--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/share/classes/java/lang/ref/Reference.java Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,238 @@
+/*
+ * Copyright 1997-2006 Sun Microsystems, Inc. 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. Sun designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.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;
+ }
+
+}