1.1 --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
     1.2 +++ b/rt/emul/compact/src/main/java/java/lang/ref/PhantomReference.java	Tue Feb 26 16:54:16 2013 +0100
     1.3 @@ -0,0 +1,83 @@
     1.4 +/*
     1.5 + * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
     1.6 + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
     1.7 + *
     1.8 + * This code is free software; you can redistribute it and/or modify it
     1.9 + * under the terms of the GNU General Public License version 2 only, as
    1.10 + * published by the Free Software Foundation.  Oracle designates this
    1.11 + * particular file as subject to the "Classpath" exception as provided
    1.12 + * by Oracle in the LICENSE file that accompanied this code.
    1.13 + *
    1.14 + * This code is distributed in the hope that it will be useful, but WITHOUT
    1.15 + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
    1.16 + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    1.17 + * version 2 for more details (a copy is included in the LICENSE file that
    1.18 + * accompanied this code).
    1.19 + *
    1.20 + * You should have received a copy of the GNU General Public License version
    1.21 + * 2 along with this work; if not, write to the Free Software Foundation,
    1.22 + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
    1.23 + *
    1.24 + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
    1.25 + * or visit www.oracle.com if you need additional information or have any
    1.26 + * questions.
    1.27 + */
    1.28 +
    1.29 +package java.lang.ref;
    1.30 +
    1.31 +
    1.32 +/**
    1.33 + * Phantom reference objects, which are enqueued after the collector
    1.34 + * determines that their referents may otherwise be reclaimed.  Phantom
    1.35 + * references are most often used for scheduling pre-mortem cleanup actions in
    1.36 + * a more flexible way than is possible with the Java finalization mechanism.
    1.37 + *
    1.38 + * <p> If the garbage collector determines at a certain point in time that the
    1.39 + * referent of a phantom reference is <a
    1.40 + * href="package-summary.html#reachability">phantom reachable</a>, then at that
    1.41 + * time or at some later time it will enqueue the reference.
    1.42 + *
    1.43 + * <p> In order to ensure that a reclaimable object remains so, the referent of
    1.44 + * a phantom reference may not be retrieved: The <code>get</code> method of a
    1.45 + * phantom reference always returns <code>null</code>.
    1.46 + *
    1.47 + * <p> Unlike soft and weak references, phantom references are not
    1.48 + * automatically cleared by the garbage collector as they are enqueued.  An
    1.49 + * object that is reachable via phantom references will remain so until all
    1.50 + * such references are cleared or themselves become unreachable.
    1.51 + *
    1.52 + * @author   Mark Reinhold
    1.53 + * @since    1.2
    1.54 + */
    1.55 +
    1.56 +public class PhantomReference<T> extends Reference<T> {
    1.57 +
    1.58 +    /**
    1.59 +     * Returns this reference object's referent.  Because the referent of a
    1.60 +     * phantom reference is always inaccessible, this method always returns
    1.61 +     * <code>null</code>.
    1.62 +     *
    1.63 +     * @return  <code>null</code>
    1.64 +     */
    1.65 +    public T get() {
    1.66 +        return null;
    1.67 +    }
    1.68 +
    1.69 +    /**
    1.70 +     * Creates a new phantom reference that refers to the given object and
    1.71 +     * is registered with the given queue.
    1.72 +     *
    1.73 +     * <p> It is possible to create a phantom reference with a <tt>null</tt>
    1.74 +     * queue, but such a reference is completely useless: Its <tt>get</tt>
    1.75 +     * method will always return null and, since it does not have a queue, it
    1.76 +     * will never be enqueued.
    1.77 +     *
    1.78 +     * @param referent the object the new phantom reference will refer to
    1.79 +     * @param q the queue with which the reference is to be registered,
    1.80 +     *          or <tt>null</tt> if registration is not required
    1.81 +     */
    1.82 +    public PhantomReference(T referent, ReferenceQueue<? super T> q) {
    1.83 +        super(referent, q);
    1.84 +    }
    1.85 +
    1.86 +}