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;
        }
    }