| /* java.lang.ref.ReferenceQueue |
| Copyright (C) 1999 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 |
| the Free Software Foundation; either version 2, or (at your option) |
| any later version. |
| |
| GNU Classpath is distributed in the hope that it will be useful, but |
| 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. |
| |
| You should have received a copy of the GNU General Public License |
| along with GNU Classpath; see the file COPYING. If not, write to the |
| Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA |
| 02111-1307 USA. |
| |
| As a special exception, if you link this library with other files to |
| produce an executable, this library does not by itself cause the |
| resulting executable to be covered by the GNU General Public License. |
| This exception does not however invalidate any other reasons why the |
| executable file might be covered by the GNU General Public License. */ |
| |
| |
| 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 |
| { |
| /** |
| * 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 first; |
| |
| /** |
| * 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 synchronized Reference poll() |
| { |
| return dequeue(); |
| } |
| |
| /** |
| * This is called by reference to enqueue itself on this queue. |
| * @param ref the reference that should be enqueued. |
| */ |
| synchronized void enqueue(Reference ref) |
| { |
| /* last reference will point to itself */ |
| ref.nextOnQueue = first == null ? ref : first; |
| first = ref; |
| /* this wakes only one remove thread. */ |
| notify(); |
| } |
| |
| /** |
| * 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 dequeue() |
| { |
| if (first == null) |
| return null; |
| |
| Reference 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 synchronized Reference remove(long timeout) |
| throws InterruptedException |
| { |
| if (first == null) |
| { |
| 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 remove() |
| throws InterruptedException |
| { |
| return remove(0L); |
| } |
| } |