java/8 : java/lang/ref/Reference.java

Reference
https://openjdk.java.net/
GPLv2 + Classpath Exception
/*
 * Copyright (c) 1997, 2017, 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/**
 * 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 = 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<? super T> queue;

    /* When active:   NULL
     *     pending:   this
     *    Enqueued:   next reference in queue (or this if last)
     *    Inactive:   this
     */
    @SuppressWarnings("rawtypes")
    volatile 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<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. The
     * list uses the discovered field to link its elements.
     */
    private static Reference<Object> 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 Reference if there is one. Return true as a hint that there might be another Reference pending or false when there are no more pending References at the moment and the program can do some other useful work instead of looping. 
Params: waitForNotify – if true and there was no pending Reference, wait until notified from VM or interrupted; if false, return immediately when there is no pending Reference.
Returns: true if there was a Reference pending and it was processed, or we waited for notification and either got it or thread was interrupted before being notified; false otherwise./**
     * Try handle pending {@link Reference} if there is one.<p>
     * 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<Object> 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<? super Object> 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.
Returns:   The object to which this reference refers, or
          null if this reference object has been cleared/**
     * 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.
 This method is invoked only by Java code; when the garbage collector
clears references it does so directly, without invoking this method.
/**
     * 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 false.
Returns:   true if and only if this reference object has
          been enqueued/**
     * 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() {
        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.
Returns:   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/**
     * 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;
    }

}
Params:	waitForNotify – if `true` and there was no pending `Reference`, wait until notified from VM or interrupted; if `false`, return immediately when there is no pending `Reference`.
Returns:	`true` if there was a `Reference` pending and it was processed, or we waited for notification and either got it or thread was interrupted before being notified; `false` otherwise.
/

java/ 8/ java/lang/ref/Reference.java
