ReferenceQueue.java

/* java.lang.ref.ReferenceQueue
   Copyright (C) 1999, 2004, 2006 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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any later version.

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WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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02110-1301 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
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or based on this library.  If you modify this library, you may extend
this exception to your version of the library, but you are not
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exception statement from your version. */


package java.lang.ref;

/**
 * This is the queue, where references can enqueue themselve on.  Each
 * reference may be registered to a queue at initialization time and
 * will be appended to the queue, when the enqueue method is called.
 *
 * The enqueue method may be automatically called by the garbage
 * collector if it detects, that the object is only reachable through
 * the Reference objects.
 *
 * @author Jochen Hoenicke
 * @see Reference#enqueue()
 */
public class ReferenceQueue<T>
{
  /**
   * This is a linked list of references.  If this is null, the list is
   * empty.  Otherwise this points to the first reference on the queue.
   * The first reference will point to the next reference via the
   * <code>nextOnQueue</code> field.  The last reference will point to
   * itself (not to null, since <code>nextOnQueue</code> is used to
   * determine if a reference is enqueued).
   */
  private Reference<? extends T> first;

  /**
   * This is the lock that protects our linked list and is used to signal
   * a thread waiting in remove().
   */
  private final Object lock = new Object();

  /**
   * Creates a new empty reference queue.
   */
  public ReferenceQueue()
  {
  }

  /**
   * Checks if there is a reference on the queue, returning it
   * immediately.  The reference will be dequeued.
   *
   * @return a reference on the queue, if there is one,
   * <code>null</code> otherwise.
   */
  public Reference<? extends T> poll()
  {
    return dequeue();
  }

  /**
   * This is called by reference to enqueue itself on this queue.
   * @param ref the reference that should be enqueued.
   * @return true if successful, false if not.
   */
  final boolean enqueue(Reference<? extends T> ref)
    {
    synchronized (lock)
      {
        if (ref.queue != this)
          return false;

        /* last reference will point to itself */
        ref.nextOnQueue = first == null ? ref : first;
        ref.queue = null;
        first = ref;
        /* this wakes only one remove thread. */
        lock.notify();
        return true;
      }
  }

  /**
   * Remove a reference from the queue, if there is one.
   * @return the first element of the queue, or null if there isn't any.
   */
  private Reference<? extends T> dequeue()
  {
    synchronized (lock)
      {
        if (first == null)
          return null;

        Reference<? extends T> result = first;
        first = (first == first.nextOnQueue) ? null : first.nextOnQueue;
        result.nextOnQueue = null;
        return result;
      }
  }

  /**
   * Removes a reference from the queue, blocking for <code>timeout</code>
   * until a reference is enqueued.
   * @param timeout the timeout period in milliseconds, <code>0</code> means
   * wait forever.
   * @return the reference removed from the queue, or
   * <code>null</code> if timeout period expired.
   * @exception InterruptedException if the wait was interrupted.
   */
  public Reference<? extends T> remove(long timeout)
    throws InterruptedException
  {
    synchronized (lock)
      {
        if (first == null)
          lock.wait(timeout);
      }

    return dequeue();
  }


  /**
   * Removes a reference from the queue, blocking until a reference is
   * enqueued.
   *
   * @return the reference removed from the queue.
   * @exception InterruptedException if the wait was interrupted.
   */
  public Reference<? extends T> remove()
    throws InterruptedException
  {
    return remove(0L);
  }
}