java 源码系列 - 带你读懂 Reference 和 ReferenceQueue

创建一个引用队列  
ReferenceQueue queue = new ReferenceQueue();  

// 创建弱引用，此时状态为Active，并且Reference.pending为空，当前Reference.queue = 上面创建的queue，并且next=null  
WeakReference reference = new WeakReference(new Object(), queue);  
System.out.println(reference);  
// 当GC执行后，由于是虚引用，所以回收该object对象，并且置于pending上，此时reference的状态为PENDING  
System.gc();  

/* ReferenceHandler从pending中取下该元素，并且将该元素放入到queue中，此时Reference状态为ENQUEUED，Reference.queue = ReferenceENQUEUED */  

/* 当从queue里面取出该元素，则变为INACTIVE，Reference.queue = Reference.NULL */  
Reference reference1 = queue.remove();  
System.out.println(reference1);

private T referent;       
volatile ReferenceQueue<? super T> queue;  

/* When active:   NULL 
 *     pending:   this 
 *    Enqueued:   next reference in queue (or this if last) 
 *    Inactive:   this 
 */  
@SuppressWarnings("rawtypes")  
Reference next;  


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


/* 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;

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);
        }
    });
}

private static class ReferenceHandler extends Thread {

   ----- // 核心代码如下

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

static boolean tryHandlePending(boolean waitForNotify) {
    Reference<Object> r;
    Cleaner c;
    try {
        synchronized (lock) {
            // 检查 pending 是否为 null，不为 null，制定 pending enqueue
            if (pending != null) {
                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 { // 为 null。等待
                // 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);
    return true;
}

/* 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;

boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
    synchronized (lock) {
        // Check that since getting the lock this reference hasn't already been
        // enqueued (and even then removed)
        ReferenceQueue<?> queue = r.queue;
        // queue 为 null 或者 queue 已经被回收了，直接返回
        if ((queue == NULL) || (queue == ENQUEUED)) {
            return false;
        }
        assert queue == this;
        // 将 refrence 的状态置为 Enqueued，表示已经被回收
        r.queue = ENQUEUED;
        // 接着，将 refrence 插入到链表
        // 判断当前链表是否为 null，不为 null，将 r.next 指向 head，为 null，head 直接指向 r
        r.next = (head == null) ? r : head;
        // head 指针指向 r
        head = r;
        queueLength++;
        if (r instanceof FinalReference) {
            sun.misc.VM.addFinalRefCount(1);
        }
        lock.notifyAll();
        return true;
    }
}

public class TestSoftReference {
        private static ReferenceQueue<Object> rq = new ReferenceQueue<Object>();
        public static void main(String[] args){
            Object obj = new Object();
            SoftReference<Object> sf = new SoftReference(obj,rq);
            System.out.println(sf.get()!=null);
            System.gc();
            obj = null;
            System.out.println(sf.get()!=null);

        }
    }

public class TestWeakReference {
        private static ReferenceQueue<Object> rq = new ReferenceQueue<Object>();
        public static void main(String[] args) {
            Object obj = new Object();
            WeakReference<Object> wr = new WeakReference(obj,rq);
            System.out.println(wr.get()!=null);
            obj = null;
            System.gc();
            System.out.println(wr.get()!=null);//false，这是因为WeakReference被回收
        }

    }

public class TestPhantomReference {

        private static ReferenceQueue<Object> rq = new ReferenceQueue<Object>();
        public static void main(String[] args){

            Object obj = new Object();
            PhantomReference<Object> pr = new PhantomReference<Object>(obj, rq);
            System.out.println(pr.get());
            obj = null;
            System.gc();
            System.out.println(pr.get());
            Reference<Object> r = (Reference<Object>)rq.poll();
            if(r!=null){
                System.out.println("回收");
            }
        }
    }