PhantomReference
一、总结
1.jdk 1.8.0
2.定义
虚引用也称为幽灵引用或者幻影引用,它是最弱的一种引用关系。一个持有虚引用的对象,和没有引用几乎是一样的,随时都有可能被垃圾回收器回收。当试图通过虚引用的get()方法取得强引用时,总是会失败。并且,虚引用必须和引用队列一起使用,它的作用在于跟踪垃圾回收过程。
二、源码分析
/* * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.lang.ref; /** * Phantom reference objects, which are enqueued after the collector * determines that their referents may otherwise be reclaimed. Phantom * references are most often used for scheduling pre-mortem cleanup actions in * a more flexible way than is possible with the Java finalization mechanism. * * <p> If the garbage collector determines at a certain point in time that the * referent of a phantom reference is <a * href="package-summary.html#reachability">phantom reachable</a>, then at that * time or at some later time it will enqueue the reference. * * <p> In order to ensure that a reclaimable object remains so, the referent of * a phantom reference may not be retrieved: The <code>get</code> method of a * phantom reference always returns <code>null</code>. * * <p> Unlike soft and weak references, phantom references are not * automatically cleared by the garbage collector as they are enqueued. An * object that is reachable via phantom references will remain so until all * such references are cleared or themselves become unreachable. * * @author Mark Reinhold * @since 1.2 */ public class PhantomReference<T> extends Reference<T> { /** * Returns this reference object's referent. Because the referent of a * phantom reference is always inaccessible, this method always returns * <code>null</code>. * * @return <code>null</code> */ public T get() { return null; } /** * Creates a new phantom reference that refers to the given object and * is registered with the given queue. * * <p> It is possible to create a phantom reference with a <tt>null</tt> * queue, but such a reference is completely useless: Its <tt>get</tt> * method will always return null and, since it does not have a queue, it * will never be enqueued. * * @param referent the object the new phantom reference will refer to * @param q the queue with which the reference is to be registered, * or <tt>null</tt> if registration is not required */ public PhantomReference(T referent, ReferenceQueue<? super T> q) { super(referent, q); } }