1 /*
2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.lang.ref;
27
28 import java.util.HashMap;
29 import java.util.Map;
30
31 /**
32 * Reference queues, to which registered reference objects are appended by the
33 * garbage collector after the appropriate reachability changes are detected.
34 *
35 * @author Mark Reinhold
36 * @since 1.2
37 */
38
39 public class ReferenceQueue<T> {
40
41 /**
42 * Constructs a new reference-object queue.
43 */
44 public ReferenceQueue() { }
45
46 private static class Null<S> extends ReferenceQueue<S> {
47 boolean enqueue(Reference<? extends S> r) {
48 return false;
49 }
50 }
51
52 static class mutableInt {
53 private int value = 1;
54
55 public void increment(){ value += 1; }
56 public int getValue(){ return value; }
57 public int compareTo(mutableInt e){ return getValue() - e.getValue(); }
58
59 public String toString(){ return Integer.toString(value); }
60 }
61
62 static ReferenceQueue<Object> NULL = new Null<>();
63 static ReferenceQueue<Object> ENQUEUED = new Null<>();
64
65 static private class Lock { };
66 private Lock lock = new Lock();
67 private volatile Reference<? extends T> head = null;
68 private long queueLength = 0;
69
70 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
71 synchronized (lock) {
72 // Check that since getting the lock this reference hasn't already been
73 // enqueued (and even then removed)
74 ReferenceQueue<?> queue = r.queue;
75 if ((queue == NULL) || (queue == ENQUEUED)) {
76 return false;
77 }
78 assert queue == this;
79 r.queue = ENQUEUED;
80 r.next = (head == null) ? r : head;
81 head = r;
82 queueLength++;
83 if (r instanceof FinalReference) {
84 sun.misc.VM.addFinalRefCount(1);
85 }
86 lock.notifyAll();
87 return true;
88 }
89 }
90
91 @SuppressWarnings("unchecked")
92 private Reference<? extends T> reallyPoll() { /* Must hold lock */
93 Reference<? extends T> r = head;
94 if (r != null) {
95 head = (r.next == r) ?
96 null :
97 r.next; // Unchecked due to the next field having a raw type in Reference
98 r.queue = NULL;
99 r.next = r;
100 queueLength--;
101 if (r instanceof FinalReference) {
102 sun.misc.VM.addFinalRefCount(-1);
103 }
104 return r;
105 }
106 return null;
107 }
108
109 /**
110 * Polls this queue to see if a reference object is available. If one is
111 * available without further delay then it is removed from the queue and
112 * returned. Otherwise this method immediately returns <tt>null</tt>.
113 *
114 * @return A reference object, if one was immediately available,
115 * otherwise <code>null</code>
116 */
117 public Reference<? extends T> poll() {
118 if (head == null)
119 return null;
120 synchronized (lock) {
121 return reallyPoll();
122 }
123 }
124
125 /**
126 * Iterate queue and cout the number of instances of each object type pending finalization
127 *
128 * @return Map of object types with associated counts
129 */
130 @SuppressWarnings("unchecked")
131 public Map<String, mutableInt> countInstances() {
132 if (head == null) {
133 return null;
134 }
135
136 HashMap<String, mutableInt> countMap = new HashMap<>();
137
138 synchronized (lock) {
139 Reference<? extends T> r = head;
140 if (r != null) {
141 Object referent = r.get();
142 if (referent != null) {
143 String typeName = referent.getClass().getName();
144 mutableInt v = countMap.get(typeName);
145 if (v == null) {
146 countMap.put(typeName, new mutableInt());
147 }
148 else {
149 v.increment();
150 }
151 }
152 r = r.next;
153 }
154 }
155
156 return countMap;
157 }
158
159
160 /**
161 * Removes the next reference object in this queue, blocking until either
162 * one becomes available or the given timeout period expires.
163 *
164 * <p> This method does not offer real-time guarantees: It schedules the
165 * timeout as if by invoking the {@link Object#wait(long)} method.
166 *
167 * @param timeout If positive, block for up to <code>timeout</code>
168 * milliseconds while waiting for a reference to be
169 * added to this queue. If zero, block indefinitely.
170 *
171 * @return A reference object, if one was available within the specified
172 * timeout period, otherwise <code>null</code>
173 *
174 * @throws IllegalArgumentException
175 * If the value of the timeout argument is negative
176 *
177 * @throws InterruptedException
178 * If the timeout wait is interrupted
179 */
180 public Reference<? extends T> remove(long timeout)
181 throws IllegalArgumentException, InterruptedException
182 {
183 if (timeout < 0) {
184 throw new IllegalArgumentException("Negative timeout value");
185 }
186 synchronized (lock) {
187 Reference<? extends T> r = reallyPoll();
188 if (r != null) return r;
189 long start = (timeout == 0) ? 0 : System.nanoTime();
190 for (;;) {
191 lock.wait(timeout);
192 r = reallyPoll();
193 if (r != null) return r;
194 if (timeout != 0) {
195 long end = System.nanoTime();
196 timeout -= (end - start) / 1000_000;
197 if (timeout <= 0) return null;
198 start = end;
199 }
200 }
201 }
202 }
203
204 /**
205 * Removes the next reference object in this queue, blocking until one
206 * becomes available.
207 *
208 * @return A reference object, blocking until one becomes available
209 * @throws InterruptedException If the wait is interrupted
210 */
211 public Reference<? extends T> remove() throws InterruptedException {
212 return remove(0);
213 }
214
215 }