java中的reference(二): jdk1.8中Reference的源码阅读

/**
 * 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.
 *
 * @author   Mark Reinhold
 * @since    1.2
 */

public abstract class Reference<T> {
    //会被GC特殊对待
    private T referent;         /* Treated specially by GC */ 
    //Reference被回收之后会被添加到这个queue
    volatile ReferenceQueue<? super T> queue;
    
    
    /* -- Constructors -- */
    //用户只需要特殊的Reference，并不关心GC状态，因此可以不需要ReferenceQueue
    Reference(T referent) {
        this(referent, null);
    }
    //构造函数中传入了queue,如果reference被GC回收，则会添加到queue中去
    Reference(T referent, ReferenceQueue<? super T> queue) {
        this.referent = referent;
        this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
    }

}

 /* A Reference instance is in one of four possible internal states:
     *
     *     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.
     *
     *     Pending: An element of the pending-Reference list, waiting to be
     *     enqueued by the Reference-handler thread.  Unregistered instances
     *     are never in this state.
     *
     *     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.
     *
     *     Inactive: Nothing more to do.  Once an instance becomes Inactive its
     *     state will never change again.
     *
     * The state is encoded in the queue and next fields as follows:
     *
     *     Active: queue = ReferenceQueue with which instance is registered, or
     *     ReferenceQueue.NULL if it was not registered with a queue; next =
     *     null.
     *
     *     Pending: queue = ReferenceQueue with which instance is registered;
     *     next = this
     *
     *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
     *     in queue, or this if at end of list.
     *
     *     Inactive: queue = 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 a 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. The discovered
     * field is also used for linking Reference objects in the pending list.
     */

    /* 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. The
     * list uses the discovered field to link its elements.
     */
    private static Reference<Object> pending = null;
    
        /* When active:   next element in a discovered reference list maintained by GC (or this if last)
     *     pending:   next element in the pending list (or null if last)
     *   otherwise:   NULL
     */
    transient private Reference<T> 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.
     */
     //GC在操作过程中需要获取reference的这个锁，与ReferenceHandler线程同步。避免造成线程不安全。
     //由于GC也要用到这个锁，因此referenceHandler中的操作必须尽快完成，不生成新的对象，也不调用用户代码。避免对GC过程造成影响。
    static private class Lock { }
    private static Lock lock = new Lock();
    /* High-priority thread to enqueue pending References
     */
    private static class ReferenceHandler extends Thread {

        private static void ensureClassInitialized(Class<?> clazz) {
            try {
                Class.forName(clazz.getName(), true, clazz.getClassLoader());
            } catch (ClassNotFoundException e) {
                throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
            }
        }

        static {
            // pre-load and initialize InterruptedException and Cleaner classes
            // so that we don't get into trouble later in the run loop if there's
            // memory shortage while loading/initializing them lazily.
            ensureClassInitialized(InterruptedException.class);
            ensureClassInitialized(Cleaner.class);
        }

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

        public void run() {
            while (true) {
                tryHandlePending(true);
            }
        }
    }

/**
     * Try handle pending {@link Reference} if there is one.<p>
     * Return {@code true} as a hint that there might be another
     * {@link Reference} pending or {@code false} when there are no more pending
     * {@link Reference}s at the moment and the program can do some other
     * useful work instead of looping.
     *
     * @param waitForNotify if {@code true} and there was no pending
     *                      {@link Reference}, wait until notified from VM
     *                      or interrupted; if {@code false}, return immediately
     *                      when there is no pending {@link Reference}.
     * @return {@code true} if there was a {@link Reference} pending and it
     *         was processed, or we waited for notification and either got it
     *         or thread was interrupted before being notified;
     *         {@code false} otherwise.
     */
    static boolean tryHandlePending(boolean waitForNotify) {
        Reference<Object> r;
        Cleaner c;
        try {
        // 获取锁，避免与垃圾回收器同时操作
            synchronized (lock) {
             //判断pending-Reference链表是否有数据
                if (pending != null) {
                 // 如果有Pending Reference，从列表中取出
                    r = pending;
                    // 'instanceof' might throw OutOfMemoryError sometimes
                    // so do this before un-linking 'r' from the 'pending' chain...
                    c = r instanceof Cleaner ? (Cleaner) r : null;
                    // unlink 'r' from 'pending' chain
                    pending = r.discovered;
                    r.discovered = null;
                } else {
                 // 如果没有Pending Reference，调用wait等待
                    // 
                    // wait等待锁，是可能抛出OOME的，
                    // 因为可能发生InterruptedException异常，然后就需要实例化这个异常对象，
                    // 如果此时内存不足，就可能抛出OOME，所以这里需要捕获OutOfMemoryError，
                    // 避免因为OOME而导致ReferenceHandler进程静默退出
                    // The waiting on the lock may cause an OutOfMemoryError
                    // because it may try to allocate exception objects.
                    if (waitForNotify) {
                        lock.wait();
                    }
                    // retry if waited
                    return waitForNotify;
                }
            }
        } catch (OutOfMemoryError x) {
            // Give other threads CPU time so they hopefully drop some live references
            // and GC reclaims some space.
            // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
            // persistently throws OOME for some time...
            Thread.yield();
            // retry
            return true;
        } catch (InterruptedException x) {
            // retry
            return true;
        }
      //调用clean方法
        // Fast path for cleaners
        if (c != null) {
            c.clean();
            return true;
        }

        ReferenceQueue<? super Object> q = r.queue;
        //如果ReferenceQueue不为null 则入队
        if (q != ReferenceQueue.NULL) q.enqueue(r);
        return true;
    }

  static {
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        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);
        handler.start();

        // provide access in SharedSecrets
        SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
            @Override
            public boolean tryHandlePendingReference() {
                return tryHandlePending(false);
            }
        });
    }

/**
 * Reference queues, to which registered reference objects are appended by the
 * garbage collector after the appropriate reachability changes are detected.
 *
 * @author   Mark Reinhold
 * @since    1.2
 */

    // 引用链表的头节点
    private volatile Reference<? extends T> head = null;
    // 引用队列长度，入队则增加1，出队则减少1
    private long queueLength = 0;

    // 静态内部类，作为锁对象
    static private class Lock { };
    /* 互斥锁，用于同步ReferenceHandler的enqueue和用户线程操作的remove和poll出队操作 */
    private Lock lock = new Lock();
    
      // 用于标识没有注册Queue
    static ReferenceQueue<Object> NULL = new Null<>();
    // 用于标识已经处于对应的Queue中
    static ReferenceQueue<Object> ENQUEUED = new Null<>();

 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
        //获得锁
        synchronized (lock) {
            //判断是否需要入队
            // Check that since getting the lock this reference hasn't already been
            // enqueued (and even then removed)
            ReferenceQueue<?> queue = r.queue;
              // 如果引用实例持有的队列为ReferenceQueue.NULL或者ReferenceQueue.ENQUEUED则入队失败返回false
            if ((queue == NULL) || (queue == ENQUEUED)) {
                return false;
            }
            assert queue == this;
            //入队之后 设置为ENQUEUED 将Reference绑定只queue改为new一个新的Enqueue队列，避免循环引用
            r.queue = ENQUEUED;
            // 如果链表没有元素，则此引用实例直接作为头节点，否则把前一个引用实例作为下一个节点
            r.next = (head == null) ? r : head;
            // 当前实例更新为头节点，也就是每一个新入队的引用实例都是作为头节点，已有的引用实例会作为后继节点
            head = r;
            // 队列长度增加1
            queueLength++;
            // 特殊处理FinalReference，VM进行计数
            if (r instanceof FinalReference) {
                sun.misc.VM.addFinalRefCount(1);
            }
            lock.notifyAll();
            return true;
        }
    }

 // 引用队列的poll操作，此方法必须在加锁情况下调用
    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
        Reference<? extends T> r = head;
        if (r != null) {
            @SuppressWarnings("unchecked")
            Reference<? extends T> rn = r.next;
            // 更新next节点为头节点，如果next节点为自身，说明已经走过一次出队，则返回null
            head = (rn == r) ? null : rn;
            r.queue = NULL;
            // 当前头节点变更为环状队列，考虑到FinalReference尚为inactive和避免重复出队的问题
            r.next = r;
            // 队列长度减少1
            queueLength--;
            if (r instanceof FinalReference) {
                sun.misc.VM.addFinalRefCount(-1);
            }
            return r;
        }
        return null;
    }

    // 队列的公有poll操作，主要是加锁后调用reallyPoll
    public Reference<? extends T> poll() {
        if (head == null)
            return null;
        synchronized (lock) {
            return reallyPoll();
        }
    }

// 移除引用队列中的下一个引用元素，实际上也是依赖于reallyPoll的Object提供的阻塞机制
    public Reference<? extends T> remove(long timeout)
        throws IllegalArgumentException, InterruptedException
    {
        if (timeout < 0) {
            throw new IllegalArgumentException("Negative timeout value");
        }
        synchronized (lock) {
            Reference<? extends T> r = reallyPoll();
            if (r != null) return r;
            long start = (timeout == 0) ? 0 : System.nanoTime();
            for (;;) {
                lock.wait(timeout);
                r = reallyPoll();
                if (r != null) return r;
                if (timeout != 0) {
                    long end = System.nanoTime();
                    timeout -= (end - start) / 1000_000;
                    if (timeout <= 0) return null;
                    start = end;
                }
            }
        }
    }

    /**
     * Timestamp clock, updated by the garbage collector
     */
    static private long clock;

    /**
     * Timestamp updated by each invocation of the get method.  The VM may use
     * this field when selecting soft references to be cleared, but it is not
     * required to do so.
     */
    private long timestamp;

    /**
     * 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
     */
    public T get() {
        T o = super.get();
        if (o != null && this.timestamp != clock)
            this.timestamp = clock;
        return o;
    }

    /**
     * Creates a new weak reference that refers to the given object.  The new
     * reference is not registered with any queue.
     *
     * @param referent object the new weak reference will refer to
     */
    public WeakReference(T referent) {
        super(referent);
    }

    /**
     * Creates a new weak reference that refers to the given object and is
     * registered with the given queue.
     *
     * @param referent object the new weak 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 WeakReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }

    /**
     * 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);
    }