java中的Reference

Object obj = new Object();
SoftReference<Object> softref = new SoftReference<Object>(obj);  
obj = null;

Object obj = new Object();
WeakReference<Object> weakRef = new WeakReference<Object>(obj);
obj = null;

static ReferenceQueue NULL = new Null();    //初始化为null
static ReferenceQueue ENQUEUED = new Null();

static private class Lock { };
private Lock lock = new Lock();    //初始化为null
private volatile Reference<? extends T> head = null;    //Queue的header设定为null
private long queueLength = 0;

boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
//本方法只能在Reference类中调用
        synchronized (r) {
 //防止将同一个Reference两次入队列
            if (r.queue == ENQUEUED) return false;
            synchronized (lock) {
//该对象入队列后，将该对象的queue对象置为 ENQUEUED
                r.queue = ENQUEUED;
                r.next = (head == null) ? r : head;
                head = r;
                queueLength++;
                if (r instanceof FinalReference) {
                    sun.misc.VM.addFinalRefCount(1);
                }
                lock.notifyAll();
                return true;
            }
        }
    }

//阻塞，直到返回结果
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;
 //死循环
            for (;;) {
                lock.wait(timeout);
                r = reallyPoll();
                if (r != null) return r;
//若传递的等待时间不为0，说明等该timeout后也没有reference入队列，返回null，
//若timeout为0，则循环，直接队列有数据
                if (timeout != 0) return null;
            }
        }
    }

    /**
     * Timestamp clock, updated by the garbage collector
     */
//GC 会更新这一个静态变量的值
    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.
     */
//每次调用get方法时会更新timestampe
// this.timestamp = clock;
    private long timestamp;

　　Reference(T referent) {
        this(referent, null);
    }

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

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

    ReferenceQueue<? super T> queue;

    Reference next;
    transient private Reference<T> discovered;  /* used by VM */


    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.
     */
//static类型，全局只有一个，GC一个Reference时，会把该Reference放到pending中，由于Reference有一个next属性，该处可能会是一些被GC过的引用的队列
    private static Reference pending = null;

    /* High-priority thread to enqueue pending References
     */
    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) {
//若pending不为null,将每个reference取出，
                        r = pending;
                        Reference rn = r.next;
                        pending = (rn == r) ? null : rn;
                        r.next = r;
                    } else {
    //若pending为null,等侍
                        try {
                            lock.wait();
                        } catch (InterruptedException x) { }
                        continue;
                    }
                }

                // Fast path for cleaners
                if (r instanceof Cleaner) {
                    ((Cleaner)r).clean();
                    continue;
                }

                ReferenceQueue q = r.queue;
//找到该Reference对象中的Queue，将自己添加到ReferenceQueue中
//这样就实现了软引用对象被回收后，在ReferenceQueue中就可以获取到。
                if (q != ReferenceQueue.NULL) q.enqueue(r);
            }
        }
    }
//启动一个线程（Reference Handler）处理引用
    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();
    }

class FinalReference<T> extends Reference<T> {

    public FinalReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }

}

    //全局一个ReferenceQueue
    private static ReferenceQueue queue = new ReferenceQueue();
    //全局只有一个unfinalized，也可以组成对象链
    private static Finalizer unfinalized = null;
    private static final Object lock = new Object();

    private Finalizer
        next = null,
        prev = null;

    // 将自己添加到unfinalized队列中
    private void add() {
        synchronized (lock) {
            if (unfinalized != null) {
                this.next = unfinalized;
                unfinalized.prev = this;
            }
            unfinalized = this;
        }
    }
//私有的构造方法
    private Finalizer(Object finalizee) {
//任何Finalizer对象的GC后都会到queue中
        super(finalizee, queue);
        add();
    }

/* Invoked by VM */
static void register(Object finalizee) {
　　new Finalizer(finalizee);
}

private static class FinalizerThread extends Thread {
        private volatile boolean running;
        FinalizerThread(ThreadGroup g) {
            super(g, "Finalizer");
        }
        public void run() {
            if (running)
                return;

            // Finalizer thread starts before System.initializeSystemClass
            // is called.  Wait until JavaLangAccess is available
            while (!VM.isBooted()) {
                // delay until VM completes initialization
                try {
                    VM.awaitBooted();
                } catch (InterruptedException x) {
                    // ignore and continue
                }
            }
            final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
            running = true;
            for (;;) {
                try {
//从ReferenceQueue中取出对象，执行对象的runFinalizer方法
                    Finalizer f = (Finalizer)queue.remove();
                    f.runFinalizer(jla);
                } catch (InterruptedException x) {
                    // ignore and continue
                }
            }
        }
    }

    static {
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());
        Thread finalizer = new FinalizerThread(tg);
    //线程优先级低
        finalizer.setPriority(Thread.MAX_PRIORITY - 2);
        finalizer.setDaemon(true);
        finalizer.start();
    }

private boolean hasBeenFinalized() {
        return (next == this);
    }
private void runFinalizer(JavaLangAccess jla) {
        synchronized (this) {
//若next==this，则表明this对象已经从unfinalized对象链中移除，已经执行过一次runFinalizer了
            if (hasBeenFinalized()) return;
//将该对象从unfinalized对象链中移除   
             remove();
        }
        try {
            Object finalizee = this.get();
            if (finalizee != null && !(finalizee instanceof java.lang.Enum)) {
//通过JDK调用对象的finalize方法
                jla.invokeFinalize(finalizee);

                /* Clear stack slot containing this variable, to decrease
                   the chances of false retention with a conservative GC */
                finalizee = null;
            }
        } catch (Throwable x) { }
        super.clear();
    }