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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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
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 * accompanied this code).
 *
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 * 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 jdk.internal.HotSpotIntrinsicCandidate;
import jdk.internal.misc.JavaLangRefAccess;
import jdk.internal.misc.SharedSecrets;
import jdk.internal.vm.annotation.DontInline;

/**
 * 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
     */
    private transient 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.
     */
    private static 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<?> pending;

    /* Discovery phase counter, guarder by above lock.
     */
    private static int discoveryPhase;

    /* Enqueue phase, guarded by enqueuePhaseLock object.
     */
    private static int enqueuePhase;
    private static final Object enqueuePhaseLock = new Object();

    /* 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 class
            // so that we don't get into trouble later in the run loop if there's
            // memory shortage while loading/initializing it lazily.
            ensureClassInitialized(InterruptedException.class);
        }

        ReferenceHandler(ThreadGroup g, String name) {
            super(g, null, name, 0, false);
        }

        public void run() {
            int[] discoveryPhase = new int[1];
            while (true) {
                Reference<?> p = getPendingReferences(discoveryPhase);
                enqueuePendingReferences(p, discoveryPhase[0]);
            }
        }
    }

    /**
     * Blocks until GC discovers some pending references, sets the 0-th element
     * of given {@code discoveryPhaseHolder} to the phase in which they were
     * discovered and returns the head of the list of discovered references.
     *
     * @param discoveryPhaseHolder a 1-element array to hold the returned
     *                             discovery phase.
     * @return the head of a list of pending references linked via
     * {@link #discovered} field with {@code null} marking the end of list.
     */
    static Reference<?> getPendingReferences(int[] discoveryPhaseHolder) {
        Reference<?> p;
        synchronized (lock) {
            while ((p = pending) == null) {
                try {
                    lock.wait();
                } catch (OutOfMemoryError x) {
                    // The waiting on the lock may cause an OutOfMemoryError
                    // because it may try to allocate InterruptedException object.
                    // Give other threads CPU time so they hopefully drop some live
                    // references and GC reclaims some space.
                    Thread.yield();
                } catch (InterruptedException x) {
                    // ignore interrupts
                }
            }
            pending = null;
            // increment discoveryPhase counter and return it in a holder array
            discoveryPhaseHolder[0] = ++discoveryPhase;
        }
        return p;
    }

    /**
     * @return current finished discovery phase.
     */
    static int getDiscoveryPhase() {
        synchronized (lock) {
            return (pending == null)
                   ? discoveryPhase      // already incremented
                   : discoveryPhase + 1; // not yet incremented
        }
    }

    /**
     * Enqueue a list of pending {@link Reference}s linked via {@link #discovered}
     * field with {@code null} marking the end of list.
     * <p>
     * The {@link #enqueuePhase} is set to given {@code discoveryPhase}
     * after all references from the list have been enqueued and any waiters on
     * {@link #enqueuePhaseLock} are notified.
     *
     * @param p a list of pending references linked via {@link #discovered}
     *          field with {@code null} marking the end of list.
     * @param discoveryPhase the phase in which given references were discovered.
     */
    static void enqueuePendingReferences(Reference<?> p, int discoveryPhase) {
        try {
            // distribute unhooked pending references to their respective queues
            while (p != null) {
                Reference<?> r = p;
                p = r.discovered;
                r.discovered = null;
                @SuppressWarnings("unchecked")
                ReferenceQueue<Object> q = (ReferenceQueue) r.queue;
                if (q != ReferenceQueue.NULL) q.enqueue(r);
            }
        } finally {
            // mark the enqueueing of references discovered in given
            // discovery phase is finished and notify waiters.
            synchronized (enqueuePhaseLock) {
                enqueuePhase = discoveryPhase;
                enqueuePhaseLock.notifyAll();
            }
        }
    }

    /**
     * Triggers discovery of new Reference(s) and returns the phase sequence number
     * in which they were discovered or previous phase sequence number if no new
     * Reference(s) were discovered.
     */
    static int discoverReferences() {
        // trigger discovery of new Reference(s)
        System.gc();
        // obtain the phase in which they were discovered (if any)
        return getDiscoveryPhase();
    }

    /**
     * Blocks until all Reference(s) that were discovered in given
     * {@code discoveryPhase} (as returned by {@link #discoverReferences()})
     * have been enqueued.
     *
     * @param discoveryPhase the discovery phase sequence number.
     * @throws InterruptedException if interrupted while waiting.
     */
    static void awaitReferencesEnqueued(int discoveryPhase) throws InterruptedException {
        // await for them to be enqueued
        synchronized (enqueuePhaseLock) {
            while (enqueuePhase - discoveryPhase < 0) {
                enqueuePhaseLock.wait();
            }
        }
    }

    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 int discoverReferences() {
                return Reference.discoverReferences();
            }

            @Override
            public void awaitReferencesEnqueued(
                int discoveryPhase) throws InterruptedException {
                Reference.awaitReferencesEnqueued(discoveryPhase);
            }
        });
    }

    /* -- 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
     */
    @HotSpotIntrinsicCandidate
    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;
    }

    /**
     * Ensures that the object referenced by the given reference remains
     * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
     * regardless of any prior actions of the program that might otherwise cause
     * the object to become unreachable; thus, the referenced object is not
     * reclaimable by garbage collection at least until after the invocation of
     * this method.  Invocation of this method does not itself initiate garbage
     * collection or finalization.
     *
     * <p> This method establishes an ordering for
     * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
     * with respect to garbage collection.  It controls relations that are
     * otherwise only implicit in a program -- the reachability conditions
     * triggering garbage collection.  This method is designed for use in
     * uncommon situations of premature finalization where using
     * {@code synchronized} blocks or methods, or using other synchronization
     * facilities are not possible or do not provide the desired control.  This
     * method is applicable only when reclamation may have visible effects,
     * which is possible for objects with finalizers (See
     * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
     * Section 12.6 17 of <cite>The Java&trade; Language Specification</cite></a>)
     * that are implemented in ways that rely on ordering control for correctness.
     *
     * @apiNote
     * Finalization may occur whenever the virtual machine detects that no
     * reference to an object will ever be stored in the heap: The garbage
     * collector may reclaim an object even if the fields of that object are
     * still in use, so long as the object has otherwise become unreachable.
     * This may have surprising and undesirable effects in cases such as the
     * following example in which the bookkeeping associated with a class is
     * managed through array indices.  Here, method {@code action} uses a
     * {@code reachabilityFence} to ensure that the {@code Resource} object is
     * not reclaimed before bookkeeping on an associated
     * {@code ExternalResource} has been performed; in particular here, to
     * ensure that the array slot holding the {@code ExternalResource} is not
     * nulled out in method {@link Object#finalize}, which may otherwise run
     * concurrently.
     *
     * <pre> {@code
     * class Resource {
     *   private static ExternalResource[] externalResourceArray = ...
     *
     *   int myIndex;
     *   Resource(...) {
     *     myIndex = ...
     *     externalResourceArray[myIndex] = ...;
     *     ...
     *   }
     *   protected void finalize() {
     *     externalResourceArray[myIndex] = null;
     *     ...
     *   }
     *   public void action() {
     *     try {
     *       // ...
     *       int i = myIndex;
     *       Resource.update(externalResourceArray[i]);
     *     } finally {
     *       Reference.reachabilityFence(this);
     *     }
     *   }
     *   private static void update(ExternalResource ext) {
     *     ext.status = ...;
     *   }
     * }}</pre>
     *
     * Here, the invocation of {@code reachabilityFence} is nonintuitively
     * placed <em>after</em> the call to {@code update}, to ensure that the
     * array slot is not nulled out by {@link Object#finalize} before the
     * update, even if the call to {@code action} was the last use of this
     * object.  This might be the case if, for example a usage in a user program
     * had the form {@code new Resource().action();} which retains no other
     * reference to this {@code Resource}.  While probably overkill here,
     * {@code reachabilityFence} is placed in a {@code finally} block to ensure
     * that it is invoked across all paths in the method.  In a method with more
     * complex control paths, you might need further precautions to ensure that
     * {@code reachabilityFence} is encountered along all of them.
     *
     * <p> It is sometimes possible to better encapsulate use of
     * {@code reachabilityFence}.  Continuing the above example, if it were
     * acceptable for the call to method {@code update} to proceed even if the
     * finalizer had already executed (nulling out slot), then you could
     * localize use of {@code reachabilityFence}:
     *
     * <pre> {@code
     * public void action2() {
     *   // ...
     *   Resource.update(getExternalResource());
     * }
     * private ExternalResource getExternalResource() {
     *   ExternalResource ext = externalResourceArray[myIndex];
     *   Reference.reachabilityFence(this);
     *   return ext;
     * }}</pre>
     *
     * <p> Method {@code reachabilityFence} is not required in constructions
     * that themselves ensure reachability.  For example, because objects that
     * are locked cannot, in general, be reclaimed, it would suffice if all
     * accesses of the object, in all methods of class {@code Resource}
     * (including {@code finalize}) were enclosed in {@code synchronized (this)}
     * blocks.  (Further, such blocks must not include infinite loops, or
     * themselves be unreachable, which fall into the corner case exceptions to
     * the "in general" disclaimer.)  However, method {@code reachabilityFence}
     * remains a better option in cases where this approach is not as efficient,
     * desirable, or possible; for example because it would encounter deadlock.
     *
     * @param ref the reference. If {@code null}, this method has no effect.
     * @since 9
     */
    @DontInline
    public static void reachabilityFence(Object ref) {
        // Does nothing, because this method is annotated with @DontInline
        // HotSpot needs to retain the ref and not GC it before a call to this
        // method
    }

}