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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

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 * 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

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 */

package java.lang.ref;

/**

 * 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 = 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;

    /* -- 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;

    }

}