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1: /* java.lang.ref.ReferenceQueue 2: Copyright (C) 1999 Free Software Foundation, Inc. 3: 4: This file is part of GNU Classpath. 5: 6: GNU Classpath is free software; you can redistribute it and/or modify 7: it under the terms of the GNU General Public License as published by 8: the Free Software Foundation; either version 2, or (at your option) 9: any later version. 10: 11: GNU Classpath is distributed in the hope that it will be useful, but 12: WITHOUT ANY WARRANTY; without even the implied warranty of 13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14: General Public License for more details. 15: 16: You should have received a copy of the GNU General Public License 17: along with GNU Classpath; see the file COPYING. If not, write to the 18: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 19: 02110-1301 USA. 20: 21: Linking this library statically or dynamically with other modules is 22: making a combined work based on this library. Thus, the terms and 23: conditions of the GNU General Public License cover the whole 24: combination. 25: 26: As a special exception, the copyright holders of this library give you 27: permission to link this library with independent modules to produce an 28: executable, regardless of the license terms of these independent 29: modules, and to copy and distribute the resulting executable under 30: terms of your choice, provided that you also meet, for each linked 31: independent module, the terms and conditions of the license of that 32: module. An independent module is a module which is not derived from 33: or based on this library. If you modify this library, you may extend 34: this exception to your version of the library, but you are not 35: obligated to do so. If you do not wish to do so, delete this 36: exception statement from your version. */ 37: 38: 39: package java.lang.ref; 40: 41: /** 42: * This is the queue, where references can enqueue themselve on. Each 43: * reference may be registered to a queue at initialization time and 44: * will be appended to the queue, when the enqueue method is called. 45: * 46: * The enqueue method may be automatically called by the garbage 47: * collector if it detects, that the object is only reachable through 48: * the Reference objects. 49: * 50: * @author Jochen Hoenicke 51: * @see Reference#enqueue() 52: */ 53: public class ReferenceQueue 54: { 55: /** 56: * This is a linked list of references. If this is null, the list is 57: * empty. Otherwise this points to the first reference on the queue. 58: * The first reference will point to the next reference via the 59: * <code>nextOnQueue</code> field. The last reference will point to 60: * itself (not to null, since <code>nextOnQueue</code> is used to 61: * determine if a reference is enqueued). 62: */ 63: private Reference first; 64: 65: /** 66: * Creates a new empty reference queue. 67: */ 68: public ReferenceQueue() 69: { 70: } 71: 72: /** 73: * Checks if there is a reference on the queue, returning it 74: * immediately. The reference will be dequeued. 75: * 76: * @return a reference on the queue, if there is one, 77: * <code>null</code> otherwise. 78: */ 79: public synchronized Reference poll() 80: { 81: return dequeue(); 82: } 83: 84: /** 85: * This is called by reference to enqueue itself on this queue. 86: * @param ref the reference that should be enqueued. 87: */ 88: synchronized void enqueue(Reference ref) 89: { 90: /* last reference will point to itself */ 91: ref.nextOnQueue = first == null ? ref : first; 92: first = ref; 93: /* this wakes only one remove thread. */ 94: notify(); 95: } 96: 97: /** 98: * Remove a reference from the queue, if there is one. 99: * @return the first element of the queue, or null if there isn't any. 100: */ 101: private Reference dequeue() 102: { 103: if (first == null) 104: return null; 105: 106: Reference result = first; 107: first = (first == first.nextOnQueue) ? null : first.nextOnQueue; 108: result.nextOnQueue = null; 109: return result; 110: } 111: 112: /** 113: * Removes a reference from the queue, blocking for <code>timeout</code> 114: * until a reference is enqueued. 115: * @param timeout the timeout period in milliseconds, <code>0</code> means 116: * wait forever. 117: * @return the reference removed from the queue, or 118: * <code>null</code> if timeout period expired. 119: * @exception InterruptedException if the wait was interrupted. 120: */ 121: public synchronized Reference remove(long timeout) 122: throws InterruptedException 123: { 124: if (first == null) 125: { 126: wait(timeout); 127: } 128: 129: return dequeue(); 130: } 131: 132: 133: /** 134: * Removes a reference from the queue, blocking until a reference is 135: * enqueued. 136: * 137: * @return the reference removed from the queue. 138: * @exception InterruptedException if the wait was interrupted. 139: */ 140: public Reference remove() 141: throws InterruptedException 142: { 143: return remove(0L); 144: } 145: }