e9207384c5
4268317: (ref) Reference.isEnqueued() can return true when instance not enqueued The reference handler now declares, and assumes, that the discovered field, rather than the next field, is (to be) used to link the entries in the pending list, thus allowing a reference object to be safely enqueued even while it is in the pending state. Also added slightly modified regression tests from the two bug reports. Reviewed-by: mchung, alanb, jcoomes
246 lines
9.0 KiB
Java
246 lines
9.0 KiB
Java
/*
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* Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.lang.ref;
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import sun.misc.Cleaner;
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/**
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* Abstract base class for reference objects. This class defines the
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* operations common to all reference objects. Because reference objects are
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* implemented in close cooperation with the garbage collector, this class may
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* not be subclassed directly.
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*
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* @author Mark Reinhold
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* @since 1.2
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*/
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public abstract class Reference<T> {
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/* A Reference instance is in one of four possible internal states:
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*
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* Active: Subject to special treatment by the garbage collector. Some
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* time after the collector detects that the reachability of the
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* referent has changed to the appropriate state, it changes the
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* instance's state to either Pending or Inactive, depending upon
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* whether or not the instance was registered with a queue when it was
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* created. In the former case it also adds the instance to the
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* pending-Reference list. Newly-created instances are Active.
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*
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* Pending: An element of the pending-Reference list, waiting to be
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* enqueued by the Reference-handler thread. Unregistered instances
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* are never in this state.
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*
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* Enqueued: An element of the queue with which the instance was
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* registered when it was created. When an instance is removed from
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* its ReferenceQueue, it is made Inactive. Unregistered instances are
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* never in this state.
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*
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* Inactive: Nothing more to do. Once an instance becomes Inactive its
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* state will never change again.
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*
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* The state is encoded in the queue and next fields as follows:
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*
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* Active: queue = ReferenceQueue with which instance is registered, or
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* ReferenceQueue.NULL if it was not registered with a queue; next =
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* null.
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*
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* Pending: queue = ReferenceQueue with which instance is registered;
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* next = this
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*
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* Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
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* in queue, or this if at end of list.
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*
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* Inactive: queue = ReferenceQueue.NULL; next = this.
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*
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* With this scheme the collector need only examine the next field in order
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* to determine whether a Reference instance requires special treatment: If
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* the next field is null then the instance is active; if it is non-null,
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* then the collector should treat the instance normally.
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*
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* To ensure that a concurrent collector can discover active Reference
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* objects without interfering with application threads that may apply
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* the enqueue() method to those objects, collectors should link
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* discovered objects through the discovered field. The discovered
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* field is also used for linking Reference objects in the pending list.
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*/
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private T referent; /* Treated specially by GC */
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ReferenceQueue<? super T> queue;
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/* When active: NULL
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* pending: this
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* Enqueued: next reference in queue (or this if last)
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* Inactive: this
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*/
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Reference next;
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/* When active: next element in a discovered reference list maintained by GC (or this if last)
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* pending: next element in the pending list (or null if last)
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* otherwise: NULL
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*/
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transient private Reference<T> discovered; /* used by VM */
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/* Object used to synchronize with the garbage collector. The collector
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* must acquire this lock at the beginning of each collection cycle. It is
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* therefore critical that any code holding this lock complete as quickly
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* as possible, allocate no new objects, and avoid calling user code.
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*/
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static private class Lock { };
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private static Lock lock = new Lock();
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/* List of References waiting to be enqueued. The collector adds
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* References to this list, while the Reference-handler thread removes
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* them. This list is protected by the above lock object. The
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* list uses the discovered field to link its elements.
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*/
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private static Reference pending = null;
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/* High-priority thread to enqueue pending References
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*/
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private static class ReferenceHandler extends Thread {
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ReferenceHandler(ThreadGroup g, String name) {
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super(g, name);
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}
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public void run() {
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for (;;) {
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Reference r;
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synchronized (lock) {
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if (pending != null) {
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r = pending;
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pending = r.discovered;
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r.discovered = null;
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} else {
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try {
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lock.wait();
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} catch (InterruptedException x) { }
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continue;
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}
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}
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// Fast path for cleaners
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if (r instanceof Cleaner) {
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((Cleaner)r).clean();
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continue;
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}
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ReferenceQueue q = r.queue;
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if (q != ReferenceQueue.NULL) q.enqueue(r);
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}
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}
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}
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static {
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ThreadGroup tg = Thread.currentThread().getThreadGroup();
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for (ThreadGroup tgn = tg;
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tgn != null;
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tg = tgn, tgn = tg.getParent());
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Thread handler = new ReferenceHandler(tg, "Reference Handler");
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/* If there were a special system-only priority greater than
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* MAX_PRIORITY, it would be used here
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*/
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handler.setPriority(Thread.MAX_PRIORITY);
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handler.setDaemon(true);
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handler.start();
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}
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/* -- Referent accessor and setters -- */
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/**
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* Returns this reference object's referent. If this reference object has
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* been cleared, either by the program or by the garbage collector, then
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* this method returns <code>null</code>.
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*
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* @return The object to which this reference refers, or
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* <code>null</code> if this reference object has been cleared
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*/
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public T get() {
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return this.referent;
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}
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/**
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* Clears this reference object. Invoking this method will not cause this
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* object to be enqueued.
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*
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* <p> This method is invoked only by Java code; when the garbage collector
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* clears references it does so directly, without invoking this method.
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*/
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public void clear() {
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this.referent = null;
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}
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/* -- Queue operations -- */
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/**
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* Tells whether or not this reference object has been enqueued, either by
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* the program or by the garbage collector. If this reference object was
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* not registered with a queue when it was created, then this method will
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* always return <code>false</code>.
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*
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* @return <code>true</code> if and only if this reference object has
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* been enqueued
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*/
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public boolean isEnqueued() {
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synchronized (this) {
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return (this.next != null && this.queue == ReferenceQueue.ENQUEUED);
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}
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}
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/**
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* Adds this reference object to the queue with which it is registered,
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* if any.
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*
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* <p> This method is invoked only by Java code; when the garbage collector
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* enqueues references it does so directly, without invoking this method.
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*
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* @return <code>true</code> if this reference object was successfully
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* enqueued; <code>false</code> if it was already enqueued or if
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* it was not registered with a queue when it was created
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*/
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public boolean enqueue() {
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return this.queue.enqueue(this);
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}
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/* -- Constructors -- */
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Reference(T referent) {
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this(referent, null);
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}
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Reference(T referent, ReferenceQueue<? super T> queue) {
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this.referent = referent;
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this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
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}
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}
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