Reference

Reference

一、总结

1.jdk 1.8.0

2.引用级别

• FinalReference，不对外提供使用，类的访问权限为默认 protect，即使抛出 out of memory 异常也不会回收所占内存
• SoftReference，在内存不够用时，在抛出 out of memory 前回收内存空间
• WeakReference，第一次GC不回收，当第二次GC时回收，有限期到第二次GC前都在内存中
• PhantomReference

3.引用对比

-	强引用	软引用	弱引用	虚引用
定义	强引用指的是，程序中有直接可达的引用，而不需要通过任何引用对象，如Object obj = new Object();中，obj为强引用	软引用，非强引用，但是可以通过软引用对象来访问。软引用的对象，只有在内存不足的时候（抛出OOM异常前），垃圾收集器会决定回收该软引用所指向的对象。软引用通常用于实现内存敏感的缓存。	弱引用，非强引用和软引用，但是可以通过弱引用对象来访问。弱引用的对象，不管内存是否足够，只要被垃圾收集器发现，该引用的对象就会被回收。实际的应用见WeakHashMap	虚引用，该引用必须和引用队列(ReferenceQueue)一起使用，一般用于实现追踪垃圾收集器的回收动作，比如在对象被回收的时候，会调用该对象的finalize方法，在使用虚引用可以实现该动作，也更加安全

4.其他

• 子类不可直接继承；处于安全考虑，Reference 类与GC交互；实现方式：构造方法的访问权限为默认，即包外的类不可见
• 对象封装了其它对象的引用，可以和普通的对象一样操作，在一定的限制条件下，支持和垃圾收集器的交互。即可以使用Reference对象来引用其它对象，但是最后还是会被垃圾收集器回收。程序有时候也需要在对象回收后被通知，以告知对象的可达性发生变更

5.流程




二、源码分析

/**
 * 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> {

类的注释：

• Reference 类是引用对象的抽象基类
• Reference 类中定义了引用对象的常用操作
• 由于引用对象是通过与垃圾回收器密切合作来实现的，因此，不能直接为此类创建子类

Reference 类不能被包外的类继承的实现：

• Reference 类的构造方法的访问权限为默认，即同包内的类可见
• 子类的构造方法默认通过  super() 调用父类的构造方法，访问不到父类的构造方法

    // 构造方法，两个
    // 区别：是否带有 ReferenceQueue 参数
    // 不带有  ReferenceQueue 参数的构造方法，当 Reference 被 GC 回收后直接由 Active 状态变为 InActive 状态
    
    Reference(T referent) {
        this(referent, null);
    }

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

    // 带有 ReferenceQueue 参数的构造方法，当 Reference 被 GC 回收后会被加入到 GC 自动加入到 pending-Reference list 中，即类中的属性 pengding  
    private static Reference<Object> pending = null;

    // 对应构造方法传入的两个参数
    private T referent;         /* Treated specially by GC */
    volatile ReferenceQueue<? super T> queue;

    // 不同状态队列的next元素
    /* When active:   NULL
     *     pending:   this
     *    Enqueued:   next reference in queue (or this if last)
     *    Inactive:   this
     */
    @SuppressWarnings("rawtypes")
    Reference next;

• 使用：访问权限为默认，当前类中未使用到该属性；为同包中其他类中调用使用，如：ReferenceQueue 中使用到了
• 作用：Reference 数据结构为链表，用于连接后续的对象
• Reference next ， 在  ReferenceQueue  中指代构造方法传入的参数中的  ReferenceQueue

Reference状态	ReferenceQueue中的next的指向
Active	NULL
Pending	本身
Enqueued	下一个元素或本身（尾部的元素指向本身）
Inactive	NULL

 /* 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.
     */
    // 用于保存对象的引用，GC会根据不同Reference来特别对待，构造方法的参数
    private T referent;         /* Treated specially by GC */
    
    // 简述：如果需要通知机制，则保存的对对应的队列
    // 使用：构造方法的参数
    // 作用：创建Reference时，将Queue注册到Reference中，当该Reference所引用的对象
    // 被垃圾收集器回收时，会将该Reference放到该队列中，相当于一种通知机制
    volatile ReferenceQueue<? super T> queue;

    /* 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
     */
    // 指向队列中的下一个对象；不同于next，为当前类中使用
    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.
     */
    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. The
     * list uses the discovered field to link its elements.
     */
    // 等待进行 enqueued 操作的对象集合；
    // 当 Reference-handler 线程删除元素后，GC 将删除的元素加入此队列中；
    // 此集合通过上述的 lock 锁实现线程安全
    // 此集合通过 discovered 属性链接本身的元素
    // pending队列中的元素由GC自动加入（对象回收后放入此队列中）
    private static Reference<Object> pending = null;

由类中的属性得知：内部的数据结果是单链表

ReferenceQueue<? super T> queue 的作用

• queue 通过构造方法传入，若无默认为 null，用于存入注册到队列上的引用对象
• queue 区分不同状态的  Reference ，不同的状态对应不同的queue

Reference有4种状态

• Active：Active状态的Reference会受到GC的特别关注，当GC察觉到引用的可达性变化为其它的状态之后，它的状态将变化为Pending或Inactive，到底转化为Pending状态还是Inactive状态取决于此Reference对象创建时是否注册了queue.如果注册了queue，则将添加此实例到pending-Reference list中。 新创建的Reference实例的状态是Active。
• Pending：在pending-Reference list中等待着被Reference-handler 线程入队列queue中的元素就处于这个状态。没有注册queue的实例是永远不可能到达这一状态
• Enqueued：队列中的对象的状态，当实例被移动到ReferenceQueue外时，Reference的状态为Inactive。没有注册ReferenceQueue的不可能到达这一状态的
• Inactive：一旦一个实例变为Inactive，则这个状态永远都不会再被改变

Reference四种状态对应的队列

• Active queue：刚创建的队列或队列中没有对象注入；next = null ；由构造方法可知，新创建的实例都处于此状态
• Pending queue：
• Enqueued queue：
• Inactive queue：

    /* 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; // PhantomReference 的子类
        try {
            synchronized (lock) {
                // pengding Reference Queue 不为空
                if (pending != null) {
                    r = pending; // 将 pending 赋值给 r 
                    // 'instanceof' might throw OutOfMemoryError sometimes
                    // so do this before un-linking 'r' from the 'pending' chain...
                    // r 是否是 Cleaner 的实例对象
                    c = r instanceof Cleaner ? (Cleaner) r : null; 
                    // unlink 'r' from 'pending' chain
                    // pengding 指向下一个元素
                    pending = r.discovered;
                    r.discovered = null;
                } else {
                    // pending Reference Queue 为空
                    // 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;
        }

        // Fast path for cleaners
        if (c != null) {
            c.clean();
            return true;
        }

        ReferenceQueue<? super Object> q = r.queue;
        if (q != ReferenceQueue.NULL) q.enqueue(r); // 入队列
        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);
            }
        });
    }

从源码中可以看出，这个线程在Reference类的static构造块中启动，并且被设置为最高优先级和daemon状态。此线程要做的事情就是不断的的检查pending是否为null，如果pending不为null，则将pending进行enqueue，否则线程进行wait状态。

    /**
     * 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
     */
    // 返回当前引用对象的指向的对象；如果对象已经被清除或被GC回收，返回NULL
    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.
     */
    // 触发这个方法则不会进行入队的操作
    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
     */
    // 判断对象是否已经入队
    public boolean isEnqueued() {
        return (this.queue == ReferenceQueue.ENQUEUED);
    }

    /**
     * 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
     */
    // 入队操作
    public boolean enqueue() {
        return this.queue.enqueue(this);
    }

博文参考：
《Java源码分析》：ReferenceQueue、Reference及其子类
Java Reference 源码分析