java.lang.ref.Reference

 Reference提供了一些引用的基本方法以及静态代码块最高优先级启动ReferenceHandler线程

 Reference提供了一些引用的基本方法以及静态代码块最高优先级启动ReferenceHandler线程

 package xxx;

 /**
  * 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.
  * 引用对象的抽象基类。这个类定义了所有引用对象的通用行为。
  * 因为引用对象是通过与垃圾回收期密切合作来实现的，所以不能直接为此类创建子类.
  *
  *
  * @version  1.43, 04/10/06
  * @author   Mark Reinhold
  * @since    1.2
  */
 public abstract class Reference<T> {
     /* A Reference instance is in one of four possible internal states:
      * 一种引用实例是可能是四种内部状态之一：
      *    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.
      *    激活：垃圾回收器特别处理的主题。有时候在回收器检测到被引用(对象)的可达性被改变成适当
      *    的状态，它会把实例的状态改变成等待状态或者未激活状态，这取决于实例是否被一个队列注册当
      *    它被创建。在前一种情况下(等待状态),它也往等待-引用集合增加实例。
      *
      *
      *    Pending: An element of the pending-Reference list, waiting to be
      *    enqueued by the Reference-handler thread.  Unregistered instances
      *    are never in this state.
      *    等待:一个等待-引用集合里的元素，等待被引用处理线程放入队列中。
      *    未注册的实例永远不会在这个状态
      *
      *    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.
      *    入队：实例被创建的时候被登记注册成一个队列的元素。当一个实例从引用队列中删除，它变成非激活状态。
      *    未注册的实例永远不会在这个状态。
      *
      *    Inactive: Nothing more to do.  Once an instance becomes Inactive its
      *    state will never change again.
      *    非激活：不会再做什么。一旦一个实例成为非激活的，它的状态永远不会被改变。
      *
      *
      * The state is encoded in the queue and next fields as follows:
      * 状态在队列里被处理并且每个状态所表现的属性如下：
      *
      *
      *    Active: queue = ReferenceQueue with which instance is registered, or
      *    ReferenceQueue.NULL if it was not registered with a queue; next =
      *    null.
      *    激活：queue=引用队列时候，实例被它注册，
      *    或者实例不被注册，当queue=ReferenceQueue.NULL时候；
      *    next=null.
      *
      *    Pending: queue = ReferenceQueue with which instance is registered;
      *    next = Following instance in queue, or this if at end of list.
      *    等待：queue=引用队列时候，实例被它注册，
      *    next=剩下的queue队列里面的实例，或者=this,如果this是队列的最后一个。
      *
      *    Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
      *    in queue, or this if at end of list.
      *    入队：queue=ReferenceQueue.ENQUEUED
      *    next=剩下的queue队列里面的实例，或者=this,如果this是队列的最后一个
      *
      *
      *    Inactive: queue = ReferenceQueue.NULL; next = this.
      *    终止：队列=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.
      *
      *
      *
      * 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.
      */
     private T referent;  /* Treated specially by GC */ //被GC引用的对象
     ReferenceQueue<? super T> queue;//引用队列
     Reference next;//下个引用
     transient private Reference<T> discovered;  /* used by VM *///被VM引用的瞬态对象

     /* 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.
      * GC线程在回收的时候的锁
      * 对象被用来和GC同步。GC必须获得锁在每个回收的生命周期。
      * 更关键的是任何持有锁的代码尽可能快的执行完，没有分配新的对象，并避免使用使用者的代码。
      *
      */
     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.
      * 排队的引用集合。当处理引用的线程删除引用时候，收集器添加引用到这个集合。
      * 这个集合受上面的对象锁保护。
      */
     private static Reference pending = null;
     /* High-priority thread to enqueue pending References
      * 处理排队等待的引用的高优先的线程
      */
     private static class ReferenceHandler extends Thread {
         ReferenceHandler(ThreadGroup g, String name) {
             super(g, name);
             }
         public void run() {
             for (;;) {
                 /*
                  * 给pending赋值
                  * 如果pending.next=pending,pending=null;否则pending=pengding.next,最后把pending.next=pending
                  * 下次执行线程里的代码时候pending=null了，再下次执行同步代码块就线程阻塞了
                  *
                  * 如果pending属性为空，释放锁的对象监视器，阻塞当前线程
                  * */
                 Reference r;
                 synchronized (lock) {
                     if (pending != null) {
                         r = pending;
                         Reference rn = r.next;
                         pending = (rn == r) ? null : rn;
                         r.next = r;
                     } else {
                         try {
                             lock.wait();
                         } catch (InterruptedException x) { }
                         continue;
                     }
                 }
                 // Fast path for cleaners
                 if (r instanceof Cleaner) {
                     ((Cleaner)r).clean();
                     continue;
                 }
                 /*
                  *  ReferenceQueue.NULL的实现：
                  *  private static class Null extends ReferenceQueue {
                  * boolean enqueue(Reference r) {
                  * return false;
             }
         }
                  * 如果Q不为空，把引用放入Queue
                  * 在刚创建一个引用，第二个参数没放Queue时候，为空。
                  * */
                 ReferenceQueue q = r.queue;
                 if (q != ReferenceQueue.NULL) q.enqueue(r);
             }
         }
     }
         static {
             //取得当前线程组
             ThreadGroup tg = Thread.currentThread().getThreadGroup();
             //取得最上层的System线程组
             for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());
             //创建线程对象
             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);//设置最高优先级
             handler.setDaemon(true);//标记守护线程或用户线程
             handler.start();//守护线程启动
         }

         /* -- 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 <code>null</code>.
          *
          * @return  The object to which this reference refers, or
          *   <code>null</code> if this reference object has been cleared
          * 获得引用对象
          */
         public T get() {
             return this.referent;
         }
         /**
          * Clears this reference object.  Invoking this method will not cause this
          * object to be enqueued.
          *
          * <p> This method is invoked only by Java code; when the garbage collector
          * clears references it does so directly, without invoking this method.
          * 清除引用对象
          */
         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 <code>false</code>.
          *
          * @return  <code>true</code> 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
             是否处于等待状态
             判断条件：队列不为空，
             它的next属性不为空。
             刚初始化一个Reference时候，next属性肯定是空的，因此肯定不处于等待状态
              */
             synchronized (this) {
                 return (this.queue != ReferenceQueue.NULL) && (this.next != null);
                 }
             }
         /**
          * Adds this reference object to the queue with which it is registered,
          * if any.
          *
          * <p> This method is invoked only by Java code; when the garbage collector
          * enqueues references it does so directly, without invoking this method.
          *
          * @return  <code>true</code> if this reference object was successfully
          *   enqueued; <code>false</code> if it was already enqueued or if
         *   it was not registered with a queue when it was created
         * 加入等待队列
             */
         public boolean enqueue() {
             return this.queue.enqueue(this);
         }

         /* -- Constructors -- */
         Reference(T referent) {
             this(referent, null);
         }
         Reference(T referent, ReferenceQueue<? super T> queue) {
             this.referent = referent;
             this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
         }
     }