/*
* @(#)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;
}
}
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