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 33package java.lang.ref; 34 35/** 36 * Implements a weak reference, which is the middle of the three types of 37 * references. Once the garbage collector decides that an object {@code obj} is 38 * is weakly-reachable, the following 39 * happens: 40 * 41 * <ul> 42 * <li> 43 * A set {@code ref} of references is determined. {@code ref} contains the 44 * following elements: 45 * <ul> 46 * <li> 47 * All weak references pointing to {@code obj}. 48 * </li> 49 * <li> 50 * All weak references pointing to objects from which {@code obj} is 51 * either strongly or softly reachable. 52 * </li> 53 * </ul> 54 * </li> 55 * <li> 56 * All references in {@code ref} are atomically cleared. 57 * </li> 58 * <li> 59 * All objects formerly being referenced by {@code ref} become eligible for 60 * finalization. 61 * </li> 62 * <li> 63 * At some future point, all references in {@code ref} will be enqueued 64 * with their corresponding reference queues, if any. 65 * </li> 66 * </ul> 67 * 68 * Weak references are useful for mappings that should have their entries 69 * removed automatically once they are not referenced any more (from outside). 70 * The difference between a {@code SoftReference} and a {@code WeakReference} is 71 * the point of time at which the decision is made to clear and enqueue the 72 * reference: 73 * 74 * <ul> 75 * <li> 76 * A {@code SoftReference} should be cleared and enqueued <em>as late as 77 * possible</em>, that is, in case the VM is in danger of running out of 78 * memory. 79 * </li> 80 * <li> 81 * A {@code WeakReference} may be cleared and enqueued as soon as is 82 * known to be weakly-referenced. 83 * </li> 84 * </ul> 85 */ 86public class WeakReference<T> extends Reference<T> { 87 88 /** 89 * Constructs a new weak reference to the given referent. The newly created 90 * reference is not registered with any reference queue. 91 * 92 * @param r the referent to track 93 */ 94 public WeakReference(T r) { 95 super(r, null); 96 } 97 98 /** 99 * Constructs a new weak reference to the given referent. The newly created 100 * reference is registered with the given reference queue. 101 * 102 * @param r the referent to track 103 * @param q the queue to register to the reference object with. A null value 104 * results in a weak reference that is not associated with any 105 * queue. 106 */ 107 public WeakReference(T r, ReferenceQueue<? super T> q) { 108 super(r, q); 109 } 110} 111