderedInt(this, QTOP, s);
                        return t;
                    }
                }
            }
            return null;
        }

        
Takes a task in FIFO order if b is base of queue and a task
can be claimed without contention. Specialized versions
appear in ForkJoinPool methods scan and helpStealer.
/**
         * Takes a task in FIFO order if b is base of queue and a task
         * can be claimed without contention. Specialized versions
         * appear in ForkJoinPool methods scan and helpStealer.
         */
        final ForkJoinTask<?> pollAt(int b) {
            ForkJoinTask<?> t; ForkJoinTask<?>[] a;
            if ((a = array) != null) {
                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
                if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null &&
                    base == b && U.compareAndSwapObject(a, j, t, null)) {
                    base = b + 1;
                    return t;
                }
            }
            return null;
        }

        
Takes next task, if one exists, in FIFO order.
/**
         * Takes next task, if one exists, in FIFO order.
         */
        final ForkJoinTask<?> poll() {
            ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t;
            while ((b = base) - top < 0 && (a = array) != null) {
                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
                t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
                if (base == b) {
                    if (t != null) {
                        if (U.compareAndSwapObject(a, j, t, null)) {
                            base = b + 1;
                            return t;
                        }
                    }
                    else if (b + 1 == top) // now empty
                        break;
                }
            }
            return null;
        }

        
Takes next task, if one exists, in order specified by mode.
/**
         * Takes next task, if one exists, in order specified by mode.
         */
        final ForkJoinTask<?> nextLocalTask() {
            return (config & FIFO_QUEUE) == 0 ? pop() : poll();
        }

        
Returns next task, if one exists, in order specified by mode.
/**
         * Returns next task, if one exists, in order specified by mode.
         */
        final ForkJoinTask<?> peek() {
            ForkJoinTask<?>[] a = array; int m;
            if (a == null || (m = a.length - 1) < 0)
                return null;
            int i = (config & FIFO_QUEUE) == 0 ? top - 1 : base;
            int j = ((i & m) << ASHIFT) + ABASE;
            return (ForkJoinTask<?>)U.getObjectVolatile(a, j);
        }

        
Pops the given task only if it is at the current top.
(A shared version is available only via FJP.tryExternalUnpush)
/**
         * Pops the given task only if it is at the current top.
         * (A shared version is available only via FJP.tryExternalUnpush)
        */
        final boolean tryUnpush(ForkJoinTask<?> t) {
            ForkJoinTask<?>[] a; int s;
            if ((a = array) != null && (s = top) != base &&
                U.compareAndSwapObject
                (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
                U.putOrderedInt(this, QTOP, s);
                return true;
            }
            return false;
        }

        
Removes and cancels all known tasks, ignoring any exceptions.
/**
         * Removes and cancels all known tasks, ignoring any exceptions.
         */
        final void cancelAll() {
            ForkJoinTask<?> t;
            if ((t = currentJoin) != null) {
                currentJoin = null;
                ForkJoinTask.cancelIgnoringExceptions(t);
            }
            if ((t = currentSteal) != null) {
                currentSteal = null;
                ForkJoinTask.cancelIgnoringExceptions(t);
            }
            while ((t = poll()) != null)
                ForkJoinTask.cancelIgnoringExceptions(t);
        }

        // Specialized execution methods

        
Polls and runs tasks until empty.
/**
         * Polls and runs tasks until empty.
         */
        final void pollAndExecAll() {
            for (ForkJoinTask<?> t; (t = poll()) != null;)
                t.doExec();
        }

        
Removes and executes all local tasks. If LIFO, invokes
pollAndExecAll. Otherwise implements a specialized pop loop
to exec until empty.
/**
         * Removes and executes all local tasks. If LIFO, invokes
         * pollAndExecAll. Otherwise implements a specialized pop loop
         * to exec until empty.
         */
        final void execLocalTasks() {
            int b = base, m, s;
            ForkJoinTask<?>[] a = array;
            if (b - (s = top - 1) <= 0 && a != null &&
                (m = a.length - 1) >= 0) {
                if ((config & FIFO_QUEUE) == 0) {
                    for (ForkJoinTask<?> t;;) {
                        if ((t = (ForkJoinTask<?>)U.getAndSetObject
                             (a, ((m & s) << ASHIFT) + ABASE, null)) == null)
                            break;
                        U.putOrderedInt(this, QTOP, s);
                        t.doExec();
                        if (base - (s = top - 1) > 0)
                            break;
                    }
                }
                else
                    pollAndExecAll();
            }
        }

        
Executes the given task and any remaining local tasks.
/**
         * Executes the given task and any remaining local tasks.
         */
        final void runTask(ForkJoinTask<?> task) {
            if (task != null) {
                scanState &= ~SCANNING; // mark as busy
                (currentSteal = task).doExec();
                U.putOrderedObject(this, QCURRENTSTEAL, null); // release for GC
                execLocalTasks();
                ForkJoinWorkerThread thread = owner;
                if (++nsteals < 0)      // collect on overflow
                    transferStealCount(pool);
                scanState |= SCANNING;
                if (thread != null)
                    thread.afterTopLevelExec();
            }
        }

        
Adds steal count to pool stealCounter if it exists, and resets.
/**
         * Adds steal count to pool stealCounter if it exists, and resets.
         */
        final void transferStealCount(ForkJoinPool p) {
            AtomicLong sc;
            if (p != null && (sc = p.stealCounter) != null) {
                int s = nsteals;
                nsteals = 0;            // if negative, correct for overflow
                sc.getAndAdd((long)(s < 0 ? Integer.MAX_VALUE : s));
            }
        }

        
If present, removes from queue and executes the given task,
or any other cancelled task. Used only by awaitJoin.
Returns:
true if queue empty and task not known to be done/**
         * If present, removes from queue and executes the given task,
         * or any other cancelled task. Used only by awaitJoin.
         *
         * @return true if queue empty and task not known to be done
         */
        final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
            ForkJoinTask<?>[] a; int m, s, b, n;
            if ((a = array) != null && (m = a.length - 1) >= 0 &&
                task != null) {
                while ((n = (s = top) - (b = base)) > 0) {
                    for (ForkJoinTask<?> t;;) {      // traverse from s to b
                        long j = ((--s & m) << ASHIFT) + ABASE;
                        if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
                            return s + 1 == top;     // shorter than expected
                        else if (t == task) {
                            boolean removed = false;
                            if (s + 1 == top) {      // pop
                                if (U.compareAndSwapObject(a, j, task, null)) {
                                    U.putOrderedInt(this, QTOP, s);
                                    removed = true;
                                }
                            }
                            else if (base == b)      // replace with proxy
                                removed = U.compareAndSwapObject(
                                    a, j, task, new EmptyTask());
                            if (removed)
                                task.doExec();
                            break;
                        }
                        else if (t.status < 0 && s + 1 == top) {
                            if (U.compareAndSwapObject(a, j, t, null))
                                U.putOrderedInt(this, QTOP, s);
                            break;                  // was cancelled
                        }
                        if (--n == 0)
                            return false;
                    }
                    if (task.status < 0)
                        return false;
                }
            }
            return true;
        }

        
Pops task if in the same CC computation as the given task,
in either shared or owned mode. Used only by helpComplete.
/**
         * Pops task if in the same CC computation as the given task,
         * in either shared or owned mode. Used only by helpComplete.
         */
        final CountedCompleter<?> popCC(CountedCompleter<?> task, int mode) {
            int s; ForkJoinTask<?>[] a; Object o;
            if (base - (s = top) < 0 && (a = array) != null) {
                long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
                if ((o = U.getObjectVolatile(a, j)) != null &&
                    (o instanceof CountedCompleter)) {
                    CountedCompleter<?> t = (CountedCompleter<?>)o;
                    for (CountedCompleter<?> r = t;;) {
                        if (r == task) {
                            if (mode < 0) { // must lock
                                if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
                                    if (top == s && array == a &&
                                        U.compareAndSwapObject(a, j, t, null)) {
                                        U.putOrderedInt(this, QTOP, s - 1);
                                        U.putOrderedInt(this, QLOCK, 0);
                                        return t;
                                    }
                                    U.compareAndSwapInt(this, QLOCK, 1, 0);
                                }
                            }
                            else if (U.compareAndSwapObject(a, j, t, null)) {
                                U.putOrderedInt(this, QTOP, s - 1);
                                return t;
                            }
                            break;
                        }
                        else if ((r = r.completer) == null) // try parent
                            break;
                    }
                }
            }
            return null;
        }

        
Steals and runs a task in the same CC computation as the
given task if one exists and can be taken without
contention. Otherwise returns a checksum/control value for
use by method helpComplete.
Returns:
1 if successful, 2 if retryable (lost to another
stealer), -1 if non-empty but no matching task found, else
the base index, forced negative./**
         * Steals and runs a task in the same CC computation as the
         * given task if one exists and can be taken without
         * contention. Otherwise returns a checksum/control value for
         * use by method helpComplete.
         *
         * @return 1 if successful, 2 if retryable (lost to another
         * stealer), -1 if non-empty but no matching task found, else
         * the base index, forced negative.
         */
        final int pollAndExecCC(CountedCompleter<?> task) {
            int b, h; ForkJoinTask<?>[] a; Object o;
            if ((b = base) - top >= 0 || (a = array) == null)
                h = b | Integer.MIN_VALUE;  // to sense movement on re-poll
            else {
                long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
                if ((o = U.getObjectVolatile(a, j)) == null)
                    h = 2;                  // retryable
                else if (!(o instanceof CountedCompleter))
                    h = -1;                 // unmatchable
                else {
                    CountedCompleter<?> t = (CountedCompleter<?>)o;
                    for (CountedCompleter<?> r = t;;) {
                        if (r == task) {
                            if (base == b &&
                                U.compareAndSwapObject(a, j, t, null)) {
                                base = b + 1;
                                t.doExec();
                                h = 1;      // success
                            }
                            else
                                h = 2;      // lost CAS
                            break;
                        }
                        else if ((r = r.completer) == null) {
                            h = -1;         // unmatched
                            break;
                        }
                    }
                }
            }
            return h;
        }

        
Returns true if owned and not known to be blocked.
/**
         * Returns true if owned and not known to be blocked.
         */
        final boolean isApparentlyUnblocked() {
            Thread wt; Thread.State s;
            return (scanState >= 0 &&
                    (wt = owner) != null &&
                    (s = wt.getState()) != Thread.State.BLOCKED &&
                    s != Thread.State.WAITING &&
                    s != Thread.State.TIMED_WAITING);
        }

        // Unsafe mechanics. Note that some are (and must be) the same as in FJP
        private static final sun.misc.Unsafe U;
        private static final int  ABASE;
        private static final int  ASHIFT;
        private static final long QTOP;
        private static final long QLOCK;
        private static final long QCURRENTSTEAL;
        static {
            try {
                U = sun.misc.Unsafe.getUnsafe();
                Class<?> wk = WorkQueue.class;
                Class<?> ak = ForkJoinTask[].class;
                QTOP = U.objectFieldOffset
                    (wk.getDeclaredField("top"));
                QLOCK = U.objectFieldOffset
                    (wk.getDeclaredField("qlock"));
                QCURRENTSTEAL = U.objectFieldOffset
                    (wk.getDeclaredField("currentSteal"));
                ABASE = U.arrayBaseOffset(ak);
                int scale = U.arrayIndexScale(ak);
                if ((scale & (scale - 1)) != 0)
                    throw new Error("data type scale not a power of two");
                ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
            } catch (Exception e) {
                throw new Error(e);
            }
        }
    }

    // static fields (initialized in static initializer below)

    
Creates a new ForkJoinWorkerThread. This factory is used unless
overridden in ForkJoinPool constructors.
/**
     * Creates a new ForkJoinWorkerThread. This factory is used unless
     * overridden in ForkJoinPool constructors.
     */
    public static final ForkJoinWorkerThreadFactory
        defaultForkJoinWorkerThreadFactory;

    
Permission required for callers of methods that may start or
kill threads.
/**
     * Permission required for callers of methods that may start or
     * kill threads.
     */
    private static final RuntimePermission modifyThreadPermission;

    
Common (static) pool. Non-null for public use unless a static
construction exception, but internal usages null-check on use
to paranoically avoid potential initialization circularities
as well as to simplify generated code.
/**
     * Common (static) pool. Non-null for public use unless a static
     * construction exception, but internal usages null-check on use
     * to paranoically avoid potential initialization circularities
     * as well as to simplify generated code.
     */
    static final ForkJoinPool common;

    
Common pool parallelism. To allow simpler use and management
when common pool threads are disabled, we allow the underlying
common.parallelism field to be zero, but in that case still report
parallelism as 1 to reflect resulting caller-runs mechanics.
/**
     * Common pool parallelism. To allow simpler use and management
     * when common pool threads are disabled, we allow the underlying
     * common.parallelism field to be zero, but in that case still report
     * parallelism as 1 to reflect resulting caller-runs mechanics.
     */
    static final int commonParallelism;

    
Limit on spare thread construction in tryCompensate.
/**
     * Limit on spare thread construction in tryCompensate.
     */
    private static int commonMaxSpares;

    
Sequence number for creating workerNamePrefix.
/**
     * Sequence number for creating workerNamePrefix.
     */
    private static int poolNumberSequence;

    
Returns the next sequence number. We don't expect this to
ever contend, so use simple builtin sync.
/**
     * Returns the next sequence number. We don't expect this to
     * ever contend, so use simple builtin sync.
     */
    private static final synchronized int nextPoolId() {
        return ++poolNumberSequence;
    }

    // static configuration constants

    
Initial timeout value (in nanoseconds) for the thread
triggering quiescence to park waiting for new work. On timeout,
the thread will instead try to shrink the number of
workers. The value should be large enough to avoid overly
aggressive shrinkage during most transient stalls (long GCs
etc).
/**
     * Initial timeout value (in nanoseconds) for the thread
     * triggering quiescence to park waiting for new work. On timeout,
     * the thread will instead try to shrink the number of
     * workers. The value should be large enough to avoid overly
     * aggressive shrinkage during most transient stalls (long GCs
     * etc).
     */
    private static final long IDLE_TIMEOUT = 2000L * 1000L * 1000L; // 2sec

    
Tolerance for idle timeouts, to cope with timer undershoots
/**
     * Tolerance for idle timeouts, to cope with timer undershoots
     */
    private static final long TIMEOUT_SLOP = 20L * 1000L * 1000L;  // 20ms

    
The initial value for commonMaxSpares during static
initialization. The value is far in excess of normal
requirements, but also far short of MAX_CAP and typical
OS thread limits, so allows JVMs to catch misuse/abuse
before running out of resources needed to do so.
/**
     * The initial value for commonMaxSpares during static
     * initialization. The value is far in excess of normal
     * requirements, but also far short of MAX_CAP and typical
     * OS thread limits, so allows JVMs to catch misuse/abuse
     * before running out of resources needed to do so.
     */
    private static final int DEFAULT_COMMON_MAX_SPARES = 256;

    
Number of times to spin-wait before blocking. The spins (in
awaitRunStateLock and awaitWork) currently use randomized
spins. Currently set to zero to reduce CPU usage.
If greater than zero the value of SPINS must be a power
of two, at least 4.  A value of 2048 causes spinning for a
small fraction of typical context-switch times.
If/when MWAIT-like intrinsics becomes available, they
may allow quieter spinning.
/**
     * Number of times to spin-wait before blocking. The spins (in
     * awaitRunStateLock and awaitWork) currently use randomized
     * spins. Currently set to zero to reduce CPU usage.
     *
     * If greater than zero the value of SPINS must be a power
     * of two, at least 4.  A value of 2048 causes spinning for a
     * small fraction of typical context-switch times.
     *
     * If/when MWAIT-like intrinsics becomes available, they
     * may allow quieter spinning.
     */
    private static final int SPINS  = 0;

    
Increment for seed generators. See class ThreadLocal for
explanation.
/**
     * Increment for seed generators. See class ThreadLocal for
     * explanation.
     */
    private static final int SEED_INCREMENT = 0x9e3779b9;

    /*
     * Bits and masks for field ctl, packed with 4 16 bit subfields:
     * AC: Number of active running workers minus target parallelism
     * TC: Number of total workers minus target parallelism
     * SS: version count and status of top waiting thread
     * ID: poolIndex of top of Treiber stack of waiters
     *
     * When convenient, we can extract the lower 32 stack top bits
     * (including version bits) as sp=(int)ctl.  The offsets of counts
     * by the target parallelism and the positionings of fields makes
     * it possible to perform the most common checks via sign tests of
     * fields: When ac is negative, there are not enough active
     * workers, when tc is negative, there are not enough total
     * workers.  When sp is non-zero, there are waiting workers.  To
     * deal with possibly negative fields, we use casts in and out of
     * "short" and/or signed shifts to maintain signedness.
     *
     * Because it occupies uppermost bits, we can add one active count
     * using getAndAddLong of AC_UNIT, rather than CAS, when returning
     * from a blocked join.  Other updates entail multiple subfields
     * and masking, requiring CAS.
     */

    // Lower and upper word masks
    private static final long SP_MASK    = 0xffffffffL;
    private static final long UC_MASK    = ~SP_MASK;

    // Active counts
    private static final int  AC_SHIFT   = 48;
    private static final long AC_UNIT    = 0x0001L << AC_SHIFT;
    private static final long AC_MASK    = 0xffffL << AC_SHIFT;

    // Total counts
    private static final int  TC_SHIFT   = 32;
    private static final long TC_UNIT    = 0x0001L << TC_SHIFT;
    private static final long TC_MASK    = 0xffffL << TC_SHIFT;
    private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign

    // runState bits: SHUTDOWN must be negative, others arbitrary powers of two
    private static final int  RSLOCK     = 1;
    private static final int  RSIGNAL    = 1 << 1;
    private static final int  STARTED    = 1 << 2;
    private static final int  STOP       = 1 << 29;
    private static final int  TERMINATED = 1 << 30;
    private static final int  SHUTDOWN   = 1 << 31;

    // Instance fields
    volatile long ctl;                   // main pool control
    volatile int runState;               // lockable status
    final int config;                    // parallelism, mode
    int indexSeed;                       // to generate worker index
    volatile WorkQueue[] workQueues;     // main registry
    final ForkJoinWorkerThreadFactory factory;
    final UncaughtExceptionHandler ueh;  // per-worker UEH
    final String workerNamePrefix;       // to create worker name string
    volatile AtomicLong stealCounter;    // also used as sync monitor

    
Acquires the runState lock; returns current (locked) runState.
/**
     * Acquires the runState lock; returns current (locked) runState.
     */
    private int lockRunState() {
        int rs;
        return ((((rs = runState) & RSLOCK) != 0 ||
                 !U.compareAndSwapInt(this, RUNSTATE, rs, rs |= RSLOCK)) ?
                awaitRunStateLock() : rs);
    }

    
Spins and/or blocks until runstate lock is available.  See
above for explanation.
/**
     * Spins and/or blocks until runstate lock is available.  See
     * above for explanation.
     */
    private int awaitRunStateLock() {
        Object lock;
        boolean wasInterrupted = false;
        for (int spins = SPINS, r = 0, rs, ns;;) {
            if (((rs = runState) & RSLOCK) == 0) {
                if (U.compareAndSwapInt(this, RUNSTATE, rs, ns = rs | RSLOCK)) {
                    if (wasInterrupted) {
                        try {
                            Thread.currentThread().interrupt();
                        } catch (SecurityException ignore) {
                        }
                    }
                    return ns;
                }
            }
            else if (r == 0)
                r = ThreadLocalRandom.nextSecondarySeed();
            else if (spins > 0) {
                r ^= r << 6; r ^= r >>> 21; r ^= r << 7; // xorshift
                if (r >= 0)
                    --spins;
            }
            else if ((rs & STARTED) == 0 || (lock = stealCounter) == null)
                Thread.yield();   // initialization race
            else if (U.compareAndSwapInt(this, RUNSTATE, rs, rs | RSIGNAL)) {
                synchronized (lock) {
                    if ((runState & RSIGNAL) != 0) {
                        try {
                            lock.wait();
                        } catch (InterruptedException ie) {
                            if (!(Thread.currentThread() instanceof
                                  ForkJoinWorkerThread))
                                wasInterrupted = true;
                        }
                    }
                    else
                        lock.notifyAll();
                }
            }
        }
    }

    
Unlocks and sets runState to newRunState.
Params:
oldRunState – a value returned from lockRunState
newRunState – the next value (must have lock bit clear)./**
     * Unlocks and sets runState to newRunState.
     *
     * @param oldRunState a value returned from lockRunState
     * @param newRunState the next value (must have lock bit clear).
     */
    private void unlockRunState(int oldRunState, int newRunState) {
        if (!U.compareAndSwapInt(this, RUNSTATE, oldRunState, newRunState)) {
            Object lock = stealCounter;
            runState = newRunState;              // clears RSIGNAL bit
            if (lock != null)
                synchronized (lock) { lock.notifyAll(); }
        }
    }

    // Creating, registering and deregistering workers

    
Tries to construct and start one worker. Assumes that total
count has already been incremented as a reservation.  Invokes
deregisterWorker on any failure.
Returns:
true if successful/**
     * Tries to construct and start one worker. Assumes that total
     * count has already been incremented as a reservation.  Invokes
     * deregisterWorker on any failure.
     *
     * @return true if successful
     */
    private boolean createWorker() {
        ForkJoinWorkerThreadFactory fac = factory;
        Throwable ex = null;
        ForkJoinWorkerThread wt = null;
        try {
            if (fac != null && (wt = fac.newThread(this)) != null) {
                wt.start();
                return true;
            }
        } catch (Throwable rex) {
            ex = rex;
        }
        deregisterWorker(wt, ex);
        return false;
    }

    
Tries to add one worker, incrementing ctl counts before doing
so, relying on createWorker to back out on failure.
Params:
c – incoming ctl value, with total count negative and no
idle workers.  On CAS failure, c is refreshed and retried if
this holds (otherwise, a new worker is not needed)./**
     * Tries to add one worker, incrementing ctl counts before doing
     * so, relying on createWorker to back out on failure.
     *
     * @param c incoming ctl value, with total count negative and no
     * idle workers.  On CAS failure, c is refreshed and retried if
     * this holds (otherwise, a new worker is not needed).
     */
    private void tryAddWorker(long c) {
        boolean add = false;
        do {
            long nc = ((AC_MASK & (c + AC_UNIT)) |
                       (TC_MASK & (c + TC_UNIT)));
            if (ctl == c) {
                int rs, stop;                 // check if terminating
                if ((stop = (rs = lockRunState()) & STOP) == 0)
                    add = U.compareAndSwapLong(this, CTL, c, nc);
                unlockRunState(rs, rs & ~RSLOCK);
                if (stop != 0)
                    break;
                if (add) {
                    createWorker();
                    break;
                }
            }
        } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0);
    }

    
Callback from ForkJoinWorkerThread constructor to establish and
record its WorkQueue.
Params:
wt – the worker thread
Returns:
the worker's queue/**
     * Callback from ForkJoinWorkerThread constructor to establish and
     * record its WorkQueue.
     *
     * @param wt the worker thread
     * @return the worker's queue
     */
    final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
        UncaughtExceptionHandler handler;
        wt.setDaemon(true);                           // configure thread
        if ((handler = ueh) != null)
            wt.setUncaughtExceptionHandler(handler);
        WorkQueue w = new WorkQueue(this, wt);
        int i = 0;                                    // assign a pool index
        int mode = config & MODE_MASK;
        int rs = lockRunState();
        try {
            WorkQueue[] ws; int n;                    // skip if no array
            if ((ws = workQueues) != null && (n = ws.length) > 0) {
                int s = indexSeed += SEED_INCREMENT;  // unlikely to collide
                int m = n - 1;
                i = ((s << 1) | 1) & m;               // odd-numbered indices
                if (ws[i] != null) {                  // collision
                    int probes = 0;                   // step by approx half n
                    int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
                    while (ws[i = (i + step) & m] != null) {
                        if (++probes >= n) {
                            workQueues = ws = Arrays.copyOf(ws, n <<= 1);
                            m = n - 1;
                            probes = 0;
                        }
                    }
                }
                w.hint = s;                           // use as random seed
                w.config = i | mode;
                w.scanState = i;                      // publication fence
                ws[i] = w;
            }
        } finally {
            unlockRunState(rs, rs & ~RSLOCK);
        }
        wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1)));
        return w;
    }

    
Final callback from terminating worker, as well as upon failure
to construct or start a worker.  Removes record of worker from
array, and adjusts counts. If pool is shutting down, tries to
complete termination.
Params:
wt – the worker thread, or null if construction failed
ex – the exception causing failure, or null if none/**
     * Final callback from terminating worker, as well as upon failure
     * to construct or start a worker.  Removes record of worker from
     * array, and adjusts counts. If pool is shutting down, tries to
     * complete termination.
     *
     * @param wt the worker thread, or null if construction failed
     * @param ex the exception causing failure, or null if none
     */
    final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
        WorkQueue w = null;
        if (wt != null && (w = wt.workQueue) != null) {
            WorkQueue[] ws;                           // remove index from array
            int idx = w.config & SMASK;
            int rs = lockRunState();
            if ((ws = workQueues) != null && ws.length > idx && ws[idx] == w)
                ws[idx] = null;
            unlockRunState(rs, rs & ~RSLOCK);
        }
        long c;                                       // decrement counts
        do {} while (!U.compareAndSwapLong
                     (this, CTL, c = ctl, ((AC_MASK & (c - AC_UNIT)) |
                                           (TC_MASK & (c - TC_UNIT)) |
                                           (SP_MASK & c))));
        if (w != null) {
            w.qlock = -1;                             // ensure set
            w.transferStealCount(this);
            w.cancelAll();                            // cancel remaining tasks
        }
        for (;;) {                                    // possibly replace
            WorkQueue[] ws; int m, sp;
            if (tryTerminate(false, false) || w == null || w.array == null ||
                (runState & STOP) != 0 || (ws = workQueues) == null ||
                (m = ws.length - 1) < 0)              // already terminating
                break;
            if ((sp = (int)(c = ctl)) != 0) {         // wake up replacement
                if (tryRelease(c, ws[sp & m], AC_UNIT))
                    break;
            }
            else if (ex != null && (c & ADD_WORKER) != 0L) {
                tryAddWorker(c);                      // create replacement
                break;
            }
            else                                      // don't need replacement
                break;
        }
        if (ex == null)                               // help clean on way out
            ForkJoinTask.helpExpungeStaleExceptions();
        else                                          // rethrow
            ForkJoinTask.rethrow(ex);
    }

    // Signalling

    
Tries to create or activate a worker if too few are active.
Params:
ws – the worker array to use to find signallees
q – a WorkQueue --if non-null, don't retry if now empty/**
     * Tries to create or activate a worker if too few are active.
     *
     * @param ws the worker array to use to find signallees
     * @param q a WorkQueue --if non-null, don't retry if now empty
     */
    final void signalWork(WorkQueue[] ws, WorkQueue q) {
        long c; int sp, i; WorkQueue v; Thread p;
        while ((c = ctl) < 0L) {                       // too few active
            if ((sp = (int)c) == 0) {                  // no idle workers
                if ((c & ADD_WORKER) != 0L)            // too few workers
                    tryAddWorker(c);
                break;
            }
            if (ws == null)                            // unstarted/terminated
                break;
            if (ws.length <= (i = sp & SMASK))         // terminated
                break;
            if ((v = ws[i]) == null)                   // terminating
                break;
            int vs = (sp + SS_SEQ) & ~INACTIVE;        // next scanState
            int d = sp - v.scanState;                  // screen CAS
            long nc = (UC_MASK & (c + AC_UNIT)) | (SP_MASK & v.stackPred);
            if (d == 0 && U.compareAndSwapLong(this, CTL, c, nc)) {
                v.scanState = vs;                      // activate v
                if ((p = v.parker) != null)
                    U.unpark(p);
                break;
            }
            if (q != null && q.base == q.top)          // no more work
                break;
        }
    }

    
Signals and releases worker v if it is top of idle worker
stack.  This performs a one-shot version of signalWork only if
there is (apparently) at least one idle worker.
Params:
c – incoming ctl value
v – if non-null, a worker
inc – the increment to active count (zero when compensating)
Returns:
true if successful/**
     * Signals and releases worker v if it is top of idle worker
     * stack.  This performs a one-shot version of signalWork only if
     * there is (apparently) at least one idle worker.
     *
     * @param c incoming ctl value
     * @param v if non-null, a worker
     * @param inc the increment to active count (zero when compensating)
     * @return true if successful
     */
    private boolean tryRelease(long c, WorkQueue v, long inc) {
        int sp = (int)c, vs = (sp + SS_SEQ) & ~INACTIVE; Thread p;
        if (v != null && v.scanState == sp) {          // v is at top of stack
            long nc = (UC_MASK & (c + inc)) | (SP_MASK & v.stackPred);
            if (U.compareAndSwapLong(this, CTL, c, nc)) {
                v.scanState = vs;
                if ((p = v.parker) != null)
                    U.unpark(p);
                return true;
            }
        }
        return false;
    }

    // Scanning for tasks

    
Top-level runloop for workers, called by ForkJoinWorkerThread.run.
/**
     * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
     */
    final void runWorker(WorkQueue w) {
        w.growArray();                   // allocate queue
        int seed = w.hint;               // initially holds randomization hint
        int r = (seed == 0) ? 1 : seed;  // avoid 0 for xorShift
        for (ForkJoinTask<?> t;;) {
            if ((t = scan(w, r)) != null)
                w.runTask(t);
            else if (!awaitWork(w, r))
                break;
            r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
        }
    }

    
Scans for and tries to steal a top-level task. Scans start at a
random location, randomly moving on apparent contention,
otherwise continuing linearly until reaching two consecutive
empty passes over all queues with the same checksum (summing
each base index of each queue, that moves on each steal), at
which point the worker tries to inactivate and then re-scans,
attempting to re-activate (itself or some other worker) if
finding a task; otherwise returning null to await work.  Scans
otherwise touch as little memory as possible, to reduce
disruption on other scanning threads.
Params:
w – the worker (via its WorkQueue)
r – a random seed
Returns:
a task, or null if none found/**
     * Scans for and tries to steal a top-level task. Scans start at a
     * random location, randomly moving on apparent contention,
     * otherwise continuing linearly until reaching two consecutive
     * empty passes over all queues with the same checksum (summing
     * each base index of each queue, that moves on each steal), at
     * which point the worker tries to inactivate and then re-scans,
     * attempting to re-activate (itself or some other worker) if
     * finding a task; otherwise returning null to await work.  Scans
     * otherwise touch as little memory as possible, to reduce
     * disruption on other scanning threads.
     *
     * @param w the worker (via its WorkQueue)
     * @param r a random seed
     * @return a task, or null if none found
     */
    private ForkJoinTask<?> scan(WorkQueue w, int r) {
        WorkQueue[] ws; int m;
        if ((ws = workQueues) != null && (m = ws.length - 1) > 0 && w != null) {
            int ss = w.scanState;                     // initially non-negative
            for (int origin = r & m, k = origin, oldSum = 0, checkSum = 0;;) {
                WorkQueue q; ForkJoinTask<?>[] a; ForkJoinTask<?> t;
                int b, n; long c;
                if ((q = ws[k]) != null) {
                    if ((n = (b = q.base) - q.top) < 0 &&
                        (a = q.array) != null) {      // non-empty
                        long i = (((a.length - 1) & b) << ASHIFT) + ABASE;
                        if ((t = ((ForkJoinTask<?>)
                                  U.getObjectVolatile(a, i))) != null &&
                            q.base == b) {
                            if (ss >= 0) {
                                if (U.compareAndSwapObject(a, i, t, null)) {
                                    q.base = b + 1;
                                    if (n < -1)       // signal others
                                        signalWork(ws, q);
                                    return t;
                                }
                            }
                            else if (oldSum == 0 &&   // try to activate
                                     w.scanState < 0)
                                tryRelease(c = ctl, ws[m & (int)c], AC_UNIT);
                        }
                        if (ss < 0)                   // refresh
                            ss = w.scanState;
                        r ^= r << 1; r ^= r >>> 3; r ^= r << 10;
                        origin = k = r & m;           // move and rescan
                        oldSum = checkSum = 0;
                        continue;
                    }
                    checkSum += b;
                }
                if ((k = (k + 1) & m) == origin) {    // continue until stable
                    if ((ss >= 0 || (ss == (ss = w.scanState))) &&
                        oldSum == (oldSum = checkSum)) {
                        if (ss < 0 || w.qlock < 0)    // already inactive
                            break;
                        int ns = ss | INACTIVE;       // try to inactivate
                        long nc = ((SP_MASK & ns) |
                                   (UC_MASK & ((c = ctl) - AC_UNIT)));
                        w.stackPred = (int)c;         // hold prev stack top
                        U.putInt(w, QSCANSTATE, ns);
                        if (U.compareAndSwapLong(this, CTL, c, nc))
                            ss = ns;
                        else
                            w.scanState = ss;         // back out
                    }
                    checkSum = 0;
                }
            }
        }
        return null;
    }

    
Possibly blocks worker w waiting for a task to steal, or
returns false if the worker should terminate.  If inactivating
w has caused the pool to become quiescent, checks for pool
termination, and, so long as this is not the only worker, waits
for up to a given duration.  On timeout, if ctl has not
changed, terminates the worker, which will in turn wake up
another worker to possibly repeat this process.
Params:
w – the calling worker
r – a random seed (for spins)
Returns:
false if the worker should terminate/**
     * Possibly blocks worker w waiting for a task to steal, or
     * returns false if the worker should terminate.  If inactivating
     * w has caused the pool to become quiescent, checks for pool
     * termination, and, so long as this is not the only worker, waits
     * for up to a given duration.  On timeout, if ctl has not
     * changed, terminates the worker, which will in turn wake up
     * another worker to possibly repeat this process.
     *
     * @param w the calling worker
     * @param r a random seed (for spins)
     * @return false if the worker should terminate
     */
    private boolean awaitWork(WorkQueue w, int r) {
        if (w == null || w.qlock < 0)                 // w is terminating
            return false;
        for (int pred = w.stackPred, spins = SPINS, ss;;) {
            if ((ss = w.scanState) >= 0)
                break;
            else if (spins > 0) {
                r ^= r << 6; r ^= r >>> 21; r ^= r << 7;
                if (r >= 0 && --spins == 0) {         // randomize spins
                    WorkQueue v; WorkQueue[] ws; int s, j; AtomicLong sc;
                    if (pred != 0 && (ws = workQueues) != null &&
                        (j = pred & SMASK) < ws.length &&
                        (v = ws[j]) != null &&        // see if pred parking
                        (v.parker == null || v.scanState >= 0))
                        spins = SPINS;                // continue spinning
                }
            }
            else if (w.qlock < 0)                     // recheck after spins
                return false;
            else if (!Thread.interrupted()) {
                long c, prevctl, parkTime, deadline;
                int ac = (int)((c = ctl) >> AC_SHIFT) + (config & SMASK);
                if ((ac <= 0 && tryTerminate(false, false)) ||
                    (runState & STOP) != 0)           // pool terminating
                    return false;
                if (ac <= 0 && ss == (int)c) {        // is last waiter
                    prevctl = (UC_MASK & (c + AC_UNIT)) | (SP_MASK & pred);
                    int t = (short)(c >>> TC_SHIFT);  // shrink excess spares
                    if (t > 2 && U.compareAndSwapLong(this, CTL, c, prevctl))
                        return false;                 // else use timed wait
                    parkTime = IDLE_TIMEOUT * ((t >= 0) ? 1 : 1 - t);
                    deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP;
                }
                else
                    prevctl = parkTime = deadline = 0L;
                Thread wt = Thread.currentThread();
                U.putObject(wt, PARKBLOCKER, this);   // emulate LockSupport
                w.parker = wt;
                if (w.scanState < 0 && ctl == c)      // recheck before park
                    U.park(false, parkTime);
                U.putOrderedObject(w, QPARKER, null);
                U.putObject(wt, PARKBLOCKER, null);
                if (w.scanState >= 0)
                    break;
                if (parkTime != 0L && ctl == c &&
                    deadline - System.nanoTime() <= 0L &&
                    U.compareAndSwapLong(this, CTL, c, prevctl))
                    return false;                     // shrink pool
            }
        }
        return true;
    }

    // Joining tasks

    
Tries to steal and run tasks within the target's computation.
Uses a variant of the top-level algorithm, restricted to tasks
with the given task as ancestor: It prefers taking and running
eligible tasks popped from the worker's own queue (via
popCC). Otherwise it scans others, randomly moving on
contention or execution, deciding to give up based on a
checksum (via return codes frob pollAndExecCC). The maxTasks
argument supports external usages; internal calls use zero,
allowing unbounded steps (external calls trap non-positive
values).
Params:
w – caller
maxTasks – if non-zero, the maximum number of other tasks to run
Returns:
task status on exit/**
     * Tries to steal and run tasks within the target's computation.
     * Uses a variant of the top-level algorithm, restricted to tasks
     * with the given task as ancestor: It prefers taking and running
     * eligible tasks popped from the worker's own queue (via
     * popCC). Otherwise it scans others, randomly moving on
     * contention or execution, deciding to give up based on a
     * checksum (via return codes frob pollAndExecCC). The maxTasks
     * argument supports external usages; internal calls use zero,
     * allowing unbounded steps (external calls trap non-positive
     * values).
     *
     * @param w caller
     * @param maxTasks if non-zero, the maximum number of other tasks to run
     * @return task status on exit
     */
    final int helpComplete(WorkQueue w, CountedCompleter<?> task,
                           int maxTasks) {
        WorkQueue[] ws; int s = 0, m;
        if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 &&
            task != null && w != null) {
            int mode = w.config;                 // for popCC
            int r = w.hint ^ w.top;              // arbitrary seed for origin
            int origin = r & m;                  // first queue to scan
            int h = 1;                           // 1:ran, >1:contended, <0:hash
            for (int k = origin, oldSum = 0, checkSum = 0;;) {
                CountedCompleter<?> p; WorkQueue q;
                if ((s = task.status) < 0)
                    break;
                if (h == 1 && (p = w.popCC(task, mode)) != null) {
                    p.doExec();                  // run local task
                    if (maxTasks != 0 && --maxTasks == 0)
                        break;
                    origin = k;                  // reset
                    oldSum = checkSum = 0;
                }
                else {                           // poll other queues
                    if ((q = ws[k]) == null)
                        h = 0;
                    else if ((h = q.pollAndExecCC(task)) < 0)
                        checkSum += h;
                    if (h > 0) {
                        if (h == 1 && maxTasks != 0 && --maxTasks == 0)
                            break;
                        r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
                        origin = k = r & m;      // move and restart
                        oldSum = checkSum = 0;
                    }
                    else if ((k = (k + 1) & m) == origin) {
                        if (oldSum == (oldSum = checkSum))
                            break;
                        checkSum = 0;
                    }
                }
            }
        }
        return s;
    }

    
Tries to locate and execute tasks for a stealer of the given
task, or in turn one of its stealers, Traces currentSteal ->
currentJoin links looking for a thread working on a descendant
of the given task and with a non-empty queue to steal back and
execute tasks from. The first call to this method upon a
waiting join will often entail scanning/search, (which is OK
because the joiner has nothing better to do), but this method
leaves hints in workers to speed up subsequent calls.
Params:
w – caller
task – the task to join/**
     * Tries to locate and execute tasks for a stealer of the given
     * task, or in turn one of its stealers, Traces currentSteal ->
     * currentJoin links looking for a thread working on a descendant
     * of the given task and with a non-empty queue to steal back and
     * execute tasks from. The first call to this method upon a
     * waiting join will often entail scanning/search, (which is OK
     * because the joiner has nothing better to do), but this method
     * leaves hints in workers to speed up subsequent calls.
     *
     * @param w caller
     * @param task the task to join
     */
    private void helpStealer(WorkQueue w, ForkJoinTask<?> task) {
        WorkQueue[] ws = workQueues;
        int oldSum = 0, checkSum, m;
        if (ws != null && (m = ws.length - 1) >= 0 && w != null &&
            task != null) {
            do {                                       // restart point
                checkSum = 0;                          // for stability check
                ForkJoinTask<?> subtask;
                WorkQueue j = w, v;                    // v is subtask stealer
                descent: for (subtask = task; subtask.status >= 0; ) {
                    for (int h = j.hint | 1, k = 0, i; ; k += 2) {
                        if (k > m)                     // can't find stealer
                            break descent;
                        if ((v = ws[i = (h + k) & m]) != null) {
                            if (v.currentSteal == subtask) {
                                j.hint = i;
                                break;
                            }
                            checkSum += v.base;
                        }
                    }
                    for (;;) {                         // help v or descend
                        ForkJoinTask<?>[] a; int b;
                        checkSum += (b = v.base);
                        ForkJoinTask<?> next = v.currentJoin;
                        if (subtask.status < 0 || j.currentJoin != subtask ||
                            v.currentSteal != subtask) // stale
                            break descent;
                        if (b - v.top >= 0 || (a = v.array) == null) {
                            if ((subtask = next) == null)
                                break descent;
                            j = v;
                            break;
                        }
                        int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
                        ForkJoinTask<?> t = ((ForkJoinTask<?>)
                                             U.getObjectVolatile(a, i));
                        if (v.base == b) {
                            if (t == null)             // stale
                                break descent;
                            if (U.compareAndSwapObject(a, i, t, null)) {
                                v.base = b + 1;
                                ForkJoinTask<?> ps = w.currentSteal;
                                int top = w.top;
                                do {
                                    U.putOrderedObject(w, QCURRENTSTEAL, t);
                                    t.doExec();        // clear local tasks too
                                } while (task.status >= 0 &&
                                         w.top != top &&
                                         (t = w.pop()) != null);
                                U.putOrderedObject(w, QCURRENTSTEAL, ps);
                                if (w.base != w.top)
                                    return;            // can't further help
                            }
                        }
                    }
                }
            } while (task.status >= 0 && oldSum != (oldSum = checkSum));
        }
    }

    
Tries to decrement active count (sometimes implicitly) and
possibly release or create a compensating worker in preparation
for blocking. Returns false (retryable by caller), on
contention, detected staleness, instability, or termination.
Params:
w – caller/**
     * Tries to decrement active count (sometimes implicitly) and
     * possibly release or create a compensating worker in preparation
     * for blocking. Returns false (retryable by caller), on
     * contention, detected staleness, instability, or termination.
     *
     * @param w caller
     */
    private boolean tryCompensate(WorkQueue w) {
        boolean canBlock;
        WorkQueue[] ws; long c; int m, pc, sp;
        if (w == null || w.qlock < 0 ||           // caller terminating
            (ws = workQueues) == null || (m = ws.length - 1) <= 0 ||
            (pc = config & SMASK) == 0)           // parallelism disabled
            canBlock = false;
        else if ((sp = (int)(c = ctl)) != 0)      // release idle worker
            canBlock = tryRelease(c, ws[sp & m], 0L);
        else {
            int ac = (int)(c >> AC_SHIFT) + pc;
            int tc = (short)(c >> TC_SHIFT) + pc;
            int nbusy = 0;                        // validate saturation
            for (int i = 0; i <= m; ++i) {        // two passes of odd indices
                WorkQueue v;
                if ((v = ws[((i << 1) | 1) & m]) != null) {
                    if ((v.scanState & SCANNING) != 0)
                        break;
                    ++nbusy;
                }
            }
            if (nbusy != (tc << 1) || ctl != c)
                canBlock = false;                 // unstable or stale
            else if (tc >= pc && ac > 1 && w.isEmpty()) {
                long nc = ((AC_MASK & (c - AC_UNIT)) |
                           (~AC_MASK & c));       // uncompensated
                canBlock = U.compareAndSwapLong(this, CTL, c, nc);
            }
            else if (tc >= MAX_CAP ||
                     (this == common && tc >= pc + commonMaxSpares))
                throw new RejectedExecutionException(
                    "Thread limit exceeded replacing blocked worker");
            else {                                // similar to tryAddWorker
                boolean add = false; int rs;      // CAS within lock
                long nc = ((AC_MASK & c) |
                           (TC_MASK & (c + TC_UNIT)));
                if (((rs = lockRunState()) & STOP) == 0)
                    add = U.compareAndSwapLong(this, CTL, c, nc);
                unlockRunState(rs, rs & ~RSLOCK);
                canBlock = add && createWorker(); // throws on exception
            }
        }
        return canBlock;
    }

    
Helps and/or blocks until the given task is done or timeout.
Params:
w – caller
task – the task
deadline – for timed waits, if nonzero
Returns:
task status on exit/**
     * Helps and/or blocks until the given task is done or timeout.
     *
     * @param w caller
     * @param task the task
     * @param deadline for timed waits, if nonzero
     * @return task status on exit
     */
    final int awaitJoin(WorkQueue w, ForkJoinTask<?> task, long deadline) {
        int s = 0;
        if (task != null && w != null) {
            ForkJoinTask<?> prevJoin = w.currentJoin;
            U.putOrderedObject(w, QCURRENTJOIN, task);
            CountedCompleter<?> cc = (task instanceof CountedCompleter) ?
                (CountedCompleter<?>)task : null;
            for (;;) {
                if ((s = task.status) < 0)
                    break;
                if (cc != null)
                    helpComplete(w, cc, 0);
                else if (w.base == w.top || w.tryRemoveAndExec(task))
                    helpStealer(w, task);
                if ((s = task.status) < 0)
                    break;
                long ms, ns;
                if (deadline == 0L)
                    ms = 0L;
                else if ((ns = deadline - System.nanoTime()) <= 0L)
                    break;
                else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
                    ms = 1L;
                if (tryCompensate(w)) {
                    task.internalWait(ms);
                    U.getAndAddLong(this, CTL, AC_UNIT);
                }
            }
            U.putOrderedObject(w, QCURRENTJOIN, prevJoin);
        }
        return s;
    }

    // Specialized scanning

    
Returns a (probably) non-empty steal queue, if one is found
during a scan, else null.  This method must be retried by
caller if, by the time it tries to use the queue, it is empty.
/**
     * Returns a (probably) non-empty steal queue, if one is found
     * during a scan, else null.  This method must be retried by
     * caller if, by the time it tries to use the queue, it is empty.
     */
    private WorkQueue findNonEmptyStealQueue() {
        WorkQueue[] ws; int m;  // one-shot version of scan loop
        int r = ThreadLocalRandom.nextSecondarySeed();
        if ((ws = workQueues) != null && (m = ws.length - 1) >= 0) {
            for (int origin = r & m, k = origin, oldSum = 0, checkSum = 0;;) {
                WorkQueue q; int b;
                if ((q = ws[k]) != null) {
                    if ((b = q.base) - q.top < 0)
                        return q;
                    checkSum += b;
                }
                if ((k = (k + 1) & m) == origin) {
                    if (oldSum == (oldSum = checkSum))
                        break;
                    checkSum = 0;
                }
            }
        }
        return null;
    }

    
Runs tasks until isQuiescent(). We piggyback on active count ctl maintenance, but rather than blocking when tasks cannot be found, we rescan until all others cannot find tasks either. /**
     * Runs tasks until {@code isQuiescent()}. We piggyback on
     * active count ctl maintenance, but rather than blocking
     * when tasks cannot be found, we rescan until all others cannot
     * find tasks either.
     */
    final void helpQuiescePool(WorkQueue w) {
        ForkJoinTask<?> ps = w.currentSteal; // save context
        for (boolean active = true;;) {
            long c; WorkQueue q; ForkJoinTask<?> t; int b;
            w.execLocalTasks();     // run locals before each scan
            if ((q = findNonEmptyStealQueue()) != null) {
                if (!active) {      // re-establish active count
                    active = true;
                    U.getAndAddLong(this, CTL, AC_UNIT);
                }
                if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) {
                    U.putOrderedObject(w, QCURRENTSTEAL, t);
                    t.doExec();
                    if (++w.nsteals < 0)
                        w.transferStealCount(this);
                }
            }
            else if (active) {      // decrement active count without queuing
                long nc = (AC_MASK & ((c = ctl) - AC_UNIT)) | (~AC_MASK & c);
                if ((int)(nc >> AC_SHIFT) + (config & SMASK) <= 0)
                    break;          // bypass decrement-then-increment
                if (U.compareAndSwapLong(this, CTL, c, nc))
                    active = false;
            }
            else if ((int)((c = ctl) >> AC_SHIFT) + (config & SMASK) <= 0 &&
                     U.compareAndSwapLong(this, CTL, c, c + AC_UNIT))
                break;
        }
        U.putOrderedObject(w, QCURRENTSTEAL, ps);
    }

    
Gets and removes a local or stolen task for the given worker.
Returns:
a task, if available/**
     * Gets and removes a local or stolen task for the given worker.
     *
     * @return a task, if available
     */
    final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
        for (ForkJoinTask<?> t;;) {
            WorkQueue q; int b;
            if ((t = w.nextLocalTask()) != null)
                return t;
            if ((q = findNonEmptyStealQueue()) == null)
                return null;
            if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
                return t;
        }
    }

    
Returns a cheap heuristic guide for task partitioning when
programmers, frameworks, tools, or languages have little or no
idea about task granularity.  In essence, by offering this
method, we ask users only about tradeoffs in overhead vs
expected throughput and its variance, rather than how finely to
partition tasks.
In a steady state strict (tree-structured) computation, each
thread makes available for stealing enough tasks for other
threads to remain active. Inductively, if all threads play by
the same rules, each thread should make available only a
constant number of tasks.
The minimum useful constant is just 1. But using a value of 1
would require immediate replenishment upon each steal to
maintain enough tasks, which is infeasible.  Further,
partitionings/granularities of offered tasks should minimize
steal rates, which in general means that threads nearer the top
of computation tree should generate more than those nearer the
bottom. In perfect steady state, each thread is at
approximately the same level of computation tree. However,
producing extra tasks amortizes the uncertainty of progress and
diffusion assumptions.
So, users will want to use values larger (but not much larger)
than 1 to both smooth over transient shortages and hedge
against uneven progress; as traded off against the cost of
extra task overhead. We leave the user to pick a threshold
value to compare with the results of this call to guide
decisions, but recommend values such as 3.
When all threads are active, it is on average OK to estimate
surplus strictly locally. In steady-state, if one thread is
maintaining say 2 surplus tasks, then so are others. So we can
just use estimated queue length.  However, this strategy alone
leads to serious mis-estimates in some non-steady-state
conditions (ramp-up, ramp-down, other stalls). We can detect
many of these by further considering the number of "idle"
threads, that are known to have zero queued tasks, so
compensate by a factor of (#idle/#active) threads.
/**
     * Returns a cheap heuristic guide for task partitioning when
     * programmers, frameworks, tools, or languages have little or no
     * idea about task granularity.  In essence, by offering this
     * method, we ask users only about tradeoffs in overhead vs
     * expected throughput and its variance, rather than how finely to
     * partition tasks.
     *
     * In a steady state strict (tree-structured) computation, each
     * thread makes available for stealing enough tasks for other
     * threads to remain active. Inductively, if all threads play by
     * the same rules, each thread should make available only a
     * constant number of tasks.
     *
     * The minimum useful constant is just 1. But using a value of 1
     * would require immediate replenishment upon each steal to
     * maintain enough tasks, which is infeasible.  Further,
     * partitionings/granularities of offered tasks should minimize
     * steal rates, which in general means that threads nearer the top
     * of computation tree should generate more than those nearer the
     * bottom. In perfect steady state, each thread is at
     * approximately the same level of computation tree. However,
     * producing extra tasks amortizes the uncertainty of progress and
     * diffusion assumptions.
     *
     * So, users will want to use values larger (but not much larger)
     * than 1 to both smooth over transient shortages and hedge
     * against uneven progress; as traded off against the cost of
     * extra task overhead. We leave the user to pick a threshold
     * value to compare with the results of this call to guide
     * decisions, but recommend values such as 3.
     *
     * When all threads are active, it is on average OK to estimate
     * surplus strictly locally. In steady-state, if one thread is
     * maintaining say 2 surplus tasks, then so are others. So we can
     * just use estimated queue length.  However, this strategy alone
     * leads to serious mis-estimates in some non-steady-state
     * conditions (ramp-up, ramp-down, other stalls). We can detect
     * many of these by further considering the number of "idle"
     * threads, that are known to have zero queued tasks, so
     * compensate by a factor of (#idle/#active) threads.
     */
    static int getSurplusQueuedTaskCount() {
        Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
        if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
            int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).
                config & SMASK;
            int n = (q = wt.workQueue).top - q.base;
            int a = (int)(pool.ctl >> AC_SHIFT) + p;
            return n - (a > (p >>>= 1) ? 0 :
                        a > (p >>>= 1) ? 1 :
                        a > (p >>>= 1) ? 2 :
                        a > (p >>>= 1) ? 4 :
                        8);
        }
        return 0;
    }

    //  Termination

    
Possibly initiates and/or completes termination.
Params:
now – if true, unconditionally terminate, else only
if no work and no active workers
enable – if true, enable shutdown when next possible
Returns:
true if now terminating or terminated/**
     * Possibly initiates and/or completes termination.
     *
     * @param now if true, unconditionally terminate, else only
     * if no work and no active workers
     * @param enable if true, enable shutdown when next possible
     * @return true if now terminating or terminated
     */
    private boolean tryTerminate(boolean now, boolean enable) {
        int rs;
        if (this == common)                       // cannot shut down
            return false;
        if ((rs = runState) >= 0) {
            if (!enable)
                return false;
            rs = lockRunState();                  // enter SHUTDOWN phase
            unlockRunState(rs, (rs & ~RSLOCK) | SHUTDOWN);
        }

        if ((rs & STOP) == 0) {
            if (!now) {                           // check quiescence
                for (long oldSum = 0L;;) {        // repeat until stable
                    WorkQueue[] ws; WorkQueue w; int m, b; long c;
                    long checkSum = ctl;
                    if ((int)(checkSum >> AC_SHIFT) + (config & SMASK) > 0)
                        return false;             // still active workers
                    if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
                        break;                    // check queues
                    for (int i = 0; i <= m; ++i) {
                        if ((w = ws[i]) != null) {
                            if ((b = w.base) != w.top || w.scanState >= 0 ||
                                w.currentSteal != null) {
                                tryRelease(c = ctl, ws[m & (int)c], AC_UNIT);
                                return false;     // arrange for recheck
                            }
                            checkSum += b;
                            if ((i & 1) == 0)
                                w.qlock = -1;     // try to disable external
                        }
                    }
                    if (oldSum == (oldSum = checkSum))
                        break;
                }
            }
            if ((runState & STOP) == 0) {
                rs = lockRunState();              // enter STOP phase
                unlockRunState(rs, (rs & ~RSLOCK) | STOP);
            }
        }

        int pass = 0;                             // 3 passes to help terminate
        for (long oldSum = 0L;;) {                // or until done or stable
            WorkQueue[] ws; WorkQueue w; ForkJoinWorkerThread wt; int m;
            long checkSum = ctl;
            if ((short)(checkSum >>> TC_SHIFT) + (config & SMASK) <= 0 ||
                (ws = workQueues) == null || (m = ws.length - 1) <= 0) {
                if ((runState & TERMINATED) == 0) {
                    rs = lockRunState();          // done
                    unlockRunState(rs, (rs & ~RSLOCK) | TERMINATED);
                    synchronized (this) { notifyAll(); } // for awaitTermination
                }
                break;
            }
            for (int i = 0; i <= m; ++i) {
                if ((w = ws[i]) != null) {
                    checkSum += w.base;
                    w.qlock = -1;                 // try to disable
                    if (pass > 0) {
                        w.cancelAll();            // clear queue
                        if (pass > 1 && (wt = w.owner) != null) {
                            if (!wt.isInterrupted()) {
                                try {             // unblock join
                                    wt.interrupt();
                                } catch (Throwable ignore) {
                                }
                            }
                            if (w.scanState < 0)
                                U.unpark(wt);     // wake up
                        }
                    }
                }
            }
            if (checkSum != oldSum) {             // unstable
                oldSum = checkSum;
                pass = 0;
            }
            else if (pass > 3 && pass > m)        // can't further help
                break;
            else if (++pass > 1) {                // try to dequeue
                long c; int j = 0, sp;            // bound attempts
                while (j++ <= m && (sp = (int)(c = ctl)) != 0)
                    tryRelease(c, ws[sp & m], AC_UNIT);
            }
        }
        return true;
    }

    // External operations

    
Full version of externalPush, handling uncommon cases, as well
as performing secondary initialization upon the first
submission of the first task to the pool.  It also detects
first submission by an external thread and creates a new shared
queue if the one at index if empty or contended.
Params:
task – the task. Caller must ensure non-null./**
     * Full version of externalPush, handling uncommon cases, as well
     * as performing secondary initialization upon the first
     * submission of the first task to the pool.  It also detects
     * first submission by an external thread and creates a new shared
     * queue if the one at index if empty or contended.
     *
     * @param task the task. Caller must ensure non-null.
     */
    private void externalSubmit(ForkJoinTask<?> task) {
        int r;                                    // initialize caller's probe
        if ((r = ThreadLocalRandom.getProbe()) == 0) {
            ThreadLocalRandom.localInit();
            r = ThreadLocalRandom.getProbe();
        }
        for (;;) {
            WorkQueue[] ws; WorkQueue q; int rs, m, k;
            boolean move = false;
            if ((rs = runState) < 0) {
                tryTerminate(false, false);     // help terminate
                throw new RejectedExecutionException();
            }
            else if ((rs & STARTED) == 0 ||     // initialize
                     ((ws = workQueues) == null || (m = ws.length - 1) < 0)) {
                int ns = 0;
                rs = lockRunState();
                try {
                    if ((rs & STARTED) == 0) {
                        U.compareAndSwapObject(this, STEALCOUNTER, null,
                                               new AtomicLong());
                        // create workQueues array with size a power of two
                        int p = config & SMASK; // ensure at least 2 slots
                        int n = (p > 1) ? p - 1 : 1;
                        n |= n >>> 1; n |= n >>> 2;  n |= n >>> 4;
                        n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1;
                        workQueues = new WorkQueue[n];
                        ns = STARTED;
                    }
                } finally {
                    unlockRunState(rs, (rs & ~RSLOCK) | ns);
                }
            }
            else if ((q = ws[k = r & m & SQMASK]) != null) {
                if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                    ForkJoinTask<?>[] a = q.array;
                    int s = q.top;
                    boolean submitted = false; // initial submission or resizing
                    try {                      // locked version of push
                        if ((a != null && a.length > s + 1 - q.base) ||
                            (a = q.growArray()) != null) {
                            int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
                            U.putOrderedObject(a, j, task);
                            U.putOrderedInt(q, QTOP, s + 1);
                            submitted = true;
                        }
                    } finally {
                        U.compareAndSwapInt(q, QLOCK, 1, 0);
                    }
                    if (submitted) {
                        signalWork(ws, q);
                        return;
                    }
                }
                move = true;                   // move on failure
            }
            else if (((rs = runState) & RSLOCK) == 0) { // create new queue
                q = new WorkQueue(this, null);
                q.hint = r;
                q.config = k | SHARED_QUEUE;
                q.scanState = INACTIVE;
                rs = lockRunState();           // publish index
                if (rs > 0 &&  (ws = workQueues) != null &&
                    k < ws.length && ws[k] == null)
                    ws[k] = q;                 // else terminated
                unlockRunState(rs, rs & ~RSLOCK);
            }
            else
                move = true;                   // move if busy
            if (move)
                r = ThreadLocalRandom.advanceProbe(r);
        }
    }

    
Tries to add the given task to a submission queue at
submitter's current queue. Only the (vastly) most common path
is directly handled in this method, while screening for need
for externalSubmit.
Params:
task – the task. Caller must ensure non-null./**
     * Tries to add the given task to a submission queue at
     * submitter's current queue. Only the (vastly) most common path
     * is directly handled in this method, while screening for need
     * for externalSubmit.
     *
     * @param task the task. Caller must ensure non-null.
     */
    final void externalPush(ForkJoinTask<?> task) {
        WorkQueue[] ws; WorkQueue q; int m;
        int r = ThreadLocalRandom.getProbe();
        int rs = runState;
        if ((ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
            (q = ws[m & r & SQMASK]) != null && r != 0 && rs > 0 &&
            U.compareAndSwapInt(q, QLOCK, 0, 1)) {
            ForkJoinTask<?>[] a; int am, n, s;
            if ((a = q.array) != null &&
                (am = a.length - 1) > (n = (s = q.top) - q.base)) {
                int j = ((am & s) << ASHIFT) + ABASE;
                U.putOrderedObject(a, j, task);
                U.putOrderedInt(q, QTOP, s + 1);
                U.putIntVolatile(q, QLOCK, 0);
                if (n <= 1)
                    signalWork(ws, q);
                return;
            }
            U.compareAndSwapInt(q, QLOCK, 1, 0);
        }
        externalSubmit(task);
    }

    
Returns common pool queue for an external thread.
/**
     * Returns common pool queue for an external thread.
     */
    static WorkQueue commonSubmitterQueue() {
        ForkJoinPool p = common;
        int r = ThreadLocalRandom.getProbe();
        WorkQueue[] ws; int m;
        return (p != null && (ws = p.workQueues) != null &&
                (m = ws.length - 1) >= 0) ?
            ws[m & r & SQMASK] : null;
    }

    
Performs tryUnpush for an external submitter: Finds queue,
locks if apparently non-empty, validates upon locking, and
adjusts top. Each check can fail but rarely does.
/**
     * Performs tryUnpush for an external submitter: Finds queue,
     * locks if apparently non-empty, validates upon locking, and
     * adjusts top. Each check can fail but rarely does.
     */
    final boolean tryExternalUnpush(ForkJoinTask<?> task) {
        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?>[] a; int m, s;
        int r = ThreadLocalRandom.getProbe();
        if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 &&
            (w = ws[m & r & SQMASK]) != null &&
            (a = w.array) != null && (s = w.top) != w.base) {
            long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
            if (U.compareAndSwapInt(w, QLOCK, 0, 1)) {
                if (w.top == s && w.array == a &&
                    U.getObject(a, j) == task &&
                    U.compareAndSwapObject(a, j, task, null)) {
                    U.putOrderedInt(w, QTOP, s - 1);
                    U.putOrderedInt(w, QLOCK, 0);
                    return true;
                }
                U.compareAndSwapInt(w, QLOCK, 1, 0);
            }
        }
        return false;
    }

    
Performs helpComplete for an external submitter.
/**
     * Performs helpComplete for an external submitter.
     */
    final int externalHelpComplete(CountedCompleter<?> task, int maxTasks) {
        WorkQueue[] ws; int n;
        int r = ThreadLocalRandom.getProbe();
        return ((ws = workQueues) == null || (n = ws.length) == 0) ? 0 :
            helpComplete(ws[(n - 1) & r & SQMASK], task, maxTasks);
    }

    // Exported methods

    // Constructors

    
Creates a ForkJoinPool with parallelism equal to Runtime.availableProcessors, using the default thread factory, no UncaughtExceptionHandler, and non-async LIFO processing mode. 
Throws:
SecurityException – if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")/**
     * Creates a {@code ForkJoinPool} with parallelism equal to {@link
     * java.lang.Runtime#availableProcessors}, using the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool() {
        this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
             defaultForkJoinWorkerThreadFactory, null, false);
    }

    
Creates a ForkJoinPool with the indicated parallelism level, the default thread factory, no UncaughtExceptionHandler, and non-async LIFO processing mode. 
Params:
parallelism – the parallelism level
Throws:
IllegalArgumentException – if parallelism less than or
        equal to zero, or greater than implementation limit
SecurityException – if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")/**
     * Creates a {@code ForkJoinPool} with the indicated parallelism
     * level, the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @param parallelism the parallelism level
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism) {
        this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
    }

    
Creates a ForkJoinPool with the given parameters. 
Params:
parallelism – the parallelism level. For default value, use Runtime.availableProcessors.
factory – the factory for creating new threads. For default value, use defaultForkJoinWorkerThreadFactory.
handler – the handler for internal worker threads that terminate due to unrecoverable errors encountered while executing tasks. For default value, use null.
asyncMode – if true, establishes local first-in-first-out scheduling mode for forked tasks that are never joined. This mode may be more appropriate than default locally stack-based mode in applications in which worker threads only process event-style asynchronous tasks. For default value, use false.
Throws:
IllegalArgumentException – if parallelism less than or
        equal to zero, or greater than implementation limit
NullPointerException – if the factory is null
SecurityException – if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")/**
     * Creates a {@code ForkJoinPool} with the given parameters.
     *
     * @param parallelism the parallelism level. For default value,
     * use {@link java.lang.Runtime#availableProcessors}.
     * @param factory the factory for creating new threads. For default value,
     * use {@link #defaultForkJoinWorkerThreadFactory}.
     * @param handler the handler for internal worker threads that
     * terminate due to unrecoverable errors encountered while executing
     * tasks. For default value, use {@code null}.
     * @param asyncMode if true,
     * establishes local first-in-first-out scheduling mode for forked
     * tasks that are never joined. This mode may be more appropriate
     * than default locally stack-based mode in applications in which
     * worker threads only process event-style asynchronous tasks.
     * For default value, use {@code false}.
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws NullPointerException if the factory is null
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism,
                        ForkJoinWorkerThreadFactory factory,
                        UncaughtExceptionHandler handler,
                        boolean asyncMode) {
        this(checkParallelism(parallelism),
             checkFactory(factory),
             handler,
             asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
             "ForkJoinPool-" + nextPoolId() + "-worker-");
        checkPermission();
    }

    private static int checkParallelism(int parallelism) {
        if (parallelism <= 0 || parallelism > MAX_CAP)
            throw new IllegalArgumentException();
        return parallelism;
    }

    private static ForkJoinWorkerThreadFactory checkFactory
        (ForkJoinWorkerThreadFactory factory) {
        if (factory == null)
            throw new NullPointerException();
        return factory;
    }

    
Creates a ForkJoinPool with the given parameters, without any security checks or parameter validation. Invoked directly by makeCommonPool. /**
     * Creates a {@code ForkJoinPool} with the given parameters, without
     * any security checks or parameter validation.  Invoked directly by
     * makeCommonPool.
     */
    private ForkJoinPool(int parallelism,
                         ForkJoinWorkerThreadFactory factory,
                         UncaughtExceptionHandler handler,
                         int mode,
                         String workerNamePrefix) {
        this.workerNamePrefix = workerNamePrefix;
        this.factory = factory;
        this.ueh = handler;
        this.config = (parallelism & SMASK) | mode;
        long np = (long)(-parallelism); // offset ctl counts
        this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
    }

    
Returns the common pool instance. This pool is statically constructed; its run state is unaffected by attempts to shutdown or shutdownNow. However this pool and any ongoing processing are automatically terminated upon program System.exit. Any program that relies on asynchronous task processing to complete before program termination should invoke commonPool().awaitQuiescence, before exit. 
Returns:
the common pool instance
Since:
1.8/**
     * Returns the common pool instance. This pool is statically
     * constructed; its run state is unaffected by attempts to {@link
     * #shutdown} or {@link #shutdownNow}. However this pool and any
     * ongoing processing are automatically terminated upon program
     * {@link System#exit}.  Any program that relies on asynchronous
     * task processing to complete before program termination should
     * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
     * before exit.
     *
     * @return the common pool instance
     * @since 1.8
     */
    public static ForkJoinPool commonPool() {
        // assert common != null : "static init error";
        return common;
    }

    // Execution methods

    
Performs the given task, returning its result upon completion. If the computation encounters an unchecked Exception or Error, it is rethrown as the outcome of this invocation. Rethrown exceptions behave in the same way as regular exceptions, but, when possible, contain stack traces (as displayed for example using ex.printStackTrace()) of both the current thread as well as the thread actually encountering the exception; minimally only the latter. 
Params:
task – the task
Type parameters:
<T> – the type of the task's result
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution
Returns:
the task's result/**
     * Performs the given task, returning its result upon completion.
     * If the computation encounters an unchecked Exception or Error,
     * it is rethrown as the outcome of this invocation.  Rethrown
     * exceptions behave in the same way as regular exceptions, but,
     * when possible, contain stack traces (as displayed for example
     * using {@code ex.printStackTrace()}) of both the current thread
     * as well as the thread actually encountering the exception;
     * minimally only the latter.
     *
     * @param task the task
     * @param <T> the type of the task's result
     * @return the task's result
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> T invoke(ForkJoinTask<T> task) {
        if (task == null)
            throw new NullPointerException();
        externalPush(task);
        return task.join();
    }

    
Arranges for (asynchronous) execution of the given task.
Params:
task – the task
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution/**
     * Arranges for (asynchronous) execution of the given task.
     *
     * @param task the task
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public void execute(ForkJoinTask<?> task) {
        if (task == null)
            throw new NullPointerException();
        externalPush(task);
    }

    // AbstractExecutorService methods

    
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution/**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public void execute(Runnable task) {
        if (task == null)
            throw new NullPointerException();
        ForkJoinTask<?> job;
        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
            job = (ForkJoinTask<?>) task;
        else
            job = new ForkJoinTask.RunnableExecuteAction(task);
        externalPush(job);
    }

    
Submits a ForkJoinTask for execution.
Params:
task – the task to submit
Type parameters:
<T> – the type of the task's result
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution
Returns:
the task/**
     * Submits a ForkJoinTask for execution.
     *
     * @param task the task to submit
     * @param <T> the type of the task's result
     * @return the task
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
        if (task == null)
            throw new NullPointerException();
        externalPush(task);
        return task;
    }

    
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution/**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(Callable<T> task) {
        ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task);
        externalPush(job);
        return job;
    }

    
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution/**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(Runnable task, T result) {
        ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result);
        externalPush(job);
        return job;
    }

    
Throws:
NullPointerException – if the task is null
RejectedExecutionException – if the task cannot be
        scheduled for execution/**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public ForkJoinTask<?> submit(Runnable task) {
        if (task == null)
            throw new NullPointerException();
        ForkJoinTask<?> job;
        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
            job = (ForkJoinTask<?>) task;
        else
            job = new ForkJoinTask.AdaptedRunnableAction(task);
        externalPush(job);
        return job;
    }

    
Throws:
NullPointerException –       {@inheritDoc}
RejectedExecutionException – {@inheritDoc}/**
     * @throws NullPointerException       {@inheritDoc}
     * @throws RejectedExecutionException {@inheritDoc}
     */
    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
        // In previous versions of this class, this method constructed
        // a task to run ForkJoinTask.invokeAll, but now external
        // invocation of multiple tasks is at least as efficient.
        ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());

        boolean done = false;
        try {
            for (Callable<T> t : tasks) {
                ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
                futures.add(f);
                externalPush(f);
            }
            for (int i = 0, size = futures.size(); i < size; i++)
                ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
            done = true;
            return futures;
        } finally {
            if (!done)
                for (int i = 0, size = futures.size(); i < size; i++)
                    futures.get(i).cancel(false);
        }
    }

    
Returns the factory used for constructing new workers.
Returns:
the factory used for constructing new workers/**
     * Returns the factory used for constructing new workers.
     *
     * @return the factory used for constructing new workers
     */
    public ForkJoinWorkerThreadFactory getFactory() {
        return factory;
    }

    
Returns the handler for internal worker threads that terminate
due to unrecoverable errors encountered while executing tasks.
Returns:
the handler, or null if none/**
     * Returns the handler for internal worker threads that terminate
     * due to unrecoverable errors encountered while executing tasks.
     *
     * @return the handler, or {@code null} if none
     */
    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return ueh;
    }

    
Returns the targeted parallelism level of this pool.
Returns:
the targeted parallelism level of this pool/**
     * Returns the targeted parallelism level of this pool.
     *
     * @return the targeted parallelism level of this pool
     */
    public int getParallelism() {
        int par;
        return ((par = config & SMASK) > 0) ? par : 1;
    }

    
Returns the targeted parallelism level of the common pool.
Returns:
the targeted parallelism level of the common pool
Since:
1.8/**
     * Returns the targeted parallelism level of the common pool.
     *
     * @return the targeted parallelism level of the common pool
     * @since 1.8
     */
    public static int getCommonPoolParallelism() {
        return commonParallelism;
    }

    
Returns the number of worker threads that have started but not yet terminated. The result returned by this method may differ from getParallelism when threads are created to maintain parallelism when others are cooperatively blocked. 
Returns:
the number of worker threads/**
     * Returns the number of worker threads that have started but not
     * yet terminated.  The result returned by this method may differ
     * from {@link #getParallelism} when threads are created to
     * maintain parallelism when others are cooperatively blocked.
     *
     * @return the number of worker threads
     */
    public int getPoolSize() {
        return (config & SMASK) + (short)(ctl >>> TC_SHIFT);
    }

    
Returns true if this pool uses local first-in-first-out scheduling mode for forked tasks that are never joined. 
Returns:
true if this pool uses async mode/**
     * Returns {@code true} if this pool uses local first-in-first-out
     * scheduling mode for forked tasks that are never joined.
     *
     * @return {@code true} if this pool uses async mode
     */
    public boolean getAsyncMode() {
        return (config & FIFO_QUEUE) != 0;
    }

    
Returns an estimate of the number of worker threads that are
not blocked waiting to join tasks or for other managed
synchronization. This method may overestimate the
number of running threads.
Returns:
the number of worker threads/**
     * Returns an estimate of the number of worker threads that are
     * not blocked waiting to join tasks or for other managed
     * synchronization. This method may overestimate the
     * number of running threads.
     *
     * @return the number of worker threads
     */
    public int getRunningThreadCount() {
        int rc = 0;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 1; i < ws.length; i += 2) {
                if ((w = ws[i]) != null && w.isApparentlyUnblocked())
                    ++rc;
            }
        }
        return rc;
    }

    
Returns an estimate of the number of threads that are currently
stealing or executing tasks. This method may overestimate the
number of active threads.
Returns:
the number of active threads/**
     * Returns an estimate of the number of threads that are currently
     * stealing or executing tasks. This method may overestimate the
     * number of active threads.
     *
     * @return the number of active threads
     */
    public int getActiveThreadCount() {
        int r = (config & SMASK) + (int)(ctl >> AC_SHIFT);
        return (r <= 0) ? 0 : r; // suppress momentarily negative values
    }

    
Returns true if all worker threads are currently idle. An idle worker is one that cannot obtain a task to execute because none are available to steal from other threads, and there are no pending submissions to the pool. This method is conservative; it might not return true immediately upon idleness of all threads, but will eventually become true if threads remain inactive. 
Returns:
true if all threads are currently idle/**
     * Returns {@code true} if all worker threads are currently idle.
     * An idle worker is one that cannot obtain a task to execute
     * because none are available to steal from other threads, and
     * there are no pending submissions to the pool. This method is
     * conservative; it might not return {@code true} immediately upon
     * idleness of all threads, but will eventually become true if
     * threads remain inactive.
     *
     * @return {@code true} if all threads are currently idle
     */
    public boolean isQuiescent() {
        return (config & SMASK) + (int)(ctl >> AC_SHIFT) <= 0;
    }

    
Returns an estimate of the total number of tasks stolen from
one thread's work queue by another. The reported value
underestimates the actual total number of steals when the pool
is not quiescent. This value may be useful for monitoring and
tuning fork/join programs: in general, steal counts should be
high enough to keep threads busy, but low enough to avoid
overhead and contention across threads.
Returns:
the number of steals/**
     * Returns an estimate of the total number of tasks stolen from
     * one thread's work queue by another. The reported value
     * underestimates the actual total number of steals when the pool
     * is not quiescent. This value may be useful for monitoring and
     * tuning fork/join programs: in general, steal counts should be
     * high enough to keep threads busy, but low enough to avoid
     * overhead and contention across threads.
     *
     * @return the number of steals
     */
    public long getStealCount() {
        AtomicLong sc = stealCounter;
        long count = (sc == null) ? 0L : sc.get();
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 1; i < ws.length; i += 2) {
                if ((w = ws[i]) != null)
                    count += w.nsteals;
            }
        }
        return count;
    }

    
Returns an estimate of the total number of tasks currently held
in queues by worker threads (but not including tasks submitted
to the pool that have not begun executing). This value is only
an approximation, obtained by iterating across all threads in
the pool. This method may be useful for tuning task
granularities.
Returns:
the number of queued tasks/**
     * Returns an estimate of the total number of tasks currently held
     * in queues by worker threads (but not including tasks submitted
     * to the pool that have not begun executing). This value is only
     * an approximation, obtained by iterating across all threads in
     * the pool. This method may be useful for tuning task
     * granularities.
     *
     * @return the number of queued tasks
     */
    public long getQueuedTaskCount() {
        long count = 0;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 1; i < ws.length; i += 2) {
                if ((w = ws[i]) != null)
                    count += w.queueSize();
            }
        }
        return count;
    }

    
Returns an estimate of the number of tasks submitted to this
pool that have not yet begun executing.  This method may take
time proportional to the number of submissions.
Returns:
the number of queued submissions/**
     * Returns an estimate of the number of tasks submitted to this
     * pool that have not yet begun executing.  This method may take
     * time proportional to the number of submissions.
     *
     * @return the number of queued submissions
     */
    public int getQueuedSubmissionCount() {
        int count = 0;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; i += 2) {
                if ((w = ws[i]) != null)
                    count += w.queueSize();
            }
        }
        return count;
    }

    
Returns true if there are any tasks submitted to this pool that have not yet begun executing. 
Returns:
true if there are any queued submissions/**
     * Returns {@code true} if there are any tasks submitted to this
     * pool that have not yet begun executing.
     *
     * @return {@code true} if there are any queued submissions
     */
    public boolean hasQueuedSubmissions() {
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; i += 2) {
                if ((w = ws[i]) != null && !w.isEmpty())
                    return true;
            }
        }
        return false;
    }

    
Removes and returns the next unexecuted submission if one is
available.  This method may be useful in extensions to this
class that re-assign work in systems with multiple pools.
Returns:
the next submission, or null if none/**
     * Removes and returns the next unexecuted submission if one is
     * available.  This method may be useful in extensions to this
     * class that re-assign work in systems with multiple pools.
     *
     * @return the next submission, or {@code null} if none
     */
    protected ForkJoinTask<?> pollSubmission() {
        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; i += 2) {
                if ((w = ws[i]) != null && (t = w.poll()) != null)
                    return t;
            }
        }
        return null;
    }

    
Removes all available unexecuted submitted and forked tasks from scheduling queues and adds them to the given collection, without altering their execution status. These may include artificially generated or wrapped tasks. This method is designed to be invoked only when the pool is known to be quiescent. Invocations at other times may not remove all tasks. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. 
Params:
c – the collection to transfer elements into
Returns:
the number of elements transferred/**
     * Removes all available unexecuted submitted and forked tasks
     * from scheduling queues and adds them to the given collection,
     * without altering their execution status. These may include
     * artificially generated or wrapped tasks. This method is
     * designed to be invoked only when the pool is known to be
     * quiescent. Invocations at other times may not remove all
     * tasks. A failure encountered while attempting to add elements
     * to collection {@code c} may result in elements being in
     * neither, either or both collections when the associated
     * exception is thrown.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.
     *
     * @param c the collection to transfer elements into
     * @return the number of elements transferred
     */
    protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
        int count = 0;
        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; ++i) {
                if ((w = ws[i]) != null) {
                    while ((t = w.poll()) != null) {
                        c.add(t);
                        ++count;
                    }
                }
            }
        }
        return count;
    }

    
Returns a string identifying this pool, as well as its state,
including indications of run state, parallelism level, and
worker and task counts.
Returns:
a string identifying this pool, as well as its state/**
     * Returns a string identifying this pool, as well as its state,
     * including indications of run state, parallelism level, and
     * worker and task counts.
     *
     * @return a string identifying this pool, as well as its state
     */
    public String toString() {
        // Use a single pass through workQueues to collect counts
        long qt = 0L, qs = 0L; int rc = 0;
        AtomicLong sc = stealCounter;
        long st = (sc == null) ? 0L : sc.get();
        long c = ctl;
        WorkQueue[] ws; WorkQueue w;
        if ((ws = workQueues) != null) {
            for (int i = 0; i < ws.length; ++i) {
                if ((w = ws[i]) != null) {
                    int size = w.queueSize();
                    if ((i & 1) == 0)
                        qs += size;
                    else {
                        qt += size;
                        st += w.nsteals;
                        if (w.isApparentlyUnblocked())
                            ++rc;
                    }
                }
            }
        }
        int pc = (config & SMASK);
        int tc = pc + (short)(c >>> TC_SHIFT);
        int ac = pc + (int)(c >> AC_SHIFT);
        if (ac < 0) // ignore transient negative
            ac = 0;
        int rs = runState;
        String level = ((rs & TERMINATED) != 0 ? "Terminated" :
                        (rs & STOP)       != 0 ? "Terminating" :
                        (rs & SHUTDOWN)   != 0 ? "Shutting down" :
                        "Running");
        return super.toString() +
            "[" + level +
            ", parallelism = " + pc +
            ", size = " + tc +
            ", active = " + ac +
            ", running = " + rc +
            ", steals = " + st +
            ", tasks = " + qt +
            ", submissions = " + qs +
            "]";
    }

    
Possibly initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no effect on execution state if this is the commonPool(), and no additional effect if already shut down. Tasks that are in the process of being submitted concurrently during the course of this method may or may not be rejected. 
Throws:
SecurityException – if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")/**
     * Possibly initiates an orderly shutdown in which previously
     * submitted tasks are executed, but no new tasks will be
     * accepted. Invocation has no effect on execution state if this
     * is the {@link #commonPool()}, and no additional effect if
     * already shut down.  Tasks that are in the process of being
     * submitted concurrently during the course of this method may or
     * may not be rejected.
     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public void shutdown() {
        checkPermission();
        tryTerminate(false, true);
    }

    
Possibly attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks. Invocation has no effect on execution state if this is the commonPool(), and no additional effect if already shut down. Otherwise, tasks that are in the process of being submitted or executed concurrently during the course of this method may or may not be rejected. This method cancels both existing and unexecuted tasks, in order to permit termination in the presence of task dependencies. So the method always returns an empty list (unlike the case for some other Executors). 
Throws:
SecurityException – if a security manager exists and the caller is not permitted to modify threads because it does not hold RuntimePermission("modifyThread")
Returns:
an empty list/**
     * Possibly attempts to cancel and/or stop all tasks, and reject
     * all subsequently submitted tasks.  Invocation has no effect on
     * execution state if this is the {@link #commonPool()}, and no
     * additional effect if already shut down. Otherwise, tasks that
     * are in the process of being submitted or executed concurrently
     * during the course of this method may or may not be
     * rejected. This method cancels both existing and unexecuted
     * tasks, in order to permit termination in the presence of task
     * dependencies. So the method always returns an empty list
     * (unlike the case for some other Executors).
     *
     * @return an empty list
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public List<Runnable> shutdownNow() {
        checkPermission();
        tryTerminate(true, true);
        return Collections.emptyList();
    }

    
Returns true if all tasks have completed following shut down. 
Returns:
true if all tasks have completed following shut down/**
     * Returns {@code true} if all tasks have completed following shut down.
     *
     * @return {@code true} if all tasks have completed following shut down
     */
    public boolean isTerminated() {
        return (runState & TERMINATED) != 0;
    }

    
Returns true if the process of termination has commenced but not yet completed. This method may be useful for debugging. A return of true reported a sufficient period after shutdown may indicate that submitted tasks have ignored or suppressed interruption, or are waiting for I/O, causing this executor not to properly terminate. (See the advisory notes for class ForkJoinTask stating that tasks should not normally entail blocking operations. But if they do, they must abort them on interrupt.) 
Returns:
true if terminating but not yet terminated/**
     * Returns {@code true} if the process of termination has
     * commenced but not yet completed.  This method may be useful for
     * debugging. A return of {@code true} reported a sufficient
     * period after shutdown may indicate that submitted tasks have
     * ignored or suppressed interruption, or are waiting for I/O,
     * causing this executor not to properly terminate. (See the
     * advisory notes for class {@link ForkJoinTask} stating that
     * tasks should not normally entail blocking operations.  But if
     * they do, they must abort them on interrupt.)
     *
     * @return {@code true} if terminating but not yet terminated
     */
    public boolean isTerminating() {
        int rs = runState;
        return (rs & STOP) != 0 && (rs & TERMINATED) == 0;
    }

    
Returns true if this pool has been shut down. 
Returns:
true if this pool has been shut down/**
     * Returns {@code true} if this pool has been shut down.
     *
     * @return {@code true} if this pool has been shut down
     */
    public boolean isShutdown() {
        return (runState & SHUTDOWN) != 0;
    }

    
Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first. Because the commonPool() never terminates until program shutdown, when applied to the common pool, this method is equivalent to awaitQuiescence(long, TimeUnit) but always returns false. 
Params:
timeout – the maximum time to wait
unit – the time unit of the timeout argument
Throws:
InterruptedException – if interrupted while waiting
Returns:
true if this executor terminated and false if the timeout elapsed before termination/**
     * Blocks until all tasks have completed execution after a
     * shutdown request, or the timeout occurs, or the current thread
     * is interrupted, whichever happens first. Because the {@link
     * #commonPool()} never terminates until program shutdown, when
     * applied to the common pool, this method is equivalent to {@link
     * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return {@code true} if this executor terminated and
     *         {@code false} if the timeout elapsed before termination
     * @throws InterruptedException if interrupted while waiting
     */
    public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (this == common) {
            awaitQuiescence(timeout, unit);
            return false;
        }
        long nanos = unit.toNanos(timeout);
        if (isTerminated())
            return true;
        if (nanos <= 0L)
            return false;
        long deadline = System.nanoTime() + nanos;
        synchronized (this) {
            for (;;) {
                if (isTerminated())
                    return true;
                if (nanos <= 0L)
                    return false;
                long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
                wait(millis > 0L ? millis : 1L);
                nanos = deadline - System.nanoTime();
            }
        }
    }

    
If called by a ForkJoinTask operating in this pool, equivalent in effect to ForkJoinTask.helpQuiesce. Otherwise, waits and/or attempts to assist performing tasks until this pool isQuiescent or the indicated timeout elapses. 
Params:
timeout – the maximum time to wait
unit – the time unit of the timeout argument
Returns:
true if quiescent; false if the timeout elapsed./**
     * If called by a ForkJoinTask operating in this pool, equivalent
     * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
     * waits and/or attempts to assist performing tasks until this
     * pool {@link #isQuiescent} or the indicated timeout elapses.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return {@code true} if quiescent; {@code false} if the
     * timeout elapsed.
     */
    public boolean awaitQuiescence(long timeout, TimeUnit unit) {
        long nanos = unit.toNanos(timeout);
        ForkJoinWorkerThread wt;
        Thread thread = Thread.currentThread();
        if ((thread instanceof ForkJoinWorkerThread) &&
            (wt = (ForkJoinWorkerThread)thread).pool == this) {
            helpQuiescePool(wt.workQueue);
            return true;
        }
        long startTime = System.nanoTime();
        WorkQueue[] ws;
        int r = 0, m;
        boolean found = true;
        while (!isQuiescent() && (ws = workQueues) != null &&
               (m = ws.length - 1) >= 0) {
            if (!found) {
                if ((System.nanoTime() - startTime) > nanos)
                    return false;
                Thread.yield(); // cannot block
            }
            found = false;
            for (int j = (m + 1) << 2; j >= 0; --j) {
                ForkJoinTask<?> t; WorkQueue q; int b, k;
                if ((k = r++ & m) <= m && k >= 0 && (q = ws[k]) != null &&
                    (b = q.base) - q.top < 0) {
                    found = true;
                    if ((t = q.pollAt(b)) != null)
                        t.doExec();
                    break;
                }
            }
        }
        return true;
    }

    
Waits and/or attempts to assist performing tasks indefinitely until the commonPool() isQuiescent. /**
     * Waits and/or attempts to assist performing tasks indefinitely
     * until the {@link #commonPool()} {@link #isQuiescent}.
     */
    static void quiesceCommonPool() {
        common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
    }

    
Interface for extending managed parallelism for tasks running in ForkJoinPools. 
A ManagedBlocker provides two methods. Method isReleasable must return true if blocking is not necessary. Method block blocks the current thread if necessary (perhaps internally invoking isReleasable before actually blocking). These actions are performed by any thread invoking ForkJoinPool.managedBlock(ManagedBlocker). The unusual methods in this API accommodate synchronizers that may, but don't usually, block for long periods. Similarly, they allow more efficient internal handling of cases in which additional workers may be, but usually are not, needed to ensure sufficient parallelism. Toward this end, implementations of method isReleasable must be amenable to repeated invocation. 
For example, here is a ManagedBlocker based on a
ReentrantLock:
 
 
class ManagedLocker implements ManagedBlocker {
  final ReentrantLock lock;
  boolean hasLock = false;
  ManagedLocker(ReentrantLock lock) { this.lock = lock; }
  public boolean block() {
    if (!hasLock)
      lock.lock();
    return true;
  }
  public boolean isReleasable() {
    return hasLock || (hasLock = lock.tryLock());
  }
 }
Here is a class that possibly blocks waiting for an
item on a given queue:
 
 
class QueueTaker<E> implements ManagedBlocker {
  final BlockingQueue<E> queue;
  volatile E item = null;
  QueueTaker(BlockingQueue<E> q) { this.queue = q; }
  public boolean block() throws InterruptedException {
    if (item == null)
      item = queue.take();
    return true;
  }
  public boolean isReleasable() {
    return item != null || (item = queue.poll()) != null;
  }
  public E getItem() { // call after pool.managedBlock completes
    return item;
  }
 }
/**
     * Interface for extending managed parallelism for tasks running
     * in {@link ForkJoinPool}s.
     *
     * <p>A {@code ManagedBlocker} provides two methods.  Method
     * {@link #isReleasable} must return {@code true} if blocking is
     * not necessary. Method {@link #block} blocks the current thread
     * if necessary (perhaps internally invoking {@code isReleasable}
     * before actually blocking). These actions are performed by any
     * thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}.
     * The unusual methods in this API accommodate synchronizers that
     * may, but don't usually, block for long periods. Similarly, they
     * allow more efficient internal handling of cases in which
     * additional workers may be, but usually are not, needed to
     * ensure sufficient parallelism.  Toward this end,
     * implementations of method {@code isReleasable} must be amenable
     * to repeated invocation.
     *
     * <p>For example, here is a ManagedBlocker based on a
     * ReentrantLock:
     *  <pre> {@code
     * class ManagedLocker implements ManagedBlocker {
     *   final ReentrantLock lock;
     *   boolean hasLock = false;
     *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
     *   public boolean block() {
     *     if (!hasLock)
     *       lock.lock();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return hasLock || (hasLock = lock.tryLock());
     *   }
     * }}</pre>
     *
     * <p>Here is a class that possibly blocks waiting for an
     * item on a given queue:
     *  <pre> {@code
     * class QueueTaker<E> implements ManagedBlocker {
     *   final BlockingQueue<E> queue;
     *   volatile E item = null;
     *   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
     *   public boolean block() throws InterruptedException {
     *     if (item == null)
     *       item = queue.take();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return item != null || (item = queue.poll()) != null;
     *   }
     *   public E getItem() { // call after pool.managedBlock completes
     *     return item;
     *   }
     * }}</pre>
     */
    public static interface ManagedBlocker {
        
Possibly blocks the current thread, for example waiting for
a lock or condition.
Throws:
InterruptedException – if interrupted while waiting
(the method is not required to do so, but is allowed to)
Returns:
true if no additional blocking is necessary (i.e., if isReleasable would return true)/**
         * Possibly blocks the current thread, for example waiting for
         * a lock or condition.
         *
         * @return {@code true} if no additional blocking is necessary
         * (i.e., if isReleasable would return true)
         * @throws InterruptedException if interrupted while waiting
         * (the method is not required to do so, but is allowed to)
         */
        boolean block() throws InterruptedException;

        
Returns true if blocking is unnecessary. 
Returns:
true if blocking is unnecessary/**
         * Returns {@code true} if blocking is unnecessary.
         * @return {@code true} if blocking is unnecessary
         */
        boolean isReleasable();
    }

    
Runs the given possibly blocking task. When running in a ForkJoinPool, this method possibly arranges for a spare thread to be activated if necessary to ensure sufficient parallelism while the current thread is blocked in blocker.block(). 
This method repeatedly calls blocker.isReleasable() and blocker.block() until either method returns true. Every call to blocker.block() is preceded by a call to blocker.isReleasable() that returned false. 
If not running in a ForkJoinPool, this method is
behaviorally equivalent to
 
 
while (!blocker.isReleasable())
  if (blocker.block())
    break;
 If running in a ForkJoinPool, the pool may first be expanded to ensure sufficient parallelism available during the call to blocker.block(). 
Params:
blocker – the blocker task
Throws:
InterruptedException – if blocker.block() did so/**
     * Runs the given possibly blocking task.  When {@linkplain
     * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
     * method possibly arranges for a spare thread to be activated if
     * necessary to ensure sufficient parallelism while the current
     * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
     *
     * <p>This method repeatedly calls {@code blocker.isReleasable()} and
     * {@code blocker.block()} until either method returns {@code true}.
     * Every call to {@code blocker.block()} is preceded by a call to
     * {@code blocker.isReleasable()} that returned {@code false}.
     *
     * <p>If not running in a ForkJoinPool, this method is
     * behaviorally equivalent to
     *  <pre> {@code
     * while (!blocker.isReleasable())
     *   if (blocker.block())
     *     break;}</pre>
     *
     * If running in a ForkJoinPool, the pool may first be expanded to
     * ensure sufficient parallelism available during the call to
     * {@code blocker.block()}.
     *
     * @param blocker the blocker task
     * @throws InterruptedException if {@code blocker.block()} did so
     */
    public static void managedBlock(ManagedBlocker blocker)
        throws InterruptedException {
        ForkJoinPool p;
        ForkJoinWorkerThread wt;
        Thread t = Thread.currentThread();
        if ((t instanceof ForkJoinWorkerThread) &&
            (p = (wt = (ForkJoinWorkerThread)t).pool) != null) {
            WorkQueue w = wt.workQueue;
            while (!blocker.isReleasable()) {
                if (p.tryCompensate(w)) {
                    try {
                        do {} while (!blocker.isReleasable() &&
                                     !blocker.block());
                    } finally {
                        U.getAndAddLong(p, CTL, AC_UNIT);
                    }
                    break;
                }
            }
        }
        else {
            do {} while (!blocker.isReleasable() &&
                         !blocker.block());
        }
    }

    // AbstractExecutorService overrides.  These rely on undocumented
    // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
    // implement RunnableFuture.

    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
    }

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new ForkJoinTask.AdaptedCallable<T>(callable);
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final int  ABASE;
    private static final int  ASHIFT;
    private static final long CTL;
    private static final long RUNSTATE;
    private static final long STEALCOUNTER;
    private static final long PARKBLOCKER;
    private static final long QTOP;
    private static final long QLOCK;
    private static final long QSCANSTATE;
    private static final long QPARKER;
    private static final long QCURRENTSTEAL;
    private static final long QCURRENTJOIN;

    static {
        // initialize field offsets for CAS etc
        try {
            U = sun.misc.Unsafe.getUnsafe();
            Class<?> k = ForkJoinPool.class;
            CTL = U.objectFieldOffset
                (k.getDeclaredField("ctl"));
            RUNSTATE = U.objectFieldOffset
                (k.getDeclaredField("runState"));
            STEALCOUNTER = U.objectFieldOffset
                (k.getDeclaredField("stealCounter"));
            Class<?> tk = Thread.class;
            PARKBLOCKER = U.objectFieldOffset
                (tk.getDeclaredField("parkBlocker"));
            Class<?> wk = WorkQueue.class;
            QTOP = U.objectFieldOffset
                (wk.getDeclaredField("top"));
            QLOCK = U.objectFieldOffset
                (wk.getDeclaredField("qlock"));
            QSCANSTATE = U.objectFieldOffset
                (wk.getDeclaredField("scanState"));
            QPARKER = U.objectFieldOffset
                (wk.getDeclaredField("parker"));
            QCURRENTSTEAL = U.objectFieldOffset
                (wk.getDeclaredField("currentSteal"));
            QCURRENTJOIN = U.objectFieldOffset
                (wk.getDeclaredField("currentJoin"));
            Class<?> ak = ForkJoinTask[].class;
            ABASE = U.arrayBaseOffset(ak);
            int scale = U.arrayIndexScale(ak);
            if ((scale & (scale - 1)) != 0)
                throw new Error("data type scale not a power of two");
            ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
        } catch (Exception e) {
            throw new Error(e);
        }

        commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
        defaultForkJoinWorkerThreadFactory =
            new DefaultForkJoinWorkerThreadFactory();
        modifyThreadPermission = new RuntimePermission("modifyThread");

        common = java.security.AccessController.doPrivileged
            (new java.security.PrivilegedAction<ForkJoinPool>() {
                public ForkJoinPool run() { return makeCommonPool(); }});
        int par = common.config & SMASK; // report 1 even if threads disabled
        commonParallelism = par > 0 ? par : 1;
    }

    
Creates and returns the common pool, respecting user settings
specified via system properties.
/**
     * Creates and returns the common pool, respecting user settings
     * specified via system properties.
     */
    private static ForkJoinPool makeCommonPool() {
        int parallelism = -1;
        ForkJoinWorkerThreadFactory factory = null;
        UncaughtExceptionHandler handler = null;
        try {  // ignore exceptions in accessing/parsing properties
            String pp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.parallelism");
            String fp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.threadFactory");
            String hp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
            if (pp != null)
                parallelism = Integer.parseInt(pp);
            if (fp != null)
                factory = ((ForkJoinWorkerThreadFactory)ClassLoader.
                           getSystemClassLoader().loadClass(fp).newInstance());
            if (hp != null)
                handler = ((UncaughtExceptionHandler)ClassLoader.
                           getSystemClassLoader().loadClass(hp).newInstance());
        } catch (Exception ignore) {
        }
        if (factory == null) {
            if (System.getSecurityManager() == null)
                factory = defaultForkJoinWorkerThreadFactory;
            else // use security-managed default
                factory = new InnocuousForkJoinWorkerThreadFactory();
        }
        if (parallelism < 0 && // default 1 less than #cores
            (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
            parallelism = 1;
        if (parallelism > MAX_CAP)
            parallelism = MAX_CAP;
        return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,
                                "ForkJoinPool.commonPool-worker-");
    }

    
Factory for innocuous worker threads
/**
     * Factory for innocuous worker threads
     */
    static final class InnocuousForkJoinWorkerThreadFactory
        implements ForkJoinWorkerThreadFactory {

        
An ACC to restrict permissions for the factory itself.
The constructed workers have no permissions set.
/**
         * An ACC to restrict permissions for the factory itself.
         * The constructed workers have no permissions set.
         */
        private static final AccessControlContext innocuousAcc;
        static {
            Permissions innocuousPerms = new Permissions();
            innocuousPerms.add(modifyThreadPermission);
            innocuousPerms.add(new RuntimePermission(
                                   "enableContextClassLoaderOverride"));
            innocuousPerms.add(new RuntimePermission(
                                   "modifyThreadGroup"));
            innocuousAcc = new AccessControlContext(new ProtectionDomain[] {
                    new ProtectionDomain(null, innocuousPerms)
                });
        }

        public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
            return (ForkJoinWorkerThread.InnocuousForkJoinWorkerThread)
                java.security.AccessController.doPrivileged(
                    new java.security.PrivilegedAction<ForkJoinWorkerThread>() {
                    public ForkJoinWorkerThread run() {
                        return new ForkJoinWorkerThread.
                            InnocuousForkJoinWorkerThread(pool);
                    }}, innocuousAcc);
        }
    }

}