Cleaner.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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package jdk.internal.ref;

import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.security.AccessController;
import java.security.PrivilegedAction;

/**
 * General-purpose phantom-reference-based cleaners.
 *
 * <p> Cleaners are a lightweight and more robust alternative to finalization.
 * They are lightweight because they are not created by the VM and thus do not
 * require a JNI upcall to be created, and because their cleanup code is
 * invoked directly by the reference-handler thread rather than by the
 * finalizer thread.  They are more robust because they use phantom references,
 * the weakest type of reference object, thereby avoiding the nasty ordering
 * problems inherent to finalization.
 *
 * <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
 * cleanup code.  Some time after the GC detects that a cleaner's referent has
 * become phantom-reachable, the reference-handler thread will run the cleaner.
 * Cleaners may also be invoked directly; they are thread safe and ensure that
 * they run their thunks at most once.
 *
 * <p> Cleaners are not a replacement for finalization.  They should be used
 * only when the cleanup code is extremely simple and straightforward.
 * Nontrivial cleaners are inadvisable since they risk blocking the
 * reference-handler thread and delaying further cleanup and finalization.
 *
 * @author Mark Reinhold
 */
/*
 * 虚引用清理器(非公开)
 *
 * 虚引用的子类，用作清理器。当虚引用被回收时，在Reference中回调此类的清理功能。
 * 一个程序可以有多个Cleaner。多个Cleaner组成了一个双向链表。
 */
public class Cleaner extends PhantomReference<Object> {
    /*
     * Dummy reference queue, needed because the PhantomReference constructor insists that we pass a queue.
     * Nothing will ever be placed on this queue since the reference handler invokes cleaners explicitly.
     *
     * 声明此字段的原因是虚引用构造方法必须使用一个ReferenceQueue参数
     * 实际上，被GC回收的对象，其相应的"虚引用清理器"不会被加入到此队列中。
     * 可参见Reference类的processPendingReferences()方法。
     */
    private static final ReferenceQueue<Object> dummyQueue = new ReferenceQueue<>();
    
    // 清理动作
    private final Runnable thunk;
    
    /* Doubly-linked list of live cleaners, which prevents the cleaners themselves from being GC'd before their referents */
    
    // 双向链表头部
    private static Cleaner first = null;
    // 双向链表前驱和后继
    private Cleaner next = null, prev = null;
    
    // 注册追踪的引用obj和清理动作thunk
    private Cleaner(Object referent, Runnable thunk) {
        super(referent, dummyQueue);
        this.thunk = thunk;
    }
    
    /**
     * Creates a new cleaner.
     *
     * @param ob    the referent object to be cleaned
     * @param thunk The cleanup code to be run when the cleaner is invoked.  The
     *              cleanup code is run directly from the reference-handler thread,
     *              so it should be as simple and straightforward as possible.
     *
     * @return The new cleaner
     */
    // 创建一个Cleaner，并将其添加到双向链表中
    public static Cleaner create(Object ob, Runnable thunk) {
        if(thunk == null)
            return null;
        return add(new Cleaner(ob, thunk));
    }
    
    // 使用头插法将Cleaner添加到双向链表中
    private static synchronized Cleaner add(Cleaner cl) {
        if(first != null) {
            cl.next = first;
            first.prev = cl;
        }
        first = cl;
        return cl;
    }
    
    // 移除一个Cleaner
    private static synchronized boolean remove(Cleaner cl) {
        // If already removed, do nothing
        if(cl.next == cl)
            return false;
        
        // Update list
        if(first == cl) {
            if(cl.next != null)
                first = cl.next;
            else
                first = cl.prev;
        }
        if(cl.next != null)
            cl.next.prev = cl.prev;
        if(cl.prev != null)
            cl.prev.next = cl.next;
        
        // Indicate removal by pointing the cleaner to itself
        cl.next = cl;
        cl.prev = cl;
        return true;
    }
    
    /**
     * Runs this cleaner, if it has not been run before.
     */
    // 对追踪对象进行清理，在Reference类中完成
    public void clean() {
        // 已移除的不再执行其功能，保证每个Cleaner只发挥一次作用
        if(!remove(this)) {
            return;
        }
        try {
            // 执行清理
            thunk.run();
        } catch(final Throwable x) {
            AccessController.doPrivileged(new PrivilegedAction<>() {
                public Void run() {
                    if(System.err != null) {
                        new Error("Cleaner terminated abnormally", x).printStackTrace();
                    }
                    System.exit(1);
                    return null;
                }
            });
        }
    }
}