| /* |
| * Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code 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 |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| package java.lang.ref; |
| |
| import sun.misc.Cleaner; |
| |
| |
| /** |
| * Abstract base class for reference objects. This class defines the |
| * operations common to all reference objects. Because reference objects are |
| * implemented in close cooperation with the garbage collector, this class may |
| * not be subclassed directly. |
| * |
| * @author Mark Reinhold |
| * @since 1.2 |
| */ |
| |
| public abstract class Reference<T> { |
| |
| /* A Reference instance is in one of four possible internal states: |
| * |
| * Active: Subject to special treatment by the garbage collector. Some |
| * time after the collector detects that the reachability of the |
| * referent has changed to the appropriate state, it changes the |
| * instance's state to either Pending or Inactive, depending upon |
| * whether or not the instance was registered with a queue when it was |
| * created. In the former case it also adds the instance to the |
| * pending-Reference list. Newly-created instances are Active. |
| * |
| * Pending: An element of the pending-Reference list, waiting to be |
| * enqueued by the Reference-handler thread. Unregistered instances |
| * are never in this state. |
| * |
| * Enqueued: An element of the queue with which the instance was |
| * registered when it was created. When an instance is removed from |
| * its ReferenceQueue, it is made Inactive. Unregistered instances are |
| * never in this state. |
| * |
| * Inactive: Nothing more to do. Once an instance becomes Inactive its |
| * state will never change again. |
| * |
| * The state is encoded in the queue and next fields as follows: |
| * |
| * Active: queue = ReferenceQueue with which instance is registered, or |
| * ReferenceQueue.NULL if it was not registered with a queue; next = |
| * null. |
| * |
| * Pending: queue = ReferenceQueue with which instance is registered; |
| * next = Following instance in queue, or this if at end of list. |
| * |
| * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance |
| * in queue, or this if at end of list. |
| * |
| * Inactive: queue = ReferenceQueue.NULL; next = this. |
| * |
| * With this scheme the collector need only examine the next field in order |
| * to determine whether a Reference instance requires special treatment: If |
| * the next field is null then the instance is active; if it is non-null, |
| * then the collector should treat the instance normally. |
| * |
| * To ensure that concurrent collector can discover active Reference |
| * objects without interfering with application threads that may apply |
| * the enqueue() method to those objects, collectors should link |
| * discovered objects through the discovered field. |
| */ |
| |
| private T referent; /* Treated specially by GC */ |
| |
| ReferenceQueue<? super T> queue; |
| |
| Reference next; |
| transient private Reference<T> discovered; /* used by VM */ |
| |
| |
| /* Object used to synchronize with the garbage collector. The collector |
| * must acquire this lock at the beginning of each collection cycle. It is |
| * therefore critical that any code holding this lock complete as quickly |
| * as possible, allocate no new objects, and avoid calling user code. |
| */ |
| static private class Lock { }; |
| private static Lock lock = new Lock(); |
| |
| |
| /* List of References waiting to be enqueued. The collector adds |
| * References to this list, while the Reference-handler thread removes |
| * them. This list is protected by the above lock object. |
| */ |
| private static Reference pending = null; |
| |
| /* High-priority thread to enqueue pending References |
| */ |
| private static class ReferenceHandler extends Thread { |
| |
| ReferenceHandler(ThreadGroup g, String name) { |
| super(g, name); |
| } |
| |
| public void run() { |
| for (;;) { |
| |
| Reference r; |
| synchronized (lock) { |
| if (pending != null) { |
| r = pending; |
| Reference rn = r.next; |
| pending = (rn == r) ? null : rn; |
| r.next = r; |
| } else { |
| try { |
| lock.wait(); |
| } catch (InterruptedException x) { } |
| continue; |
| } |
| } |
| |
| // Fast path for cleaners |
| if (r instanceof Cleaner) { |
| ((Cleaner)r).clean(); |
| continue; |
| } |
| |
| ReferenceQueue q = r.queue; |
| if (q != ReferenceQueue.NULL) q.enqueue(r); |
| } |
| } |
| } |
| |
| static { |
| ThreadGroup tg = Thread.currentThread().getThreadGroup(); |
| for (ThreadGroup tgn = tg; |
| tgn != null; |
| tg = tgn, tgn = tg.getParent()); |
| Thread handler = new ReferenceHandler(tg, "Reference Handler"); |
| /* If there were a special system-only priority greater than |
| * MAX_PRIORITY, it would be used here |
| */ |
| handler.setPriority(Thread.MAX_PRIORITY); |
| handler.setDaemon(true); |
| handler.start(); |
| } |
| |
| |
| /* -- Referent accessor and setters -- */ |
| |
| /** |
| * Returns this reference object's referent. If this reference object has |
| * been cleared, either by the program or by the garbage collector, then |
| * this method returns <code>null</code>. |
| * |
| * @return The object to which this reference refers, or |
| * <code>null</code> if this reference object has been cleared |
| */ |
| public T get() { |
| return this.referent; |
| } |
| |
| /** |
| * Clears this reference object. Invoking this method will not cause this |
| * object to be enqueued. |
| * |
| * <p> This method is invoked only by Java code; when the garbage collector |
| * clears references it does so directly, without invoking this method. |
| */ |
| public void clear() { |
| this.referent = null; |
| } |
| |
| |
| /* -- Queue operations -- */ |
| |
| /** |
| * Tells whether or not this reference object has been enqueued, either by |
| * the program or by the garbage collector. If this reference object was |
| * not registered with a queue when it was created, then this method will |
| * always return <code>false</code>. |
| * |
| * @return <code>true</code> if and only if this reference object has |
| * been enqueued |
| */ |
| public boolean isEnqueued() { |
| /* In terms of the internal states, this predicate actually tests |
| whether the instance is either Pending or Enqueued */ |
| synchronized (this) { |
| return (this.queue != ReferenceQueue.NULL) && (this.next != null); |
| } |
| } |
| |
| /** |
| * Adds this reference object to the queue with which it is registered, |
| * if any. |
| * |
| * <p> This method is invoked only by Java code; when the garbage collector |
| * enqueues references it does so directly, without invoking this method. |
| * |
| * @return <code>true</code> if this reference object was successfully |
| * enqueued; <code>false</code> if it was already enqueued or if |
| * it was not registered with a queue when it was created |
| */ |
| public boolean enqueue() { |
| return this.queue.enqueue(this); |
| } |
| |
| |
| /* -- Constructors -- */ |
| |
| Reference(T referent) { |
| this(referent, null); |
| } |
| |
| Reference(T referent, ReferenceQueue<? super T> queue) { |
| this.referent = referent; |
| this.queue = (queue == null) ? ReferenceQueue.NULL : queue; |
| } |
| |
| } |