diff -r 3fcc279c921b -r d382dacfd73f rt/emul/compact/src/main/java/java/lang/ref/Reference.java --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/rt/emul/compact/src/main/java/java/lang/ref/Reference.java Tue Feb 26 16:54:16 2013 +0100 @@ -0,0 +1,185 @@ +/* + * 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; + + +/** + * 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 { + + /* 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 queue; + + Reference next; + transient private Reference 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; + + + + /* -- 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 null. + * + * @return The object to which this reference refers, or + * null 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. + * + *

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 false. + * + * @return true 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. + * + *

This method is invoked only by Java code; when the garbage collector + * enqueues references it does so directly, without invoking this method. + * + * @return true if this reference object was successfully + * enqueued; false 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 queue) { + this.referent = referent; + this.queue = (queue == null) ? ReferenceQueue.NULL : queue; + } + +}