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/* * @(#)Reference.java 1.43 06/04/10 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ 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. * * @version 1.43, 04/10/06 * @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; } }