16_02_ReferenceQueue_引用队列

ReferenceQueue引用队列——被

类的原注释，翻译过来是：

引用队列，在检测到适当的可达性更改后，垃圾收集器会将已注册的参考对象，附加到这些队列中。

public class ReferenceQueue<T> {
    // ...
    // 1. 静态内容，
    static final ReferenceQueue<Object> NULL = new Null();
    static final ReferenceQueue<Object> ENQUEUED = new Null();
    private static class Null extends ReferenceQueue<Object> {
        boolean enqueue(Reference<?> r) {
            return false;
        }
    }
    // 当前对象的锁操作
    private static class Lock { };
    private final Lock lock = new Lock();
    // 链表标识
    private volatile Reference<? extends T> head;
    private long queueLength = 0;
    // 入队，仅被Reference操作
    boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
        // 同步方法锁
        synchronized (lock) {
            // Check that since getting the lock this reference hasn't already been
            // enqueued (and even then removed)
            // 1. 拿到引用队列，如果支持
            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 {@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.
     *
     * <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.
     *
     * @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.
     */
    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;
            }
        }
    }
}