ReferenceQueue<T>随笔

参考资料:

ReferenceQueue食用手册

java引用食用手册

ReferenceQueue源代码里面很好的展示了java队列的实现思路， 以及多线程观察者的实现思路

多线程观察者实现思路：

入队的时候， 调用notify()方法

remove方法调用的时候，如果对列不为空， 出队列， 方法结束， 否则调用wait()方法(这里notify和wait()使用的监视器都是ReferenceQueue.class)

remove(long), remove(), remove()方法调用wait(0)（wait参数为0， 表示一直等待), remove(long timeout), 这里参数代表超时时间， 超过时间没有获取到，就结束啦(return null) , 这里超时的实现也很特殊，详见下面贴出来的代码吧

  public synchronized Reference<? extends T> remove(long timeoutMillis)
            throws InterruptedException {
        if (timeoutMillis < 0) {
            throw new IllegalArgumentException("timeout < 0: " + timeoutMillis);
        }

        if (head != null) {
            return poll();
        }

        // avoid overflow: if total > 292 years, just wait forever
        if (timeoutMillis == 0 || (timeoutMillis > Long.MAX_VALUE / NANOS_PER_MILLI)) {
            do {
                wait(0);
            } while (head == null);
            return poll();
        }

        // guaranteed to not overflow
        long nanosToWait = timeoutMillis * NANOS_PER_MILLI;
        int timeoutNanos = 0;

        // wait until notified or the timeout has elapsed
        long startTime = System.nanoTime();
        while (true) {
            wait(timeoutMillis, timeoutNanos);
            if (head != null) {
                break;
            }
            long nanosElapsed = System.nanoTime() - startTime;
            long nanosRemaining = nanosToWait - nanosElapsed;
            if (nanosRemaining <= 0) {
                break;
            }
            timeoutMillis = nanosRemaining / NANOS_PER_MILLI;
            timeoutNanos = (int) (nanosRemaining - timeoutMillis * NANOS_PER_MILLI);
        }
        return poll();
    }

同时从这个类里面也可以看到java对列的基本实现

入队:

   synchronized void enqueue(Reference<? extends T> reference) {

//对列为空，那么直接将head指向新的入队元素
        if (tail == null) {
            head = reference;
        } else { //对列不为空， 新元素放入队尾
            tail.queueNext = reference;
        }

        // The newly enqueued reference becomes the new tail, and always
        // points to itself.
        tail = reference; //队尾指向后移
        tail.queueNext = reference;
        notify();
    }

出队:

 public synchronized Reference<? extends T> poll() {
        if (head == null) { //空对列返回null
            return null;
        }

        Reference<? extends T> ret = head;

        if (head == tail) {//head == tail也是空对列
            tail = null;
            head = null;
        } else { //不是空对列， head指向后移
            head = head.queueNext;
        }

        ret.queueNext = null;
        return ret;
    }