https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Strings.isNullOrEmpty;
import static com.google.common.base.Throwables.throwIfUnchecked;
import static com.google.common.util.concurrent.Futures.getDone;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Ascii;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.ForOverride;
import com.google.j2objc.annotations.ReflectionSupport;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import java.util.concurrent.locks.LockSupport;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.checkerframework.checker.nullness.qual.Nullable;


An abstract implementation of ListenableFuture, intended for advanced users only. More common ways to create a ListenableFuture include instantiating a SettableFuture, submitting a task to a ListeningExecutorService, and deriving a Future from an existing one, typically using methods like Futures.transform and Futures.catching. 
This class implements all methods in ListenableFuture. Subclasses should provide a way to set the result of the computation through the protected methods set(Object), setFuture(ListenableFuture) and setException(Throwable). Subclasses may also override afterDone(), which will be invoked automatically when the future completes. Subclasses should rarely override other methods. 
Author:Sven Mawson, Luke Sandberg
Since:1.0
/**
 * An abstract implementation of {@link ListenableFuture}, intended for advanced users only. More
 * common ways to create a {@code ListenableFuture} include instantiating a {@link SettableFuture},
 * submitting a task to a {@link ListeningExecutorService}, and deriving a {@code Future} from an
 * existing one, typically using methods like {@link Futures#transform(ListenableFuture,
 * com.google.common.base.Function, java.util.concurrent.Executor) Futures.transform} and {@link
 * Futures#catching(ListenableFuture, Class, com.google.common.base.Function,
 * java.util.concurrent.Executor) Futures.catching}.
 *
 * <p>This class implements all methods in {@code ListenableFuture}. Subclasses should provide a way
 * to set the result of the computation through the protected methods {@link #set(Object)}, {@link
 * #setFuture(ListenableFuture)} and {@link #setException(Throwable)}. Subclasses may also override
 * {@link #afterDone()}, which will be invoked automatically when the future completes. Subclasses
 * should rarely override other methods.
 *
 * @author Sven Mawson
 * @author Luke Sandberg
 * @since 1.0
 */
@SuppressWarnings("ShortCircuitBoolean") // we use non-short circuiting comparisons intentionally
@GwtCompatible(emulated = true)
@ReflectionSupport(value = ReflectionSupport.Level.FULL)
public abstract class AbstractFuture<V> extends FluentFuture<V> {
  // NOTE: Whenever both tests are cheap and functional, it's faster to use &, | instead of &&, ||

  private static final boolean GENERATE_CANCELLATION_CAUSES =
      Boolean.parseBoolean(
          System.getProperty("guava.concurrent.generate_cancellation_cause", "false"));

  
A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring that get calls exactly the implementation of AbstractFuture.get. 
/**
   * A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring
   * that {@link #get} calls exactly the implementation of {@link AbstractFuture#get}.
   */
  abstract static class TrustedFuture<V> extends AbstractFuture<V> {
    @CanIgnoreReturnValue
    @Override
    public final V get() throws InterruptedException, ExecutionException {
      return super.get();
    }

    @CanIgnoreReturnValue
    @Override
    public final V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
      return super.get(timeout, unit);
    }

    @Override
    public final boolean isDone() {
      return super.isDone();
    }

    @Override
    public final boolean isCancelled() {
      return super.isCancelled();
    }

    @Override
    public final void addListener(Runnable listener, Executor executor) {
      super.addListener(listener, executor);
    }

    @CanIgnoreReturnValue
    @Override
    public final boolean cancel(boolean mayInterruptIfRunning) {
      return super.cancel(mayInterruptIfRunning);
    }
  }

  // Logger to log exceptions caught when running listeners.
  private static final Logger log = Logger.getLogger(AbstractFuture.class.getName());

  // A heuristic for timed gets. If the remaining timeout is less than this, spin instead of
  // blocking. This value is what AbstractQueuedSynchronizer uses.
  private static final long SPIN_THRESHOLD_NANOS = 1000L;

  private static final AtomicHelper ATOMIC_HELPER;

  static {
    AtomicHelper helper;
    Throwable thrownUnsafeFailure = null;
    Throwable thrownAtomicReferenceFieldUpdaterFailure = null;

    try {
      helper = new UnsafeAtomicHelper();
    } catch (Throwable unsafeFailure) {
      thrownUnsafeFailure = unsafeFailure;
      // catch absolutely everything and fall through to our 'SafeAtomicHelper'
      // The access control checks that ARFU does means the caller class has to be AbstractFuture
      // instead of SafeAtomicHelper, so we annoyingly define these here
      try {
        helper =
            new SafeAtomicHelper(
                newUpdater(Waiter.class, Thread.class, "thread"),
                newUpdater(Waiter.class, Waiter.class, "next"),
                newUpdater(AbstractFuture.class, Waiter.class, "waiters"),
                newUpdater(AbstractFuture.class, Listener.class, "listeners"),
                newUpdater(AbstractFuture.class, Object.class, "value"));
      } catch (Throwable atomicReferenceFieldUpdaterFailure) {
        // Some Android 5.0.x Samsung devices have bugs in JDK reflection APIs that cause
        // getDeclaredField to throw a NoSuchFieldException when the field is definitely there.
        // For these users fallback to a suboptimal implementation, based on synchronized. This will
        // be a definite performance hit to those users.
        thrownAtomicReferenceFieldUpdaterFailure = atomicReferenceFieldUpdaterFailure;
        helper = new SynchronizedHelper();
      }
    }
    ATOMIC_HELPER = helper;

    // Prevent rare disastrous classloading in first call to LockSupport.park.
    // See: https://bugs.openjdk.java.net/browse/JDK-8074773
    @SuppressWarnings("unused")
    Class<?> ensureLoaded = LockSupport.class;

    // Log after all static init is finished; if an installed logger uses any Futures methods, it
    // shouldn't break in cases where reflection is missing/broken.
    if (thrownAtomicReferenceFieldUpdaterFailure != null) {
      log.log(Level.SEVERE, "UnsafeAtomicHelper is broken!", thrownUnsafeFailure);
      log.log(
          Level.SEVERE, "SafeAtomicHelper is broken!", thrownAtomicReferenceFieldUpdaterFailure);
    }
  }

  
Waiter links form a Treiber stack, in the AbstractFuture<V>.waiters field. 
/** Waiter links form a Treiber stack, in the {@link #waiters} field. */
  private static final class Waiter {
    static final Waiter TOMBSTONE = new Waiter(false /* ignored param */);

    volatile @Nullable Thread thread;
    volatile @Nullable Waiter next;

    
Constructor for the TOMBSTONE, avoids use of ATOMIC_HELPER in case this class is loaded
before the ATOMIC_HELPER. Apparently this is possible on some android platforms.

/**
     * Constructor for the TOMBSTONE, avoids use of ATOMIC_HELPER in case this class is loaded
     * before the ATOMIC_HELPER. Apparently this is possible on some android platforms.
     */
    Waiter(boolean unused) {}

    Waiter() {
      // avoid volatile write, write is made visible by subsequent CAS on waiters field
      ATOMIC_HELPER.putThread(this, Thread.currentThread());
    }

    // non-volatile write to the next field. Should be made visible by subsequent CAS on waiters
    // field.
    void setNext(Waiter next) {
      ATOMIC_HELPER.putNext(this, next);
    }

    void unpark() {
      // This is racy with removeWaiter. The consequence of the race is that we may spuriously call
      // unpark even though the thread has already removed itself from the list. But even if we did
      // use a CAS, that race would still exist (it would just be ever so slightly smaller).
      Thread w = thread;
      if (w != null) {
        thread = null;
        LockSupport.unpark(w);
      }
    }
  }

  
Marks the given node as 'deleted' (null waiter) and then scans the list to unlink all deleted
nodes. This is an O(n) operation in the common case (and O(n^2) in the worst), but we are saved
by two things.

    
  
  • This is only called when a waiting thread times out or is interrupted. Both of which
      should be rare.
  
  • The waiters list should be very short.



/**
   * Marks the given node as 'deleted' (null waiter) and then scans the list to unlink all deleted
   * nodes. This is an O(n) operation in the common case (and O(n^2) in the worst), but we are saved
   * by two things.
   *
   * <ul>
   *   <li>This is only called when a waiting thread times out or is interrupted. Both of which
   *       should be rare.
   *   <li>The waiters list should be very short.
   * </ul>
   */
  private void removeWaiter(Waiter node) {
    node.thread = null; // mark as 'deleted'
    restart:
    while (true) {
      Waiter pred = null;
      Waiter curr = waiters;
      if (curr == Waiter.TOMBSTONE) {
        return; // give up if someone is calling complete
      }
      Waiter succ;
      while (curr != null) {
        succ = curr.next;
        if (curr.thread != null) { // we aren't unlinking this node, update pred.
          pred = curr;
        } else if (pred != null) { // We are unlinking this node and it has a predecessor.
          pred.next = succ;
          if (pred.thread == null) { // We raced with another node that unlinked pred. Restart.
            continue restart;
          }
        } else if (!ATOMIC_HELPER.casWaiters(this, curr, succ)) { // We are unlinking head
          continue restart; // We raced with an add or complete
        }
        curr = succ;
      }
      break;
    }
  }

  
Listeners also form a stack through the AbstractFuture<V>.listeners field. 
/** Listeners also form a stack through the {@link #listeners} field. */
  private static final class Listener {
    static final Listener TOMBSTONE = new Listener(null, null);
    final Runnable task;
    final Executor executor;

    // writes to next are made visible by subsequent CAS's on the listeners field
    @Nullable Listener next;

    Listener(Runnable task, Executor executor) {
      this.task = task;
      this.executor = executor;
    }
  }

  
A special value to represent null. 
/** A special value to represent {@code null}. */
  private static final Object NULL = new Object();

  
A special value to represent failure, when setException is called successfully. 
/** A special value to represent failure, when {@link #setException} is called successfully. */
  private static final class Failure {
    static final Failure FALLBACK_INSTANCE =
        new Failure(
            new Throwable("Failure occurred while trying to finish a future.") {
              @Override
              public synchronized Throwable fillInStackTrace() {
                return this; // no stack trace
              }
            });
    final Throwable exception;

    Failure(Throwable exception) {
      this.exception = checkNotNull(exception);
    }
  }

  
A special value to represent cancellation and the 'wasInterrupted' bit. 
/** A special value to represent cancellation and the 'wasInterrupted' bit. */
  private static final class Cancellation {
    // constants to use when GENERATE_CANCELLATION_CAUSES = false
    static final Cancellation CAUSELESS_INTERRUPTED;
    static final Cancellation CAUSELESS_CANCELLED;

    static {
      if (GENERATE_CANCELLATION_CAUSES) {
        CAUSELESS_CANCELLED = null;
        CAUSELESS_INTERRUPTED = null;
      } else {
        CAUSELESS_CANCELLED = new Cancellation(false, null);
        CAUSELESS_INTERRUPTED = new Cancellation(true, null);
      }
    }

    final boolean wasInterrupted;
    final @Nullable Throwable cause;

    Cancellation(boolean wasInterrupted, @Nullable Throwable cause) {
      this.wasInterrupted = wasInterrupted;
      this.cause = cause;
    }
  }

  
A special value that encodes the 'setFuture' state. 
/** A special value that encodes the 'setFuture' state. */
  private static final class SetFuture<V> implements Runnable {
    final AbstractFuture<V> owner;
    final ListenableFuture<? extends V> future;

    SetFuture(AbstractFuture<V> owner, ListenableFuture<? extends V> future) {
      this.owner = owner;
      this.future = future;
    }

    @Override
    public void run() {
      if (owner.value != this) {
        // nothing to do, we must have been cancelled, don't bother inspecting the future.
        return;
      }
      Object valueToSet = getFutureValue(future);
      if (ATOMIC_HELPER.casValue(owner, this, valueToSet)) {
        complete(owner);
      }
    }
  }

  // TODO(lukes): investigate using the @Contended annotation on these fields when jdk8 is
  // available.
  
This field encodes the current state of the future.

The valid values are:

    
  
  • null initial state, nothing has happened. 
  • Cancellation terminal state, cancel was called. 
  • Failure terminal state, setException was called. 
  • SetFuture intermediate state, setFuture was called. 
  • AbstractFuture<V>.NULL terminal state, set(null) was called. 
  • Any other non-null value, terminal state, set was called with a non-null argument. 


/**
   * This field encodes the current state of the future.
   *
   * <p>The valid values are:
   *
   * <ul>
   *   <li>{@code null} initial state, nothing has happened.
   *   <li>{@link Cancellation} terminal state, {@code cancel} was called.
   *   <li>{@link Failure} terminal state, {@code setException} was called.
   *   <li>{@link SetFuture} intermediate state, {@code setFuture} was called.
   *   <li>{@link #NULL} terminal state, {@code set(null)} was called.
   *   <li>Any other non-null value, terminal state, {@code set} was called with a non-null
   *       argument.
   * </ul>
   */
  private volatile @Nullable Object value;

  
All listeners. 
/** All listeners. */
  private volatile @Nullable Listener listeners;

  
All waiting threads. 
/** All waiting threads. */
  private volatile @Nullable Waiter waiters;

  
Constructor for use by subclasses. 
/** Constructor for use by subclasses. */
  protected AbstractFuture() {}

  // Gets and Timed Gets
  //
  // * Be responsive to interruption
  // * Don't create Waiter nodes if you aren't going to park, this helps reduce contention on the
  //   waiters field.
  // * Future completion is defined by when #value becomes non-null/non SetFuture
  // * Future completion can be observed if the waiters field contains a TOMBSTONE

  // Timed Get
  // There are a few design constraints to consider
  // * We want to be responsive to small timeouts, unpark() has non trivial latency overheads (I
  //   have observed 12 micros on 64 bit linux systems to wake up a parked thread). So if the
  //   timeout is small we shouldn't park(). This needs to be traded off with the cpu overhead of
  //   spinning, so we use SPIN_THRESHOLD_NANOS which is what AbstractQueuedSynchronizer uses for
  //   similar purposes.
  // * We want to behave reasonably for timeouts of 0
  // * We are more responsive to completion than timeouts. This is because parkNanos depends on
  //   system scheduling and as such we could either miss our deadline, or unpark() could be delayed
  //   so that it looks like we timed out even though we didn't. For comparison FutureTask respects
  //   completion preferably and AQS is non-deterministic (depends on where in the queue the waiter
  //   is). If we wanted to be strict about it, we could store the unpark() time in the Waiter node
  //   and we could use that to make a decision about whether or not we timed out prior to being
  //   unparked.

  
{@inheritDoc}

The default AbstractFuture implementation throws InterruptedException if the current thread is interrupted during the call, even if the value is already available. 
Throws:
/**
   * {@inheritDoc}
   *
   * <p>The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
   * current thread is interrupted during the call, even if the value is already available.
   *
   * @throws CancellationException {@inheritDoc}
   */
  @CanIgnoreReturnValue
  @Override
  public V get(long timeout, TimeUnit unit)
      throws InterruptedException, TimeoutException, ExecutionException {
    // NOTE: if timeout < 0, remainingNanos will be < 0 and we will fall into the while(true) loop
    // at the bottom and throw a timeoutexception.
    long remainingNanos = unit.toNanos(timeout); // we rely on the implicit null check on unit.
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    Object localValue = value;
    if (localValue != null & !(localValue instanceof SetFuture)) {
      return getDoneValue(localValue);
    }
    // we delay calling nanoTime until we know we will need to either park or spin
    final long endNanos = remainingNanos > 0 ? System.nanoTime() + remainingNanos : 0;
    long_wait_loop:
    if (remainingNanos >= SPIN_THRESHOLD_NANOS) {
      Waiter oldHead = waiters;
      if (oldHead != Waiter.TOMBSTONE) {
        Waiter node = new Waiter();
        do {
          node.setNext(oldHead);
          if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
            while (true) {
              LockSupport.parkNanos(this, remainingNanos);
              // Check interruption first, if we woke up due to interruption we need to honor that.
              if (Thread.interrupted()) {
                removeWaiter(node);
                throw new InterruptedException();
              }

              // Otherwise re-read and check doneness. If we loop then it must have been a spurious
              // wakeup
              localValue = value;
              if (localValue != null & !(localValue instanceof SetFuture)) {
                return getDoneValue(localValue);
              }

              // timed out?
              remainingNanos = endNanos - System.nanoTime();
              if (remainingNanos < SPIN_THRESHOLD_NANOS) {
                // Remove the waiter, one way or another we are done parking this thread.
                removeWaiter(node);
                break long_wait_loop; // jump down to the busy wait loop
              }
            }
          }
          oldHead = waiters; // re-read and loop.
        } while (oldHead != Waiter.TOMBSTONE);
      }
      // re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
      // waiter.
      return getDoneValue(value);
    }
    // If we get here then we have remainingNanos < SPIN_THRESHOLD_NANOS and there is no node on the
    // waiters list
    while (remainingNanos > 0) {
      localValue = value;
      if (localValue != null & !(localValue instanceof SetFuture)) {
        return getDoneValue(localValue);
      }
      if (Thread.interrupted()) {
        throw new InterruptedException();
      }
      remainingNanos = endNanos - System.nanoTime();
    }

    String futureToString = toString();
    // It's confusing to see a completed future in a timeout message; if isDone() returns false,
    // then we know it must have given a pending toString value earlier. If not, then the future
    // completed after the timeout expired, and the message might be success.
    if (isDone()) {
      throw new TimeoutException(
          "Waited "
              + timeout
              + " "
              + Ascii.toLowerCase(unit.toString())
              + " but future completed as timeout expired");
    }
    throw new TimeoutException(
        "Waited " + timeout + " " + Ascii.toLowerCase(unit.toString()) + " for " + futureToString);
  }

  
{@inheritDoc}

The default AbstractFuture implementation throws InterruptedException if the current thread is interrupted during the call, even if the value is already available. 
Throws:
/**
   * {@inheritDoc}
   *
   * <p>The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
   * current thread is interrupted during the call, even if the value is already available.
   *
   * @throws CancellationException {@inheritDoc}
   */
  @CanIgnoreReturnValue
  @Override
  public V get() throws InterruptedException, ExecutionException {
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    Object localValue = value;
    if (localValue != null & !(localValue instanceof SetFuture)) {
      return getDoneValue(localValue);
    }
    Waiter oldHead = waiters;
    if (oldHead != Waiter.TOMBSTONE) {
      Waiter node = new Waiter();
      do {
        node.setNext(oldHead);
        if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
          // we are on the stack, now wait for completion.
          while (true) {
            LockSupport.park(this);
            // Check interruption first, if we woke up due to interruption we need to honor that.
            if (Thread.interrupted()) {
              removeWaiter(node);
              throw new InterruptedException();
            }
            // Otherwise re-read and check doneness. If we loop then it must have been a spurious
            // wakeup
            localValue = value;
            if (localValue != null & !(localValue instanceof SetFuture)) {
              return getDoneValue(localValue);
            }
          }
        }
        oldHead = waiters; // re-read and loop.
      } while (oldHead != Waiter.TOMBSTONE);
    }
    // re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
    // waiter.
    return getDoneValue(value);
  }

  
Unboxes obj. Assumes that obj is not null or a SetFuture. 
/** Unboxes {@code obj}. Assumes that obj is not {@code null} or a {@link SetFuture}. */
  private V getDoneValue(Object obj) throws ExecutionException {
    // While this seems like it might be too branch-y, simple benchmarking proves it to be
    // unmeasurable (comparing done AbstractFutures with immediateFuture)
    if (obj instanceof Cancellation) {
      throw cancellationExceptionWithCause("Task was cancelled.", ((Cancellation) obj).cause);
    } else if (obj instanceof Failure) {
      throw new ExecutionException(((Failure) obj).exception);
    } else if (obj == NULL) {
      return null;
    } else {
      @SuppressWarnings("unchecked") // this is the only other option
      V asV = (V) obj;
      return asV;
    }
  }

  @Override
  public boolean isDone() {
    final Object localValue = value;
    return localValue != null & !(localValue instanceof SetFuture);
  }

  @Override
  public boolean isCancelled() {
    final Object localValue = value;
    return localValue instanceof Cancellation;
  }

  
{@inheritDoc}

If a cancellation attempt succeeds on a Future that had previously been set asynchronously, then the cancellation will also be propagated to the delegate Future that was supplied in the setFuture call. 
Rather than override this method to perform additional cancellation work or cleanup, subclasses should override afterDone, consulting isCancelled and wasInterrupted as necessary. This ensures that the work is done even if the future is cancelled without a call to cancel, such as by calling 
setFuture(cancelledFuture). 
/**
   * {@inheritDoc}
   *
   * <p>If a cancellation attempt succeeds on a {@code Future} that had previously been {@linkplain
   * #setFuture set asynchronously}, then the cancellation will also be propagated to the delegate
   * {@code Future} that was supplied in the {@code setFuture} call.
   *
   * <p>Rather than override this method to perform additional cancellation work or cleanup,
   * subclasses should override {@link #afterDone}, consulting {@link #isCancelled} and {@link
   * #wasInterrupted} as necessary. This ensures that the work is done even if the future is
   * cancelled without a call to {@code cancel}, such as by calling {@code
   * setFuture(cancelledFuture)}.
   */
  @CanIgnoreReturnValue
  @Override
  public boolean cancel(boolean mayInterruptIfRunning) {
    Object localValue = value;
    boolean rValue = false;
    if (localValue == null | localValue instanceof SetFuture) {
      // Try to delay allocating the exception. At this point we may still lose the CAS, but it is
      // certainly less likely.
      Object valueToSet =
          GENERATE_CANCELLATION_CAUSES
              ? new Cancellation(
                  mayInterruptIfRunning, new CancellationException("Future.cancel() was called."))
              : (mayInterruptIfRunning
                  ? Cancellation.CAUSELESS_INTERRUPTED
                  : Cancellation.CAUSELESS_CANCELLED);
      AbstractFuture<?> abstractFuture = this;
      while (true) {
        if (ATOMIC_HELPER.casValue(abstractFuture, localValue, valueToSet)) {
          rValue = true;
          // We call interuptTask before calling complete(), which is consistent with
          // FutureTask
          if (mayInterruptIfRunning) {
            abstractFuture.interruptTask();
          }
          complete(abstractFuture);
          if (localValue instanceof SetFuture) {
            // propagate cancellation to the future set in setfuture, this is racy, and we don't
            // care if we are successful or not.
            ListenableFuture<?> futureToPropagateTo = ((SetFuture) localValue).future;
            if (futureToPropagateTo instanceof TrustedFuture) {
              // If the future is a TrustedFuture then we specifically avoid calling cancel()
              // this has 2 benefits
              // 1. for long chains of futures strung together with setFuture we consume less stack
              // 2. we avoid allocating Cancellation objects at every level of the cancellation
              //    chain
              // We can only do this for TrustedFuture, because TrustedFuture.cancel is final and
              // does nothing but delegate to this method.
              AbstractFuture<?> trusted = (AbstractFuture<?>) futureToPropagateTo;
              localValue = trusted.value;
              if (localValue == null | localValue instanceof SetFuture) {
                abstractFuture = trusted;
                continue; // loop back up and try to complete the new future
              }
            } else {
              // not a TrustedFuture, call cancel directly.
              futureToPropagateTo.cancel(mayInterruptIfRunning);
            }
          }
          break;
        }
        // obj changed, reread
        localValue = abstractFuture.value;
        if (!(localValue instanceof SetFuture)) {
          // obj cannot be null at this point, because value can only change from null to non-null.
          // So if value changed (and it did since we lost the CAS), then it cannot be null and
          // since it isn't a SetFuture, then the future must be done and we should exit the loop
          break;
        }
      }
    }
    return rValue;
  }

  
Subclasses can override this method to implement interruption of the future's computation. The method is invoked automatically by a successful call to cancel(true). 
The default implementation does nothing.

This method is likely to be deprecated. Prefer to override afterDone, checking wasInterrupted to decide whether to interrupt your task. 
Since:10.0
/**
   * Subclasses can override this method to implement interruption of the future's computation. The
   * method is invoked automatically by a successful call to {@link #cancel(boolean) cancel(true)}.
   *
   * <p>The default implementation does nothing.
   *
   * <p>This method is likely to be deprecated. Prefer to override {@link #afterDone}, checking
   * {@link #wasInterrupted} to decide whether to interrupt your task.
   *
   * @since 10.0
   */
  protected void interruptTask() {}

  
Returns true if this future was cancelled with mayInterruptIfRunning set to 
true. 
Since:14.0
/**
   * Returns true if this future was cancelled with {@code mayInterruptIfRunning} set to {@code
   * true}.
   *
   * @since 14.0
   */
  protected final boolean wasInterrupted() {
    final Object localValue = value;
    return (localValue instanceof Cancellation) && ((Cancellation) localValue).wasInterrupted;
  }

  
{@inheritDoc}

Since:10.0
/**
   * {@inheritDoc}
   *
   * @since 10.0
   */
  @Override
  public void addListener(Runnable listener, Executor executor) {
    checkNotNull(listener, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");
    Listener oldHead = listeners;
    if (oldHead != Listener.TOMBSTONE) {
      Listener newNode = new Listener(listener, executor);
      do {
        newNode.next = oldHead;
        if (ATOMIC_HELPER.casListeners(this, oldHead, newNode)) {
          return;
        }
        oldHead = listeners; // re-read
      } while (oldHead != Listener.TOMBSTONE);
    }
    // If we get here then the Listener TOMBSTONE was set, which means the future is done, call
    // the listener.
    executeListener(listener, executor);
  }

  
Sets the result of this Future unless this Future has already been cancelled or set (including set asynchronously). When a call to this method returns, the Future is guaranteed to be done only if the call was accepted (in which case it returns true). If it returns false, the 
Future may have previously been set asynchronously, in which case its result may not be known yet. That result, though not yet known, cannot be overridden by a call to a set* method, only by a call to cancel. 
Params:
  • value – the value to be used as the result
Returns:true if the attempt was accepted, completing the Future
/**
   * Sets the result of this {@code Future} unless this {@code Future} has already been cancelled or
   * set (including {@linkplain #setFuture set asynchronously}). When a call to this method returns,
   * the {@code Future} is guaranteed to be {@linkplain #isDone done} <b>only if</b> the call was
   * accepted (in which case it returns {@code true}). If it returns {@code false}, the {@code
   * Future} may have previously been set asynchronously, in which case its result may not be known
   * yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
   * method, only by a call to {@link #cancel}.
   *
   * @param value the value to be used as the result
   * @return true if the attempt was accepted, completing the {@code Future}
   */
  @CanIgnoreReturnValue
  protected boolean set(@Nullable V value) {
    Object valueToSet = value == null ? NULL : value;
    if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
      complete(this);
      return true;
    }
    return false;
  }

  
Sets the failed result of this Future unless this Future has already been cancelled or set (including set asynchronously). When a call to this method returns, the Future is guaranteed to be done only if the call was accepted (in which case it returns true). If it returns false, the Future may have previously been set asynchronously, in which case its result may not be known yet. That result, though not yet known, cannot be overridden by a call to a set* method, only by a call to cancel. 
Params:
  • throwable – the exception to be used as the failed result
Returns:true if the attempt was accepted, completing the Future
/**
   * Sets the failed result of this {@code Future} unless this {@code Future} has already been
   * cancelled or set (including {@linkplain #setFuture set asynchronously}). When a call to this
   * method returns, the {@code Future} is guaranteed to be {@linkplain #isDone done} <b>only if</b>
   * the call was accepted (in which case it returns {@code true}). If it returns {@code false}, the
   * {@code Future} may have previously been set asynchronously, in which case its result may not be
   * known yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
   * method, only by a call to {@link #cancel}.
   *
   * @param throwable the exception to be used as the failed result
   * @return true if the attempt was accepted, completing the {@code Future}
   */
  @CanIgnoreReturnValue
  protected boolean setException(Throwable throwable) {
    Object valueToSet = new Failure(checkNotNull(throwable));
    if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
      complete(this);
      return true;
    }
    return false;
  }

  
Sets the result of this Future to match the supplied input Future once the supplied Future is done, unless this Future has already been cancelled or set (including "set asynchronously," defined below). 
If the supplied future is done when this method is called and the call is accepted, then this future is guaranteed to have been completed with the supplied future by the time this method returns. If the supplied future is not done and the call is accepted, then the future will be set asynchronously. Note that such a result, though not yet known, cannot be overridden by a call to a set* method, only by a call to cancel. 
If the call setFuture(delegate) is accepted and this Future is later cancelled, cancellation will be propagated to delegate. Additionally, any call to setFuture after any cancellation will propagate cancellation to the supplied 
Future. 
Note that, even if the supplied future is cancelled and it causes this future to complete, it will never trigger interruption behavior. In particular, it will not cause this future to invoke the interruptTask method, and the wasInterrupted method will not return true. 
Params:
  • future – the future to delegate to
Returns:true if the attempt was accepted, indicating that the Future was not previously cancelled or set.
Since:19.0
/**
   * Sets the result of this {@code Future} to match the supplied input {@code Future} once the
   * supplied {@code Future} is done, unless this {@code Future} has already been cancelled or set
   * (including "set asynchronously," defined below).
   *
   * <p>If the supplied future is {@linkplain #isDone done} when this method is called and the call
   * is accepted, then this future is guaranteed to have been completed with the supplied future by
   * the time this method returns. If the supplied future is not done and the call is accepted, then
   * the future will be <i>set asynchronously</i>. Note that such a result, though not yet known,
   * cannot be overridden by a call to a {@code set*} method, only by a call to {@link #cancel}.
   *
   * <p>If the call {@code setFuture(delegate)} is accepted and this {@code Future} is later
   * cancelled, cancellation will be propagated to {@code delegate}. Additionally, any call to
   * {@code setFuture} after any cancellation will propagate cancellation to the supplied {@code
   * Future}.
   *
   * <p>Note that, even if the supplied future is cancelled and it causes this future to complete,
   * it will never trigger interruption behavior. In particular, it will not cause this future to
   * invoke the {@link #interruptTask} method, and the {@link #wasInterrupted} method will not
   * return {@code true}.
   *
   * @param future the future to delegate to
   * @return true if the attempt was accepted, indicating that the {@code Future} was not previously
   *     cancelled or set.
   * @since 19.0
   */
  @Beta
  @CanIgnoreReturnValue
  protected boolean setFuture(ListenableFuture<? extends V> future) {
    checkNotNull(future);
    Object localValue = value;
    if (localValue == null) {
      if (future.isDone()) {
        Object value = getFutureValue(future);
        if (ATOMIC_HELPER.casValue(this, null, value)) {
          complete(this);
          return true;
        }
        return false;
      }
      SetFuture valueToSet = new SetFuture<V>(this, future);
      if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
        // the listener is responsible for calling completeWithFuture, directExecutor is appropriate
        // since all we are doing is unpacking a completed future which should be fast.
        try {
          future.addListener(valueToSet, directExecutor());
        } catch (Throwable t) {
          // addListener has thrown an exception! SetFuture.run can't throw any exceptions so this
          // must have been caused by addListener itself. The most likely explanation is a
          // misconfigured mock. Try to switch to Failure.
          Failure failure;
          try {
            failure = new Failure(t);
          } catch (Throwable oomMostLikely) {
            failure = Failure.FALLBACK_INSTANCE;
          }
          // Note: The only way this CAS could fail is if cancel() has raced with us. That is ok.
          boolean unused = ATOMIC_HELPER.casValue(this, valueToSet, failure);
        }
        return true;
      }
      localValue = value; // we lost the cas, fall through and maybe cancel
    }
    // The future has already been set to something. If it is cancellation we should cancel the
    // incoming future.
    if (localValue instanceof Cancellation) {
      // we don't care if it fails, this is best-effort.
      future.cancel(((Cancellation) localValue).wasInterrupted);
    }
    return false;
  }

  
Returns a value that satisfies the contract of the AbstractFuture<V>.value field based on the state of given future. 
This is approximately the inverse of getDoneValue(Object) 
/**
   * Returns a value that satisfies the contract of the {@link #value} field based on the state of
   * given future.
   *
   * <p>This is approximately the inverse of {@link #getDoneValue(Object)}
   */
  private static Object getFutureValue(ListenableFuture<?> future) {
    Object valueToSet;
    if (future instanceof TrustedFuture) {
      // Break encapsulation for TrustedFuture instances since we know that subclasses cannot
      // override .get() (since it is final) and therefore this is equivalent to calling .get()
      // and unpacking the exceptions like we do below (just much faster because it is a single
      // field read instead of a read, several branches and possibly creating exceptions).
      Object v = ((AbstractFuture<?>) future).value;
      if (v instanceof Cancellation) {
        // If the other future was interrupted, clear the interrupted bit while preserving the cause
        // this will make it consistent with how non-trustedfutures work which cannot propagate the
        // wasInterrupted bit
        Cancellation c = (Cancellation) v;
        if (c.wasInterrupted) {
          v =
              c.cause != null
                  ? new Cancellation(/* wasInterrupted= */ false, c.cause)
                  : Cancellation.CAUSELESS_CANCELLED;
        }
      }
      return v;
    } else {
      // Otherwise calculate valueToSet by calling .get()
      try {
        Object v = getDone(future);
        valueToSet = v == null ? NULL : v;
      } catch (ExecutionException exception) {
        valueToSet = new Failure(exception.getCause());
      } catch (CancellationException cancellation) {
        valueToSet = new Cancellation(false, cancellation);
      } catch (Throwable t) {
        valueToSet = new Failure(t);
      }
    }
    return valueToSet;
  }

  
Unblocks all threads and runs all listeners. 
/** Unblocks all threads and runs all listeners. */
  private static void complete(AbstractFuture<?> future) {
    Listener next = null;
    outer:
    while (true) {
      future.releaseWaiters();
      // We call this before the listeners in order to avoid needing to manage a separate stack data
      // structure for them.
      // afterDone() should be generally fast and only used for cleanup work... but in theory can
      // also be recursive and create StackOverflowErrors
      future.afterDone();
      // push the current set of listeners onto next
      next = future.clearListeners(next);
      future = null;
      while (next != null) {
        Listener curr = next;
        next = next.next;
        Runnable task = curr.task;
        if (task instanceof SetFuture) {
          SetFuture<?> setFuture = (SetFuture<?>) task;
          // We unwind setFuture specifically to avoid StackOverflowErrors in the case of long
          // chains of SetFutures
          // Handling this special case is important because there is no way to pass an executor to
          // setFuture, so a user couldn't break the chain by doing this themselves.  It is also
          // potentially common if someone writes a recursive Futures.transformAsync transformer.
          future = setFuture.owner;
          if (future.value == setFuture) {
            Object valueToSet = getFutureValue(setFuture.future);
            if (ATOMIC_HELPER.casValue(future, setFuture, valueToSet)) {
              continue outer;
            }
          }
          // other wise the future we were trying to set is already done.
        } else {
          executeListener(task, curr.executor);
        }
      }
      break;
    }
  }

  
Callback method that is called exactly once after the future is completed.

If interruptTask is also run during completion, afterDone runs after it. 
The default implementation of this method in AbstractFuture does nothing. This is intended for very lightweight cleanup work, for example, timing statistics or clearing fields. If your task does anything heavier consider, just using a listener with an executor. 
Since:20.0
/**
   * Callback method that is called exactly once after the future is completed.
   *
   * <p>If {@link #interruptTask} is also run during completion, {@link #afterDone} runs after it.
   *
   * <p>The default implementation of this method in {@code AbstractFuture} does nothing. This is
   * intended for very lightweight cleanup work, for example, timing statistics or clearing fields.
   * If your task does anything heavier consider, just using a listener with an executor.
   *
   * @since 20.0
   */
  @Beta
  @ForOverride
  protected void afterDone() {}

  
Returns the exception that this Future completed with. This includes completion through a call to setException or setFuture(failedFuture) but not cancellation. 
Throws:
/**
   * Returns the exception that this {@code Future} completed with. This includes completion through
   * a call to {@link #setException} or {@link #setFuture setFuture}{@code (failedFuture)} but not
   * cancellation.
   *
   * @throws RuntimeException if the {@code Future} has not failed
   */
  final Throwable trustedGetException() {
    return ((Failure) value).exception;
  }

  
If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts)
the given future (if available).

/**
   * If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts)
   * the given future (if available).
   */
  final void maybePropagateCancellationTo(@Nullable Future<?> related) {
    if (related != null & isCancelled()) {
      related.cancel(wasInterrupted());
    }
  }

  
Releases all threads in the AbstractFuture<V>.waiters list, and clears the list. 
/** Releases all threads in the {@link #waiters} list, and clears the list. */
  private void releaseWaiters() {
    Waiter head;
    do {
      head = waiters;
    } while (!ATOMIC_HELPER.casWaiters(this, head, Waiter.TOMBSTONE));
    for (Waiter currentWaiter = head; currentWaiter != null; currentWaiter = currentWaiter.next) {
      currentWaiter.unpark();
    }
  }

  
Clears the AbstractFuture<V>.listeners list and prepends its contents to onto, least recently added first. 
/**
   * Clears the {@link #listeners} list and prepends its contents to {@code onto}, least recently
   * added first.
   */
  private Listener clearListeners(Listener onto) {
    // We need to
    // 1. atomically swap the listeners with TOMBSTONE, this is because addListener uses that to
    //    to synchronize with us
    // 2. reverse the linked list, because despite our rather clear contract, people depend on us
    //    executing listeners in the order they were added
    // 3. push all the items onto 'onto' and return the new head of the stack
    Listener head;
    do {
      head = listeners;
    } while (!ATOMIC_HELPER.casListeners(this, head, Listener.TOMBSTONE));
    Listener reversedList = onto;
    while (head != null) {
      Listener tmp = head;
      head = head.next;
      tmp.next = reversedList;
      reversedList = tmp;
    }
    return reversedList;
  }

  // TODO(user) move this up into FluentFuture, or parts as a default method on ListenableFuture?
  @Override
  public String toString() {
    StringBuilder builder = new StringBuilder().append(super.toString()).append("[status=");
    if (isCancelled()) {
      builder.append("CANCELLED");
    } else if (isDone()) {
      addDoneString(builder);
    } else {
      String pendingDescription;
      try {
        pendingDescription = pendingToString();
      } catch (RuntimeException e) {
        // Don't call getMessage or toString() on the exception, in case the exception thrown by the
        // subclass is implemented with bugs similar to the subclass.
        pendingDescription = "Exception thrown from implementation: " + e.getClass();
      }
      // The future may complete during or before the call to getPendingToString, so we use null
      // as a signal that we should try checking if the future is done again.
      if (!isNullOrEmpty(pendingDescription)) {
        builder.append("PENDING, info=[").append(pendingDescription).append("]");
      } else if (isDone()) {
        addDoneString(builder);
      } else {
        builder.append("PENDING");
      }
    }
    return builder.append("]").toString();
  }

  
Provide a human-readable explanation of why this future has not yet completed.

Returns:null if an explanation cannot be provided because the future is done.
Since:23.0
/**
   * Provide a human-readable explanation of why this future has not yet completed.
   *
   * @return null if an explanation cannot be provided because the future is done.
   * @since 23.0
   */
  protected @Nullable String pendingToString() {
    Object localValue = value;
    if (localValue instanceof SetFuture) {
      return "setFuture=[" + userObjectToString(((SetFuture) localValue).future) + "]";
    } else if (this instanceof ScheduledFuture) {
      return "remaining delay=["
          + ((ScheduledFuture) this).getDelay(TimeUnit.MILLISECONDS)
          + " ms]";
    }
    return null;
  }

  private void addDoneString(StringBuilder builder) {
    try {
      V value = getDone(this);
      builder.append("SUCCESS, result=[").append(userObjectToString(value)).append("]");
    } catch (ExecutionException e) {
      builder.append("FAILURE, cause=[").append(e.getCause()).append("]");
    } catch (CancellationException e) {
      builder.append("CANCELLED"); // shouldn't be reachable
    } catch (RuntimeException e) {
      builder.append("UNKNOWN, cause=[").append(e.getClass()).append(" thrown from get()]");
    }
  }

  
Helper for printing user supplied objects into our toString method. 
/** Helper for printing user supplied objects into our toString method. */
  private String userObjectToString(Object o) {
    // This is some basic recursion detection for when people create cycles via set/setFuture
    // This is however only partial protection though since it only detects self loops.  We could
    // detect arbitrary cycles using a thread local or possibly by catching StackOverflowExceptions
    // but this should be a good enough solution (it is also what jdk collections do in these cases)
    if (o == this) {
      return "this future";
    }
    return String.valueOf(o);
  }

  
Submits the given runnable to the given Executor catching and logging all runtime exceptions thrown by the executor. 
/**
   * Submits the given runnable to the given {@link Executor} catching and logging all {@linkplain
   * RuntimeException runtime exceptions} thrown by the executor.
   */
  private static void executeListener(Runnable runnable, Executor executor) {
    try {
      executor.execute(runnable);
    } catch (RuntimeException e) {
      // Log it and keep going -- bad runnable and/or executor. Don't punish the other runnables if
      // we're given a bad one. We only catch RuntimeException because we want Errors to propagate
      // up.
      log.log(
          Level.SEVERE,
          "RuntimeException while executing runnable " + runnable + " with executor " + executor,
          e);
    }
  }

  private abstract static class AtomicHelper {
    
Non volatile write of the thread to the Waiter.thread field. 
/** Non volatile write of the thread to the {@link Waiter#thread} field. */
    abstract void putThread(Waiter waiter, Thread newValue);

    
Non volatile write of the waiter to the Waiter.next field. 
/** Non volatile write of the waiter to the {@link Waiter#next} field. */
    abstract void putNext(Waiter waiter, Waiter newValue);

    
Performs a CAS operation on the AbstractFuture<V>.waiters field. 
/** Performs a CAS operation on the {@link #waiters} field. */
    abstract boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update);

    
Performs a CAS operation on the AbstractFuture<V>.listeners field. 
/** Performs a CAS operation on the {@link #listeners} field. */
    abstract boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update);

    
Performs a CAS operation on the AbstractFuture<V>.value field. 
/** Performs a CAS operation on the {@link #value} field. */
    abstract boolean casValue(AbstractFuture<?> future, Object expect, Object update);
  }

  
AtomicHelper based on Unsafe. 
Static initialization of this class will fail if the Unsafe object cannot be accessed. 
/**
   * {@link AtomicHelper} based on {@link sun.misc.Unsafe}.
   *
   * <p>Static initialization of this class will fail if the {@link sun.misc.Unsafe} object cannot
   * be accessed.
   */
  private static final class UnsafeAtomicHelper extends AtomicHelper {
    static final sun.misc.Unsafe UNSAFE;
    static final long LISTENERS_OFFSET;
    static final long WAITERS_OFFSET;
    static final long VALUE_OFFSET;
    static final long WAITER_THREAD_OFFSET;
    static final long WAITER_NEXT_OFFSET;

    static {
      sun.misc.Unsafe unsafe = null;
      try {
        unsafe = sun.misc.Unsafe.getUnsafe();
      } catch (SecurityException tryReflectionInstead) {
        try {
          unsafe =
              AccessController.doPrivileged(
                  new PrivilegedExceptionAction<sun.misc.Unsafe>() {
                    @Override
                    public sun.misc.Unsafe run() throws Exception {
                      Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                      for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                        f.setAccessible(true);
                        Object x = f.get(null);
                        if (k.isInstance(x)) {
                          return k.cast(x);
                        }
                      }
                      throw new NoSuchFieldError("the Unsafe");
                    }
                  });
        } catch (PrivilegedActionException e) {
          throw new RuntimeException("Could not initialize intrinsics", e.getCause());
        }
      }
      try {
        Class<?> abstractFuture = AbstractFuture.class;
        WAITERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("waiters"));
        LISTENERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("listeners"));
        VALUE_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("value"));
        WAITER_THREAD_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("thread"));
        WAITER_NEXT_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("next"));
        UNSAFE = unsafe;
      } catch (Exception e) {
        throwIfUnchecked(e);
        throw new RuntimeException(e);
      }
    }

    @Override
    void putThread(Waiter waiter, Thread newValue) {
      UNSAFE.putObject(waiter, WAITER_THREAD_OFFSET, newValue);
    }

    @Override
    void putNext(Waiter waiter, Waiter newValue) {
      UNSAFE.putObject(waiter, WAITER_NEXT_OFFSET, newValue);
    }

    
Performs a CAS operation on the AbstractFuture<V>.waiters field. 
/** Performs a CAS operation on the {@link #waiters} field. */
    @Override
    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
      return UNSAFE.compareAndSwapObject(future, WAITERS_OFFSET, expect, update);
    }

    
Performs a CAS operation on the AbstractFuture<V>.listeners field. 
/** Performs a CAS operation on the {@link #listeners} field. */
    @Override
    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
      return UNSAFE.compareAndSwapObject(future, LISTENERS_OFFSET, expect, update);
    }

    
Performs a CAS operation on the AbstractFuture<V>.value field. 
/** Performs a CAS operation on the {@link #value} field. */
    @Override
    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
      return UNSAFE.compareAndSwapObject(future, VALUE_OFFSET, expect, update);
    }
  }

  /** {@link AtomicHelper} based on {@link AtomicReferenceFieldUpdater}. */
  private static final class SafeAtomicHelper extends AtomicHelper {
    final AtomicReferenceFieldUpdater<Waiter, Thread> waiterThreadUpdater;
    final AtomicReferenceFieldUpdater<Waiter, Waiter> waiterNextUpdater;
    final AtomicReferenceFieldUpdater<AbstractFuture, Waiter> waitersUpdater;
    final AtomicReferenceFieldUpdater<AbstractFuture, Listener> listenersUpdater;
    final AtomicReferenceFieldUpdater<AbstractFuture, Object> valueUpdater;

    SafeAtomicHelper(
        AtomicReferenceFieldUpdater<Waiter, Thread> waiterThreadUpdater,
        AtomicReferenceFieldUpdater<Waiter, Waiter> waiterNextUpdater,
        AtomicReferenceFieldUpdater<AbstractFuture, Waiter> waitersUpdater,
        AtomicReferenceFieldUpdater<AbstractFuture, Listener> listenersUpdater,
        AtomicReferenceFieldUpdater<AbstractFuture, Object> valueUpdater) {
      this.waiterThreadUpdater = waiterThreadUpdater;
      this.waiterNextUpdater = waiterNextUpdater;
      this.waitersUpdater = waitersUpdater;
      this.listenersUpdater = listenersUpdater;
      this.valueUpdater = valueUpdater;
    }

    @Override
    void putThread(Waiter waiter, Thread newValue) {
      waiterThreadUpdater.lazySet(waiter, newValue);
    }

    @Override
    void putNext(Waiter waiter, Waiter newValue) {
      waiterNextUpdater.lazySet(waiter, newValue);
    }

    @Override
    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
      return waitersUpdater.compareAndSet(future, expect, update);
    }

    @Override
    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
      return listenersUpdater.compareAndSet(future, expect, update);
    }

    @Override
    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
      return valueUpdater.compareAndSet(future, expect, update);
    }
  }

  
AtomicHelper based on synchronized and volatile writes. 
This is an implementation of last resort for when certain basic VM features are broken (like
AtomicReferenceFieldUpdater).

/**
   * {@link AtomicHelper} based on {@code synchronized} and volatile writes.
   *
   * <p>This is an implementation of last resort for when certain basic VM features are broken (like
   * AtomicReferenceFieldUpdater).
   */
  private static final class SynchronizedHelper extends AtomicHelper {
    @Override
    void putThread(Waiter waiter, Thread newValue) {
      waiter.thread = newValue;
    }

    @Override
    void putNext(Waiter waiter, Waiter newValue) {
      waiter.next = newValue;
    }

    @Override
    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
      synchronized (future) {
        if (future.waiters == expect) {
          future.waiters = update;
          return true;
        }
        return false;
      }
    }

    @Override
    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
      synchronized (future) {
        if (future.listeners == expect) {
          future.listeners = update;
          return true;
        }
        return false;
      }
    }

    @Override
    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
      synchronized (future) {
        if (future.value == expect) {
          future.value = update;
          return true;
        }
        return false;
      }
    }
  }

  private static CancellationException cancellationExceptionWithCause(
      @Nullable String message, @Nullable Throwable cause) {
    CancellationException exception = new CancellationException(message);
    exception.initCause(cause);
    return exception;
  }
}