| /* |
| * 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. |
| */ |
| /* |
| * 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. |
| * <p> |
| * 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<T> extends Reference<T> { |
| |
| /** |
| * 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<? super T> 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; |
| } |
| } |