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package java.lang.ref;

import java.util.function.Consumer;
import jdk.internal.misc.VM;

Reference queues, to which registered reference objects are appended by the garbage collector after the appropriate reachability changes are detected.
Author: Mark Reinhold
Since: 1.2
/** * Reference queues, to which registered reference objects are appended by the * garbage collector after the appropriate reachability changes are detected. * * @author Mark Reinhold * @since 1.2 */
public class ReferenceQueue<T> {
Constructs a new reference-object queue.
/** * Constructs a new reference-object queue. */
public ReferenceQueue() { } private static class Null extends ReferenceQueue<Object> { boolean enqueue(Reference<?> r) { return false; } } static final ReferenceQueue<Object> NULL = new Null(); static final ReferenceQueue<Object> ENQUEUED = new Null(); private static class Lock { }; private final Lock lock = new Lock(); private volatile Reference<? extends T> head; private long queueLength = 0; boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */ synchronized (lock) { // Check that since getting the lock this reference hasn't already been // enqueued (and even then removed) ReferenceQueue<?> queue = r.queue; if ((queue == NULL) || (queue == ENQUEUED)) { return false; } assert queue == this; // Self-loop end, so if a FinalReference it remains inactive. r.next = (head == null) ? r : head; head = r; queueLength++; // Update r.queue *after* adding to list, to avoid race // with concurrent enqueued checks and fast-path poll(). // Volatiles ensure ordering. r.queue = ENQUEUED; if (r instanceof FinalReference) { VM.addFinalRefCount(1); } lock.notifyAll(); return true; } } private Reference<? extends T> reallyPoll() { /* Must hold lock */ Reference<? extends T> r = head; if (r != null) { r.queue = NULL; // Update r.queue *before* removing from list, to avoid // race with concurrent enqueued checks and fast-path // poll(). Volatiles ensure ordering. @SuppressWarnings("unchecked") Reference<? extends T> rn = r.next; // Handle self-looped next as end of list designator. head = (rn == r) ? null : rn; // Self-loop next rather than setting to null, so if a // FinalReference it remains inactive. r.next = r; queueLength--; if (r instanceof FinalReference) { VM.addFinalRefCount(-1); } return r; } return null; }
Polls this queue to see if a reference object is available. If one is available without further delay then it is removed from the queue and returned. Otherwise this method immediately returns null.
Returns: A reference object, if one was immediately available, otherwise null
/** * Polls this queue to see if a reference object is available. If one is * available without further delay then it is removed from the queue and * returned. Otherwise this method immediately returns {@code null}. * * @return A reference object, if one was immediately available, * otherwise {@code null} */
public Reference<? extends T> poll() { if (head == null) return null; synchronized (lock) { return reallyPoll(); } }
Removes the next reference object in this queue, blocking until either one becomes available or the given timeout period expires.

This method does not offer real-time guarantees: It schedules the timeout as if by invoking the Object.wait(long) method.

Params:
  • timeout – If positive, block for up to timeout milliseconds while waiting for a reference to be added to this queue. If zero, block indefinitely.
Throws:
Returns: A reference object, if one was available within the specified timeout period, otherwise null
/** * Removes the next reference object in this queue, blocking until either * one becomes available or the given timeout period expires. * * <p> This method does not offer real-time guarantees: It schedules the * timeout as if by invoking the {@link Object#wait(long)} method. * * @param timeout If positive, block for up to {@code timeout} * milliseconds while waiting for a reference to be * added to this queue. If zero, block indefinitely. * * @return A reference object, if one was available within the specified * timeout period, otherwise {@code null} * * @throws IllegalArgumentException * If the value of the timeout argument is negative * * @throws InterruptedException * If the timeout wait is interrupted */
public Reference<? extends T> remove(long timeout) throws IllegalArgumentException, InterruptedException { if (timeout < 0) { throw new IllegalArgumentException("Negative timeout value"); } synchronized (lock) { Reference<? extends T> r = reallyPoll(); if (r != null) return r; long start = (timeout == 0) ? 0 : System.nanoTime(); for (;;) { lock.wait(timeout); r = reallyPoll(); if (r != null) return r; if (timeout != 0) { long end = System.nanoTime(); timeout -= (end - start) / 1000_000; if (timeout <= 0) return null; start = end; } } } }
Removes the next reference object in this queue, blocking until one becomes available.
Throws:
Returns:A reference object, blocking until one becomes available
/** * Removes the next reference object in this queue, blocking until one * becomes available. * * @return A reference object, blocking until one becomes available * @throws InterruptedException If the wait is interrupted */
public Reference<? extends T> remove() throws InterruptedException { return remove(0); }
Iterate queue and invoke given action with each Reference. Suitable for diagnostic purposes. WARNING: any use of this method should make sure to not retain the referents of iterated references (in case of FinalReference(s)) so that their life is not prolonged more than necessary.
/** * Iterate queue and invoke given action with each Reference. * Suitable for diagnostic purposes. * WARNING: any use of this method should make sure to not * retain the referents of iterated references (in case of * FinalReference(s)) so that their life is not prolonged more * than necessary. */
void forEach(Consumer<? super Reference<? extends T>> action) { for (Reference<? extends T> r = head; r != null;) { action.accept(r); @SuppressWarnings("unchecked") Reference<? extends T> rn = r.next; if (rn == r) { if (r.queue == ENQUEUED) { // still enqueued -> we reached end of chain r = null; } else { // already dequeued: r.queue == NULL; -> // restart from head when overtaken by queue poller(s) r = head; } } else { // next in chain r = rn; } } } }