java.lang.ref.PhantomReference Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
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/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package java.lang.ref;
/**
* Implements a phantom reference, which is the weakest of the three types of
* references. Once the garbage collector decides that an object {@code obj} is
* phantom-reachable, it is being enqueued
* on the corresponding queue, but its referent is not cleared. That is, the
* reference queue of the phantom reference must explicitly be processed by some
* application code. As a consequence, a phantom reference that is not
* registered with any reference queue does not make any sense.
*
* Phantom references are useful for implementing cleanup operations that are
* necessary before an object gets garbage-collected. They are sometimes more
* flexible than the {@link Object#finalize()} method.
*/
public class PhantomReference extends Reference {
/**
* Constructs a new phantom reference and registers it with the given
* reference queue. The reference queue may be {@code null}, but this case
* does not make any sense, since the reference will never be enqueued, and
* the {@link #get()} method always returns {@code null}.
*
* @param r the referent to track
* @param q the queue to register the phantom reference object with
*/
public PhantomReference(T r, ReferenceQueue q) {
super(r, q);
}
/**
* Returns {@code null}. The referent of a phantom reference is not
* accessible.
*
* @return {@code null} (always)
*/
@Override
public T get() {
return null;
}
}