diff -r 836adbf7a2cd -r 3317bb8137f4 jdk/src/java.base/share/classes/java/lang/ref/Reference.java --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/jdk/src/java.base/share/classes/java/lang/ref/Reference.java Sun Aug 17 15:54:13 2014 +0100 @@ -0,0 +1,311 @@ +/* + * Copyright (c) 1997, 2013, 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; +import sun.misc.JavaLangRefAccess; +import sun.misc.SharedSecrets; + +/** + * 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 = this + * + * 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 a 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. The discovered + * field is also used for linking Reference objects in the pending list. + */ + + private T referent; /* Treated specially by GC */ + + volatile ReferenceQueue queue; + + /* When active: NULL + * pending: this + * Enqueued: next reference in queue (or this if last) + * Inactive: this + */ + @SuppressWarnings("rawtypes") + Reference next; + + /* When active: next element in a discovered reference list maintained by GC (or this if last) + * pending: next element in the pending list (or null if last) + * otherwise: NULL + */ + 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. The + * list uses the discovered field to link its elements. + */ + private static Reference pending = null; + + /* High-priority thread to enqueue pending References + */ + private static class ReferenceHandler extends Thread { + + private static void ensureClassInitialized(Class clazz) { + try { + Class.forName(clazz.getName(), true, clazz.getClassLoader()); + } catch (ClassNotFoundException e) { + throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e); + } + } + + static { + // pre-load and initialize InterruptedException and Cleaner classes + // so that we don't get into trouble later in the run loop if there's + // memory shortage while loading/initializing them lazily. + ensureClassInitialized(InterruptedException.class); + ensureClassInitialized(Cleaner.class); + } + + ReferenceHandler(ThreadGroup g, String name) { + super(g, name); + } + + public void run() { + while (true) { + tryHandlePending(true); + } + } + } + + /** + * Try handle pending {@link Reference} if there is one.

+ * Return {@code true} as a hint that there might be another + * {@link Reference} pending or {@code false} when there are no more pending + * {@link Reference}s at the moment and the program can do some other + * useful work instead of looping. + * + * @param waitForNotify if {@code true} and there was no pending + * {@link Reference}, wait until notified from VM + * or interrupted; if {@code false}, return immediately + * when there is no pending {@link Reference}. + * @return {@code true} if there was a {@link Reference} pending and it + * was processed, or we waited for notification and either got it + * or thread was interrupted before being notified; + * {@code false} otherwise. + */ + static boolean tryHandlePending(boolean waitForNotify) { + Reference r; + Cleaner c; + try { + synchronized (lock) { + if (pending != null) { + r = pending; + // 'instanceof' might throw OutOfMemoryError sometimes + // so do this before un-linking 'r' from the 'pending' chain... + c = r instanceof Cleaner ? (Cleaner) r : null; + // unlink 'r' from 'pending' chain + pending = r.discovered; + r.discovered = null; + } else { + // The waiting on the lock may cause an OutOfMemoryError + // because it may try to allocate exception objects. + if (waitForNotify) { + lock.wait(); + } + // retry if waited + return waitForNotify; + } + } + } catch (OutOfMemoryError x) { + // Give other threads CPU time so they hopefully drop some live references + // and GC reclaims some space. + // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above + // persistently throws OOME for some time... + Thread.yield(); + // retry + return true; + } catch (InterruptedException x) { + // retry + return true; + } + + // Fast path for cleaners + if (c != null) { + c.clean(); + return true; + } + + ReferenceQueue q = r.queue; + if (q != ReferenceQueue.NULL) q.enqueue(r); + return true; + } + + 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(); + + // provide access in SharedSecrets + SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() { + @Override + public boolean tryHandlePendingReference() { + return tryHandlePending(false); + } + }); + } + + /* -- 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() { + return (this.queue == ReferenceQueue.ENQUEUED); + } + + /** + * 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; + } + +}