Reference定义（PhantomReference，Cleaner）

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Java NIO ByteBuffer详解： http://donald-draper.iteye.com/blog/2357084
MappedByteBuffer定义: http://donald-draper.iteye.com/blog/2371594
整理 Java 引用（Reference）的概念 ： https://www.oschina.net/question/12_8662
Java 中的 Reference： http://www.cnblogs.com/newcj/archive/2011/05/15/2046882.html
引言：
这一篇文章，我们没有打算深入研究java的Reference的机制和及各种类型引用与垃圾回收器的关系，我们仅仅看一下Reference的定义，由于本人的能力有限，现在还不能够完全理解Reference，我们仅仅窥探一下内部的结构，在简单看一下PhantomReference，Cleaner。

//Reference

package java.lang.ref;
import sun.misc.Cleaner;
/**
 * Abstract base class for reference objects.  This class defines the
 * operations common to all reference objects.  Because reference objects are
 * implemented in close cooperation with the garbage collector, this class may
 * not be subclassed directly.
 *Reference为引用对象的抽象基础类。Reference定义了所有引用对象的一般操作。Reference用于垃圾
 回收器先关操作中，Reference不能够直接subclassed。
 * @author   Mark Reinhold
 * @since    1.2
 */

public abstract class Reference {

    /* A Reference instance is in one of four possible internal states:
     *一个引用实例可能有以下四种内部状态
     *     Active: Subject to special treatment by the garbage collector.  Some
     *     time after the collector detects that the reachability of the
     *     referent has changed to the appropriate state, it changes the
     *     instance's state to either Pending or Inactive, depending upon
     *     whether or not the instance was registered with a queue when it was
     *     created.  In the former case it also adds the instance to the
     *     pending-Reference list.  Newly-created instances are Active.
     *     Active：被垃圾回收器对待为一个Subject，如果垃圾回收器探测到引用的可达性
     达到一个适当状态，将改变实例的状态为Pending or Inactive，这个依赖于，在实例创建时，
     实例是否注册到队列。Pending状态，一个实例将会添加到pending-Reference集合。新创建的
     实例处于Active状态。
    
     *     Pending: An element of the pending-Reference list, waiting to be
     *     enqueued by the Reference-handler thread.  Unregistered instances
     *     are never in this state.
     *     Pending:pending-Reference集合中的元素，等待通过 Reference-handler线程入队列。
     已经反注册的实例不会处于此状态。
     *     Enqueued: An element of the queue with which the instance was
     *     registered when it was created.  When an instance is removed from
     *     its ReferenceQueue, it is made Inactive.  Unregistered instances are
     *     never in this state.
     *     Enqueued:实例创建时注册到队列。当实例从ReferenceQueue中移除，则为Inactive。
      已经反注册的实例不会处于此状态。
     *     Inactive: Nothing more to do.  Once an instance becomes Inactive its
     *     state will never change again.
     *     Inactive:一单一个实例处理此状态，就不会在改变。
     * The state is encoded in the queue and next fields as follows:
     *各状态在队列中的编码和next指针如下：
     *     Active: queue = ReferenceQueue with which instance is registered, or
     *     ReferenceQueue.NULL if it was not registered with a queue; next =
     *     null.
     *     Active：queue为注册实例的ReferenceQueue或者ReferenceQueue.NULL（还没有注册到队列），
     next指针为null
     *     Pending: queue = ReferenceQueue with which instance is registered;
     *     next = Following instance in queue, or this if at end of list.
     *      Pending:queue为实例注册到的队列ReferenceQueue，next执行队列中的紧跟着的元素，
     在队列尾部，next为this。
     *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
     *     in queue, or this if at end of list.
     *      Enqueued:queue 为ReferenceQueue.ENQUEUED;next执行队列中的紧跟着的元素，
     在队列尾部，next为this。
     *     Inactive: queue = ReferenceQueue.NULL; next = this.
     *      Inactive: queue为ReferenceQueue.NULL，next = this.
     * With this scheme the collector need only examine the next field in order
     * to determine whether a Reference instance requires special treatment: If
     * the next field is null then the instance is active; if it is non-null,
     * then the collector should treat the instance normally.
     *使用此状态和队列机制，垃圾回收器仅仅需要检查next，就可以确定一个引用的实例对象是否
     需要特殊对待：若果next为null，实例处于存活状态，如果非空，则正常对待为一个实例。
     * To ensure that concurrent collector can discover active Reference
     * objects without interfering with application threads that may apply
     * the enqueue() method to those objects, collectors should link
     * discovered objects through the discovered field.
     为确保并发垃圾回收器在没有其他应用线程对这些对象调用enqueue的情况下，发现
     active引用对象，垃圾回收应该通过discovered连接到需要发现或查找的对象。
     */

    private T referent;         /* Treated specially by GC */

    ReferenceQueue queue;//对象的引用队列

    Reference next;//下一个引用对象
    transient private Reference discovered;  /* used by VM */


    /* Object used to synchronize with the garbage collector.  The collector
     * must acquire this lock at the beginning of each collection cycle.  It is
     * therefore critical that any code holding this lock complete as quickly
     * as possible, allocate no new objects, and avoid calling user code.
     lock用于垃圾回收器同步。在每次回收垃圾的开始时，必须获取此锁。因此任何拥有此锁的
     代码必须尽可能快的完成， allocate no new objects, and avoid calling user code.
     */
    static private class Lock { };
    private static Lock lock = new Lock();


    /* List of References waiting to be enqueued.  The collector adds
     * References to this list, while the Reference-handler thread removes
     * them.  This list is protected by the above lock object.
     等待入队列的引用集合。当引用被 Reference-handler线程移除时，垃圾回收器将会添加
     引用到pending集合。集合使用lock对象保护。
     */
    private static Reference pending = null;

    /* High-priority thread to enqueue pending References
    高优先级pending References入队列线程
     */
    private static class ReferenceHandler extends Thread {

        ReferenceHandler(ThreadGroup g, String name) {
            super(g, name);
        }

        public void run() {
            for (;;) {

                Reference r;
                synchronized (lock) {
                    if (pending != null) {
                        r = pending;
                        Reference rn = r.next;
			//引用指向自己则为null，否则为pending的next
                        pending = (rn == r) ? null : rn;
                        r.next = r;
                    } else {
                        try {
			    //否则等待
                            lock.wait();
                        } catch (InterruptedException x) { }
                        continue;
                    }
                }
                // Fast path for cleaners
                if (r instanceof Cleaner) {
		    //如果pending为Cleaner，则clean，这个我们在后面在讲
                    ((Cleaner)r).clean();
                    continue;
                }
                
                ReferenceQueue q = r.queue;
		//如果引用队列不为空，则入队列
                if (q != ReferenceQueue.NULL) q.enqueue(r);
            }
        }
    }

    static {
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
	//获取当前现成的顶级父线程组
        for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());
        Thread handler = new ReferenceHandler(tg, "Reference Handler");
        /* If there were a special system-only priority greater than
         * MAX_PRIORITY, it would be used here
         */
        handler.setPriority(Thread.MAX_PRIORITY);
        handler.setDaemon(true);
	//启动pending集合处理线程
        handler.start();
    }


    /* -- Referent accessor and setters -- */

    /**
     * Returns this reference object's referent.  If this reference object has
     * been cleared, either by the program or by the garbage collector, then
     * this method returns null.
     *返回对象的引用者。如果这个引用对象被应用或者来及回收器清除，将会返回null。
     * @return   The object to which this reference refers, or
     *           null if this reference object has been cleared
     */
    public T get() {
        return this.referent;
    }

    /**
     * Clears this reference object.  Invoking this method will not cause this
     * object to be enqueued.
     *清除引用对象，调用此方不会引起对象入队列。
     * 
 This method is invoked only by Java code; when the garbage collector
     * clears references it does so directly, without invoking this method.
     此方法通过Java代码调用，垃圾回收器直接清除引用，不会调用此方法
     */
    public void clear() {
        this.referent = null;
    }


    /* -- Queue operations -- */

    /**
     * Tells whether or not this reference object has been enqueued, either by
     * the program or by the garbage collector.  If this reference object was
     * not registered with a queue when it was created, then this method will
     * always return false.
     *判断一个引用对象是否通过程序或垃圾回收器入队列，如果引用对象在创建时没有注册
     到队列，则此方返回fasle。
     * @return   true if and only if this reference object has
     *           been enqueued
     */
    public boolean isEnqueued() {
        /* In terms of the internal states, this predicate actually tests
           whether the instance is either Pending or Enqueued */
        //如果引用队列不为空，则next不为null
        synchronized (this) {
            return (this.queue != ReferenceQueue.NULL) && (this.next != null);
        }
    }

    /**
     * Adds this reference object to the queue with which it is registered,
     * if any.
     *引用对象注册时添加到队列
     * 
 This method is invoked only by Java code; when the garbage collector
     * enqueues references it does so directly, without invoking this method.
     *此方通过java代码调用，垃圾回收器直接入队列引用，不会调用此方法
     * @return   true if this reference object was successfully
     *           enqueued; false if it was already enqueued or if
     *           it was not registered with a queue when it was created
     */
    public boolean enqueue() {
        return this.queue.enqueue(this);
    }
    /* -- Constructors -- */

    Reference(T referent) {
        this(referent, null);
    }
    Reference(T referent, ReferenceQueue queue) {
        this.referent = referent;
        this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
    }
}
再来看ReferenceQueue
package java.lang.ref;

/**
 * Reference queues, to which registered reference objects are appended by the
 * garbage collector after the appropriate reachability changes are detected.
 *在引用对象路径可达状态改变被垃圾回收器探测到时，垃圾回收器注册引用对象到引用队列。
 * @author   Mark Reinhold
 * @since    1.2
 */

public class ReferenceQueue {

    /**
     * Constructs a new reference-object queue.
     */
    public ReferenceQueue() { }

    private static class Null extends ReferenceQueue {
        boolean enqueue(Reference r) {
            return false;
        }
    }
    //NULL引用队列
    static ReferenceQueue NULL = new Null();
    //已经入队列的引用
    static ReferenceQueue ENQUEUED = new Null();

    static private class Lock { };
    private Lock lock = new Lock();//同步锁
    private volatile Reference head = null;//队列头部
    private long queueLength = 0;//队列长度
    //将引用添加到引用队列头部
    boolean enqueue(Reference r) { /* Called only by Reference class */
        synchronized (r) {
	    //如果已经入队列则返回false
            if (r.queue == ENQUEUED) return false;
            synchronized (lock) {
                r.queue = ENQUEUED;
		//获取队列头部
                r.next = (head == null) ? r : head;
                head = r;
                queueLength++;//队列长度自增
                if (r instanceof FinalReference) {
		    //如果引用为FinalReference，则虚拟机不可变引用计数器+1
                    sun.misc.VM.addFinalRefCount(1);
                }
		//唤醒所有等待lock的操作
                lock.notifyAll();
                return true;
            }
        }
    }
    //从引用队列取出一个引用对象
    private Reference reallyPoll() {       /* Must hold lock */
        if (head != null) {
            Reference r = head;
            head = (r.next == r) ? null : r.next;
	    //重置引用对象的引用队列为NUll
            r.queue = NULL;
            r.next = r;
            queueLength--;
            if (r instanceof FinalReference) {
	    //如果引用为FinalReference，则虚拟机不可变引用计数器-1
                sun.misc.VM.addFinalRefCount(-1);
            }
            return r;
        }
        return null;
    }

    /**
     * Polls this queue to see if a reference object is available.  If one is
     * available without further delay then it is removed from the queue and
     * returned.  Otherwise this method immediately returns null.
     *从引用队列取一个引用对象，查看引用对象是否可用。如果可用，则从引用队列移除，
     否则返回null
     * @return  A reference object, if one was immediately available,
     *          otherwise null
     */
    public Reference poll() {
        if (head == null)
            return null;
        synchronized (lock) {
            return reallyPoll();
        }
    }

    /**
     * Removes the next reference object in this queue, blocking until either
     * one becomes available or the given timeout period expires.
     *移除队列的下一个引用对象，阻塞到对象可用，或者超时
     * 
 This method does not offer real-time guarantees: It schedules the
     * timeout as if by invoking the {@link Object#wait(long)} method.
     *此方法不能保证超时时间的准确性。通过Object#wait(long)方法调度时间。
     * @param  timeout  If positive, block for up to timeout
     *                  milliseconds while waiting for a reference to be
     *                  added to this queue.  If zero, block indefinitely.
     *
     * @return  A reference object, if one was available within the specified
     *          timeout period, otherwise null
     *
     * @throws  IllegalArgumentException
     *          If the value of the timeout argument is negative
     *
     * @throws  InterruptedException
     *          If the timeout wait is interrupted
     */
    public Reference remove(long timeout)
        throws IllegalArgumentException, InterruptedException
    {
        if (timeout < 0) {
            throw new IllegalArgumentException("Negative timeout value");
        }
        synchronized (lock) {
            Reference r = reallyPoll();
	    //如果对象可用，理解返回
            if (r != null) return r;
            for (;;) {
	        //否则自旋超时等地下一个引用对象可用。
                lock.wait(timeout);
                r = reallyPoll();
                if (r != null) return r;
                if (timeout != 0) return null;
            }
        }
    }

    /**
     * Removes the next reference object in this queue, blocking until one
     * becomes available.
     *移除队列的下一个引用对象，阻塞到引用对象可用
     * @return A reference object, blocking until one becomes available
     * @throws  InterruptedException  If the wait is interrupted
     */
    public Reference remove() throws InterruptedException {
        return remove(0);
    }

}

在Reference的pending集合处理线程ReferenceHandler中，入队列后，
如果引用对象为Cleaner，则Clean，下面我们来看clean操作做了什么

// Fast path for cleaners
   if (r instanceof Cleaner) {
    //如果pending为Cleaner，则clean，这个我们在后面在讲
       ((Cleaner)r).clean();
       continue;
   }
//Reference-ReferenceHandler
 private static class ReferenceHandler extends Thread {

        ReferenceHandler(ThreadGroup g, String name) {
            super(g, name);
        }

        public void run() {
            for (;;) {

                Reference r;
                synchronized (lock) {
                    if (pending != null) {
                        r = pending;
                        Reference rn = r.next;
			//引用指向自己则为null，否则为pending的next
                        pending = (rn == r) ? null : rn;
                        r.next = r;
                    } else {
                        try {
			    //否则等待
                            lock.wait();
                        } catch (InterruptedException x) { }
                        continue;
                    }
                }
                // Fast path for cleaners
                if (r instanceof Cleaner) {
		    //如果pending为Cleaner，则clean，这个我们在后面在讲
                    ((Cleaner)r).clean();
                    continue;
                }
                
                ReferenceQueue q = r.queue;
		//如果引用队列不为空，则入队列
                if (q != ReferenceQueue.NULL) q.enqueue(r);
            }
        }

//Cleaner

package sun.misc;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.security.AccessController;
import java.security.PrivilegedAction;

public class Cleaner extends PhantomReference
{

再看Cleaner的clean操作之前，我们先来看一下PhantomReference
//PhantomReference

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.
 *
 * 
 If the garbage collector determines at a certain point in time that the
 * referent of a phantom reference is phantom reachable, then at that
 * time or at some later time it will enqueue the reference.
 *
 * 
 In order to ensure that a reclaimable object remains so, the referent of
 * a phantom reference may not be retrieved: The get method of a
 * phantom reference always returns null.
 *
 * 
 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 extends Reference {

    /**
     * Returns this reference object's referent.  Because the referent of a
     * phantom reference is always inaccessible, this method always returns
     * null.
     *返回引用对象的引用者。由于PhantomReference总是不可访问的，所以总是返回null。
     * @return  null
     */
    public T get() {
        return null;
    }
    /**
     * Creates a new phantom reference that refers to the given object and
     * is registered with the given queue.
     *创建一个PhantomReference引用注册到指定队列的指定对象。
     * 
 It is possible to create a phantom reference with a null
     * queue, but such a reference is completely useless: Its get
     * method will always return null and, since it does not have a queue, it
     * will never be enqueued.
     *有可能创建一个PhantomReference，它的引用队列为null，此引用完全无用，
     由于没有入引用队列，所以get方法总是返回null
     * @param referent the object the new phantom reference will refer to
     * @param q the queue with which the reference is to be registered,
     *          or null if registration is not required
     */
    public PhantomReference(T referent, ReferenceQueue q) {
        super(referent, q);
    }
}

当GC一但发现了虚引用对象，将会将PhantomReference对象插入ReferenceQueue队列，
而此时PhantomReference所指向的对象并没有被GC回收，而是要等到ReferenceQueue被你真正的处理后才会被回收。

回到Cleaner，我们来看他的clear操作：

package sun.misc;

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

public class Cleaner extends PhantomReference
{
    private static final ReferenceQueue dummyQueue = new ReferenceQueue();//引用队列
    private static Cleaner first = null;//头部
    private Cleaner next;//后继
    private Cleaner prev;//先驱
    private final Runnable thunk;
    //构造Cleaner
      private Cleaner(Object obj, Runnable runnable)
    {
        super(obj, dummyQueue);
        next = null;
        prev = null;
        thunk = runnable;
    }
    //根据给定的obj和线程创建Cleaner，并添加到Cleaner链表中
     public static Cleaner create(Object obj, Runnable runnable)
    {
        if(runnable == null)
            return null;
        else
            return add(new Cleaner(obj, runnable));
    }
    //添加Cleaner到Cleaner链表中
     private static synchronized Cleaner add(Cleaner cleaner)
    {
        if(first != null)
        {
            cleaner.next = first;
            first.prev = cleaner;
        }
        first = cleaner;
        return cleaner;
    }
    从Cleaner的链表中移除cleaner
    private static synchronized boolean remove(Cleaner cleaner)
    {
        if(cleaner.next == cleaner)
            return false;
        if(first == cleaner)
            if(cleaner.next != null)
                first = cleaner.next;
            else
                first = cleaner.prev;
        if(cleaner.next != null)
            cleaner.next.prev = cleaner.prev;
        if(cleaner.prev != null)
            cleaner.prev.next = cleaner.next;
        cleaner.next = cleaner;
        cleaner.prev = cleaner;
        return true;
    }
    //清除操作
    public void clean()
    {
        //如果移除clear失败，则直接返回
        if(!remove(this))
            return;
        try
        {    //运行清除线程
            thunk.run();
        }
        catch(final Throwable x)
        {
            AccessController.doPrivileged(new PrivilegedAction() {

                public Void run()
                {
		    //如果清除异常，则抛出错误
                    if(System.err != null)
                        (new Error("Cleaner terminated abnormally", x)).printStackTrace();
                    System.exit(1);
                    return null;
                }

                public volatile Object run()
                {
                    return run();
                }

                final Throwable val$x;
                final Cleaner this$0;

            
            {
                this$0 = Cleaner.this;
                x = throwable;
                super();
            }
            });
        }
    }
}

总结：
Cleaner关联一个清除线程thunk和cleaner，在手动clean一个引用对象cleaner（PhantomReference）时，首先从引用对应队列ReferenceQueue移除引用对象，再执行清除线程thunk，完成实际的清除工作。了解Cleaner才是我们写这篇文章的目的，这个会在DirectByteBuffer中用到，我们在后面的文章会在讲，以便更深刻的理解Cleaner的作用。