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 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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,
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 *
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package java.lang.ref;

import sun.misc.Cleaner;
import sun.misc.JavaLangRefAccess;
import sun.misc.SharedSecrets;
import sun.misc.VM;

/**
 * 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
     */
    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 final 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;

    /* Max. number of references to unhook from pending chain in one chunk
     * before releasing the lock, handling them and grabbing the
     * lock again.
     */
    private static final int CHUNK_SIZE = 256;

    /**
     * Try handle a chunk of pending {@link Reference}s if there are any.<p>
     * Return {@code true} as a hint that there are more
     * {@link Reference}s 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 is be more {@link Reference}s pending.
     */
    static boolean tryHandlePending(boolean waitForNotify) {
        Reference<?> r = pollPendingChunk(waitForNotify, null);
        if (r == null) return false;
        handlePendingChunk(r);
        synchronized (lock) {
            return pending != null;
        }
    }

    /**
     * Polls a chunk of max. {@link #CHUNK_SIZE} references from pending chain and
     * returns the head of the chunk; others can be reached using {@link #next} pointer; the
     * last in chunk is linked to itself.
     *
     * @param waitForNotify if {@code true} and there were no pending
     *                      {@link Reference}s, wait until notified from VM
     *                      or interrupted; if {@code false}, return immediately
     *                      when there are no pending {@link Reference}s.
     * @param morePending   if non null, it should be a boolean array with length 1
     *                      to hold the additional result - a flag indicating that
     *                      there are more pending references waiting after a chunk
     *                      of them has been returned.
     * @return the head of the chunk of max. {@link #CHUNK_SIZE} pending references or
     * null if there are none pending.
     */
    static Reference<?> pollPendingChunk(boolean waitForNotify, boolean[] morePending) {
        Reference<?> r = null;
        try {
            synchronized (lock) {
                if ((r = pending) != null) {
                    // pending state invariant established by VM:
                    // assert r.next == r;
                    // move a chunk of pending/discovered references to a
                    // temporary local r/next chain
                    Reference<?> rd = r.discovered;
                    for (int i = 0; rd != null; rd = r.discovered) {
                        r.discovered = null;
                        if (++i >= CHUNK_SIZE) {
                            break;
                        }
                        rd.next = r;
                        r = rd;
                    }
                    pending = (Reference) rd;
                    if (morePending != null) morePending[0] = (rd != null);
                } else {
                    if (morePending != null) morePending[0] = false;
                    // The waiting on the lock may cause an OutOfMemoryError
                    // because it may try to allocate exception objects.
                    if (waitForNotify) {
                        lock.wait();
                    }
                }
            }
        } catch (OutOfMemoryError x) {
            // Give other threads CPU time so they hopefully drop some live references
            // and GC reclaims some space.
            Thread.yield();
        } catch (InterruptedException x) {
            // ignore
        }
        return r;
    }

    /**
     * Handles a non-null chunk of pending references
     * (obtained using {@link #pollPendingChunk(boolean, boolean[])}) and handles
     * them as following:
     * <ul>
     *     <li>Cleaner(s) are executed immediately</li>
     *     <li>Finalizer(s) are submitted as ForkJoinTask(s)</li>
     *     <li>all other Reference(s) are enqueued in their respected queues</li>
     * </ul>
     * @param r the head of a chunk of pending references
     */
    static void handlePendingChunk(Reference<?> r) {
        // dispatch temporary local r/next chain to appropriate queues
        for (Reference<?> rn = r.next; ; r = rn, rn = r.next) {
            // make 'r' appear to be just taken off the pending chain
            r.next = r;
            // Fast path for cleaners
            if (r instanceof Cleaner) {
                ((Cleaner) r).clean();
            } else if (r instanceof Finalizer) {
                // submit task for finalizers
                new ReferenceHandling.FinalizerHandler((Finalizer) r).submit();
            } else {
                // Enqueue all other references
                ReferenceQueue<?> q = r.queue;
                if (q != ReferenceQueue.NULL) q.enqueue((Reference)r);
            }
            if (rn == r) { // last in chain
                break;
            }
        }
    }

    /* -- 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() {
        return (this.queue == ReferenceQueue.ENQUEUED);
    }

    /**
     * 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;
    }

    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);
        }
    }

    // Unsafe machinery

    @SuppressWarnings("unchecked")
    T getReferentVolatile() {
        return (T) UNSAFE.getObjectVolatile(this, referentOffset);
    }

    boolean casReferent(T cmp, T val) {
        return UNSAFE.compareAndSwapObject(this, referentOffset, cmp, val);
    }

    void lazySetQueue(ReferenceQueue<? super T> val) {
        UNSAFE.putOrderedObject(this, queueOffset, val);
    }

    boolean casQueue(ReferenceQueue<?> cmp, ReferenceQueue<? super T> val) {
        return UNSAFE.compareAndSwapObject(this, queueOffset, cmp, val);
    }

    private static final sun.misc.Unsafe UNSAFE;
    private static final long referentOffset;
    private static final long queueOffset;

    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<Reference> rc = Reference.class;
            referentOffset = UNSAFE.objectFieldOffset(rc.getDeclaredField("referent"));
            queueOffset = UNSAFE.objectFieldOffset(rc.getDeclaredField("queue"));
        } catch (Exception e) {
            throw new Error(e);
        }

        // pre-load and initialize InterruptedException and Cleaner classes
        // so that we don't get into trouble later if there's
        // memory shortage while loading/initializing them lazily.
        ensureClassInitialized(InterruptedException.class);
        ensureClassInitialized(Cleaner.class);

        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());

        // must wait for VM to boot-up before starting ReferenceHandling
        // as ForkJoinPool initialization accesses system properties
        new Thread(tg, "ReferenceHandlingStarter") {
            @Override
            public void run() {
                while (true) {
                    try {
                        VM.awaitBooted();
                        break;
                    } catch (InterruptedException e) {
                        // ignore
                    }
                }
                ReferenceHandling.start();
            }
        }.start();

        // provide access in SharedSecrets
        SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
            @Override
            public boolean tryHandlePendingReference() {
                return tryHandlePending(false);
            }
        });
    }
}