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package java.lang.ref;

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 {
        boolean enqueue(Reference r) {
            return false;
        }
    }

    static ReferenceQueue NULL = new Null();
    static ReferenceQueue ENQUEUED = new Null();

    static private class Lock { };
    private Lock lock = new Lock();
    private Reference<? extends T> head = null;
    private long queueLength = 0;

    boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
        synchronized (r) {
            if (r.queue == ENQUEUED) return false;
            synchronized (lock) {
                r.queue = ENQUEUED;
                r.next = (head == null) ? r : head;
                head = r;
                queueLength++;
                if (r instanceof FinalReference) {
                    sun.misc.VM.addFinalRefCount(1);
                }
                lock.notifyAll();
                return true;
            }
        }
    }

    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
        if (head != null) {
            Reference<? extends T> r = head;
            head = (r.next == r) ? null : r.next;
            r.queue = NULL;
            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 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 <tt>null</tt>.
     *
     * @return  A reference object, if one was immediately available,
     *          otherwise <code>null</code>
     */
    public Reference<? extends T> poll() {
        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:
IllegalArgumentException – 
         If the value of the timeout argument is negative
InterruptedException – 
         If the timeout wait is interrupted
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</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;
            for (;;) {
                lock.wait(timeout);
                r = reallyPoll();
                if (r != null) return r;
                if (timeout != 0) return null;
            }
        }
    }

    
Removes the next reference object in this queue, blocking until one
becomes available.
Throws:
InterruptedException –  If the wait is interrupted
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);
    }

}