ReferenceQueue

ReferenceQueue

一、总结

1.jkd 1.8.0

2.作用
该队列作为引用中的一员，可以和上述三种引用类型组合使用，该队列的作用是：创建Reference时，将Queue注册到Reference中，当该Reference所引用的对象被垃圾收集器回收时，会将该Reference放到该队列中，相当于一种通知机制。

二、源码分析

/**
 * 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> {

}

• Reference queues,在适当的时候检测到对象的可达性发生改变后，垃圾回收器就将已注册的引用对象添加到此队列中。

    /**
     * Constructs a new reference-object queue.
     */
    public ReferenceQueue() { }

• 构造方法，需指定泛型

    // 私有内部静态类 Null ，继承 ReferenceQueue ，并覆盖实现父类中的方法 enqueue
    private static class Null<S> extends ReferenceQueue<S> {
        boolean enqueue(Reference<? extends S> r) {
            return false;
        }
    }
    // Reference 对象出队列标识，即 Reference 状态，由 Enqueued 变为 Inactive
    static ReferenceQueue<Object> NULL = new Null<>();
    // Reference 对象入队列标识，即 Reference 状态，由 Pending 变为 Enqueued 
    static ReferenceQueue<Object> ENQUEUED = new Null<>();

• 出队、入队标识，队列指的是  Reference 构造方法中 ReferenceQueue 参数

    static private class Lock { };
    private Lock lock = new Lock();

• 锁：类对象锁

    private volatile Reference<? extends T> head = null;

• 列表头部
• 作用：出队、入队操作时，作为临时变量保存数据

    private long queueLength = 0;

• 队列中元素的个数
• 类中的属性，实例对象私有，Reference 构造方法需要传入 ReferenceQueue 的实例对象，此属性指的是传入对象的队列中的对象的数量

    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;
            // 检查需要入队的 Reference 对象，对象的 queue 属性是否已经有标识
            // 即已经做过入队、出队的处理
            if ((queue == NULL) || (queue == ENQUEUED)) {
                return false;
            }
            // 断言
            assert queue == this;
            // 入队处理，入队标识
            r.queue = ENQUEUED;
            // 新入队对象放在头部
            r.next = (head == null) ? r : head;
            head = r;
            queueLength++;
            // 如果是强引用对象
            if (r instanceof FinalReference) {
                sun.misc.VM.addFinalRefCount(1);
            }
            lock.notifyAll(); // 释放锁对象
            return true;
        }
    }

• 添加入队标识
• 新加入的元素在队列头部

    @SuppressWarnings("unchecked")
    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
        Reference<? extends T> r = head;
        if (r != null) {
            // r.next == r 仅有一个元素
            head = (r.next == r) ?
                null :
                r.next; // Unchecked due to the next field having a raw type in Reference
            r.queue = NULL;
            // 出队元素的next 指向自己
            r.next = r;
            queueLength--;
            if (r instanceof FinalReference) {
                sun.misc.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 <tt>null</tt>.
     *
     * @return  A reference object, if one was immediately available,
     *          otherwise <code>null</code>
     */
    public Reference<? extends T> poll() {
        if (head == null)
            return null;
        synchronized (lock) {
            return reallyPoll();
        }
    }

• 添加出队标识
• 从头部开始出队，先入后出

    /**
     * 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</code>
     *                  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</code>
     *
     * @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);
    }

}

• 删除队列中的下一个引用对象，阻塞队列直到一个引用对象变为可达

博文参考：
Java Reference 源码分析