ReferenceQueue.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * published by the Free Software Foundation.  Oracle designates this
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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package java.lang.ref;

import jdk.internal.misc.VM;

import java.util.function.Consumer;

/**
 * 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
 */
// 存储GC之后报废的Reference（该引用指向的对象被回收），配合Reference类完成一些清理操作
public class ReferenceQueue<T> {
    
    static final ReferenceQueue<Object> NULL = new Null();      // 此处代表Reference没有关联的ReferenceQueue，无法入队
    static final ReferenceQueue<Object> ENQUEUED = new Null();  // 此处代表Reference已经在队列中，无法再入队
    
    private final Lock lock = new Lock();
    
    private volatile Reference<? extends T> head;
    private long queueLength = 0;
    
    /**
     * Constructs a new reference-object queue.
     */
    public ReferenceQueue() {
    }
    
    /**
     * 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}
     */
    // 从ReferenceQueue中删除一个Reference并将其返回
    public Reference<? extends T> poll() {
        if(head == null) {
            return null;
        }
        
        synchronized(lock) {
            return reallyPoll();
        }
    }
    
    
    // 将“报废引用”入队
    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;
        }
    }
    
    /**
     * 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
     */
    // 从ReferenceQueue中删除并返回一个Reference（如果没有就陷入阻塞，直到队列里有引用就取出）
    public Reference<? extends T> remove() throws InterruptedException {
        return remove(0);
    }
    
    /**
     * 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
     */
    // 轮询，在指定时间内，直到找到一个非空的Reference才返回
    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;
                }
            }
        }
    }
    
    // 从ReferenceQueue中删除一个Reference并将其返回
    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;
    }
    
    /**
     * 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;
            }
        }
    }
    
    
    private static class Null extends ReferenceQueue<Object> {
        boolean enqueue(Reference<?> r) {
            return false;
        }
    }
    
    private static class Lock {
    }
}