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| 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 sun.misc.Cleaner; |
| 29 | + |
| 30 | +/** |
| 31 | + * Abstract base class for reference objects. This class defines the |
| 32 | + * operations common to all reference objects. Because reference objects are |
| 33 | + * implemented in close cooperation with the garbage collector, this class may |
| 34 | + * not be subclassed directly. |
| 35 | + * |
| 36 | + * @author Mark Reinhold |
| 37 | + * @since 1.2 |
| 38 | + */ |
| 39 | + |
| 40 | +public abstract class Reference<T> { |
| 41 | + |
| 42 | + /* A Reference instance is in one of four possible internal states: |
| 43 | + * |
| 44 | + * Active: Subject to special treatment by the garbage collector. Some |
| 45 | + * time after the collector detects that the reachability of the |
| 46 | + * referent has changed to the appropriate state, it changes the |
| 47 | + * instance's state to either Pending or Inactive, depending upon |
| 48 | + * whether or not the instance was registered with a queue when it was |
| 49 | + * created. In the former case it also adds the instance to the |
| 50 | + * pending-Reference list. Newly-created instances are Active. |
| 51 | + * |
| 52 | + * Pending: An element of the pending-Reference list, waiting to be |
| 53 | + * enqueued by the Reference-handler thread. Unregistered instances |
| 54 | + * are never in this state. |
| 55 | + * |
| 56 | + * Enqueued: An element of the queue with which the instance was |
| 57 | + * registered when it was created. When an instance is removed from |
| 58 | + * its ReferenceQueue, it is made Inactive. Unregistered instances are |
| 59 | + * never in this state. |
| 60 | + * |
| 61 | + * Inactive: Nothing more to do. Once an instance becomes Inactive its |
| 62 | + * state will never change again. |
| 63 | + * |
| 64 | + * The state is encoded in the queue and next fields as follows: |
| 65 | + * |
| 66 | + * Active: queue = ReferenceQueue with which instance is registered, or |
| 67 | + * ReferenceQueue.NULL if it was not registered with a queue; next = |
| 68 | + * null. |
| 69 | + * |
| 70 | + * Pending: queue = ReferenceQueue with which instance is registered; |
| 71 | + * next = this |
| 72 | + * |
| 73 | + * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance |
| 74 | + * in queue, or this if at end of list. |
| 75 | + * |
| 76 | + * Inactive: queue = ReferenceQueue.NULL; next = this. |
| 77 | + * |
| 78 | + * With this scheme the collector need only examine the next field in order |
| 79 | + * to determine whether a Reference instance requires special treatment: If |
| 80 | + * the next field is null then the instance is active; if it is non-null, |
| 81 | + * then the collector should treat the instance normally. |
| 82 | + * |
| 83 | + * To ensure that a concurrent collector can discover active Reference |
| 84 | + * objects without interfering with application threads that may apply |
| 85 | + * the enqueue() method to those objects, collectors should link |
| 86 | + * discovered objects through the discovered field. The discovered |
| 87 | + * field is also used for linking Reference objects in the pending list. |
| 88 | + */ |
| 89 | + |
| 90 | + private T referent; /* Treated specially by GC */ |
| 91 | +// |
| 92 | +// volatile ReferenceQueue<? super T> queue; |
| 93 | +// |
| 94 | +// /* When active: NULL |
| 95 | +// * pending: this |
| 96 | +// * Enqueued: next reference in queue (or this if last) |
| 97 | +// * Inactive: this |
| 98 | +// */ |
| 99 | +// @SuppressWarnings("rawtypes") |
| 100 | +// Reference next; |
| 101 | +// |
| 102 | +// /* When active: next element in a discovered reference list maintained by GC (or this if last) |
| 103 | +// * pending: next element in the pending list (or null if last) |
| 104 | +// * otherwise: NULL |
| 105 | +// */ |
| 106 | +// transient private Reference<T> discovered; /* used by VM */ |
| 107 | +// |
| 108 | +// |
| 109 | +// /* Object used to synchronize with the garbage collector. The collector |
| 110 | +// * must acquire this lock at the beginning of each collection cycle. It is |
| 111 | +// * therefore critical that any code holding this lock complete as quickly |
| 112 | +// * as possible, allocate no new objects, and avoid calling user code. |
| 113 | +// */ |
| 114 | +// static private class Lock { }; |
| 115 | +// private static Lock lock = new Lock(); |
| 116 | +// |
| 117 | +// |
| 118 | +// /* List of References waiting to be enqueued. The collector adds |
| 119 | +// * References to this list, while the Reference-handler thread removes |
| 120 | +// * them. This list is protected by the above lock object. The |
| 121 | +// * list uses the discovered field to link its elements. |
| 122 | +// */ |
| 123 | +// private static Reference<Object> pending = null; |
| 124 | +// |
| 125 | +// /* High-priority thread to enqueue pending References |
| 126 | +// */ |
| 127 | +// private static class ReferenceHandler extends Thread { |
| 128 | +// |
| 129 | +// ReferenceHandler(ThreadGroup g, String name) { |
| 130 | +// super(g, name); |
| 131 | +// } |
| 132 | +// |
| 133 | +// public void run() { |
| 134 | +// for (;;) { |
| 135 | +// Reference<Object> r; |
| 136 | +// synchronized (lock) { |
| 137 | +// if (pending != null) { |
| 138 | +// r = pending; |
| 139 | +// pending = r.discovered; |
| 140 | +// r.discovered = null; |
| 141 | +// } else { |
| 142 | +// // The waiting on the lock may cause an OOME because it may try to allocate |
| 143 | +// // exception objects, so also catch OOME here to avoid silent exit of the |
| 144 | +// // reference handler thread. |
| 145 | +// // |
| 146 | +// // Explicitly define the order of the two exceptions we catch here |
| 147 | +// // when waiting for the lock. |
| 148 | +// // |
| 149 | +// // We do not want to try to potentially load the InterruptedException class |
| 150 | +// // (which would be done if this was its first use, and InterruptedException |
| 151 | +// // were checked first) in this situation. |
| 152 | +// // |
| 153 | +// // This may lead to the VM not ever trying to load the InterruptedException |
| 154 | +// // class again. |
| 155 | +// try { |
| 156 | +// try { |
| 157 | +// lock.wait(); |
| 158 | +// } catch (OutOfMemoryError x) { } |
| 159 | +// } catch (InterruptedException x) { } |
| 160 | +// continue; |
| 161 | +// } |
| 162 | +// } |
| 163 | +// |
| 164 | +// // Fast path for cleaners |
| 165 | +// if (r instanceof Cleaner) { |
| 166 | +// ((Cleaner)r).clean(); |
| 167 | +// continue; |
| 168 | +// } |
| 169 | +// |
| 170 | +// ReferenceQueue<Object> q = r.queue; |
| 171 | +// if (q != ReferenceQueue.NULL) q.enqueue(r); |
| 172 | +// } |
| 173 | +// } |
| 174 | +// } |
| 175 | +// |
| 176 | +// static { |
| 177 | +// ThreadGroup tg = Thread.currentThread().getThreadGroup(); |
| 178 | +// for (ThreadGroup tgn = tg; |
| 179 | +// tgn != null; |
| 180 | +// tg = tgn, tgn = tg.getParent()); |
| 181 | +// Thread handler = new ReferenceHandler(tg, "Reference Handler"); |
| 182 | +// /* If there were a special system-only priority greater than |
| 183 | +// * MAX_PRIORITY, it would be used here |
| 184 | +// */ |
| 185 | +// handler.setPriority(Thread.MAX_PRIORITY); |
| 186 | +// handler.setDaemon(true); |
| 187 | +// handler.start(); |
| 188 | +// } |
| 189 | +// |
| 190 | + |
| 191 | + /* -- Referent accessor and setters -- */ |
| 192 | + |
| 193 | + /** |
| 194 | + * Returns this reference object's referent. If this reference object has |
| 195 | + * been cleared, either by the program or by the garbage collector, then |
| 196 | + * this method returns <code>null</code>. |
| 197 | + * |
| 198 | + * @return The object to which this reference refers, or |
| 199 | + * <code>null</code> if this reference object has been cleared |
| 200 | + */ |
| 201 | + public T get() { |
| 202 | + return this.referent; |
| 203 | + } |
| 204 | + |
| 205 | + /** |
| 206 | + * Clears this reference object. Invoking this method will not cause this |
| 207 | + * object to be enqueued. |
| 208 | + * |
| 209 | + * <p> This method is invoked only by Java code; when the garbage collector |
| 210 | + * clears references it does so directly, without invoking this method. |
| 211 | + */ |
| 212 | + public void clear() { |
| 213 | + this.referent = null; |
| 214 | + } |
| 215 | + |
| 216 | + |
| 217 | + /* -- Queue operations -- */ |
| 218 | + |
| 219 | + /** |
| 220 | + * Tells whether or not this reference object has been enqueued, either by |
| 221 | + * the program or by the garbage collector. If this reference object was |
| 222 | + * not registered with a queue when it was created, then this method will |
| 223 | + * always return <code>false</code>. |
| 224 | + * |
| 225 | + * @return <code>true</code> if and only if this reference object has |
| 226 | + * been enqueued |
| 227 | + */ |
| 228 | + public boolean isEnqueued() { |
| 229 | + return false; |
| 230 | +// return (this.queue == ReferenceQueue.ENQUEUED); |
| 231 | + } |
| 232 | + |
| 233 | + /** |
| 234 | + * Adds this reference object to the queue with which it is registered, |
| 235 | + * if any. |
| 236 | + * |
| 237 | + * <p> This method is invoked only by Java code; when the garbage collector |
| 238 | + * enqueues references it does so directly, without invoking this method. |
| 239 | + * |
| 240 | + * @return <code>true</code> if this reference object was successfully |
| 241 | + * enqueued; <code>false</code> if it was already enqueued or if |
| 242 | + * it was not registered with a queue when it was created |
| 243 | + */ |
| 244 | + public boolean enqueue() { |
| 245 | + return false; |
| 246 | +// return this.queue.enqueue(this); |
| 247 | + } |
| 248 | + |
| 249 | + |
| 250 | + /* -- Constructors -- */ |
| 251 | + |
| 252 | + Reference(T referent) { |
| 253 | + this(referent, null); |
| 254 | + } |
| 255 | + |
| 256 | + Reference(T referent, ReferenceQueue<? super T> queue) { |
| 257 | + this.referent = referent; |
| 258 | +// this.queue = (queue == null) ? ReferenceQueue.NULL : queue; |
| 259 | + } |
| 260 | + |
| 261 | +} |
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