sdk/sources/android-23/java/lang/ref/PhantomReference.java

   1 /*
   2  * Licensed to the Apache Software Foundation (ASF) under one or more
   3  * contributor license agreements.  See the NOTICE file distributed with
   4  * this work for additional information regarding copyright ownership.
   5  * The ASF licenses this file to You under the Apache License, Version 2.0
   6  * (the "License"); you may not use this file except in compliance with
   7  * the License.  You may obtain a copy of the License at
   8  *
   9  *     http://www.apache.org/licenses/LICENSE-2.0
  10  *
  11  * Unless required by applicable law or agreed to in writing, software
  12  * distributed under the License is distributed on an "AS IS" BASIS,
  13  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14  * See the License for the specific language governing permissions and
  15  * limitations under the License.
  16  */
  17 /*
  18  * Copyright (C) 2008 The Android Open Source Project
  19  *
  20  * Licensed under the Apache License, Version 2.0 (the "License");
  21  * you may not use this file except in compliance with the License.
  22  * You may obtain a copy of the License at
  23  *
  24  *      http://www.apache.org/licenses/LICENSE-2.0
  25  *
  26  * Unless required by applicable law or agreed to in writing, software
  27  * distributed under the License is distributed on an "AS IS" BASIS,
  28  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  29  * See the License for the specific language governing permissions and
  30  * limitations under the License.
  31  */
  32
  33 package java.lang.ref;
  34
  35 /**
  36  * Implements a phantom reference, which is the weakest of the three types of
  37  * references. Once the garbage collector decides that an object {@code obj} is
  38  * phantom-reachable, it is being enqueued
  39  * on the corresponding queue, but its referent is not cleared. That is, the
  40  * reference queue of the phantom reference must explicitly be processed by some
  41  * application code. As a consequence, a phantom reference that is not
  42  * registered with any reference queue does not make any sense.
  43  * <p>
  44  * Phantom references are useful for implementing cleanup operations that are
  45  * necessary before an object gets garbage-collected. They are sometimes more
  46  * flexible than the {@link Object#finalize()} method.
  47  */
  48 public class PhantomReference<T> extends Reference<T> {
  49
  50     /**
  51      * Constructs a new phantom reference and registers it with the given
  52      * reference queue. The reference queue may be {@code null}, but this case
  53      * does not make any sense, since the reference will never be enqueued, and
  54      * the {@link #get()} method always returns {@code null}.
  55      *
  56      * @param r the referent to track
  57      * @param q the queue to register the phantom reference object with
  58      */
  59     public PhantomReference(T r, ReferenceQueue<? super T> q) {
  60         super(r, q);
  61     }
  62
  63     /**
  64      * Returns {@code null}.  The referent of a phantom reference is not
  65      * accessible.
  66      *
  67      * @return {@code null} (always)
  68      */
  69     @Override
  70     public T get() {
  71         return null;
  72     }
  73 }