JUC工具类

CountDownLatch(线程计数器)

countDown()计数器递减

  public void countDown() {
        sync.releaseShared(1);
    }

  public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

唤醒共享锁队列的线程

   private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                //如果节点状态为 SIGNAL 表示可以被唤醒
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

await()阻塞等待 CountDownLatch变成0在释放执行后面逻辑

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
         //state 不为 0 的时候 进行阻塞
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

 protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    //根据判断结果获取锁
                    int r = tryAcquireShared(arg);
                    //这里不会走 因为 外面判断了 r<0
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    //阻塞等待
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

Semaphore(信号量)

假设初始时 是5 每次调用 release()方法 都针对 state进行递减 因此当 state令牌==5的时候 意味着所有的令牌都被使用完了 后续调用的线程都会以共享类型加入到 CLH队列中 而当 state<5时 说明有其他线程释放了令牌 可以从clh队列中唤醒头部的线程

限制对某个资源同时访问的线程数

acquire()获取令牌

获取指定数量的令牌 如果不足会阻塞

release()释放令牌

释放指定数量的令牌

tryAcquire()

尝试获取指定数量的令牌 此过程是非阻塞的 如果令牌数不够 则返回false否则返回true

drainPermits()

当前线程获得剩下的所有可用令牌

hasQueuedThreads()

判断当前 Semaphore实例是否存在正在等待令牌的线程

CyclicBarrier

当四个线程都执行结束 唤醒main方法的线程继续执行

基本使用

    public static void main(String[] args) {
        int parties =4;
        CyclicBarrier cyclicBarrier = new CyclicBarrier(parties,() ->{
            System.out.println("所有线程执行完开始执行");
        });
        for (int i = 0; i <parties ; i++) {
            new ImportDataTask(cyclicBarrier).start();
        }
    }
   static class ImportDataTask extends Thread {
        private CyclicBarrier cyclicBarrier;
        public ImportDataTask(CyclicBarrier cyclicBarrier) {
            this.cyclicBarrier = cyclicBarrier;
        }
        @SneakyThrows
        @Override
        public void run() {
           Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName()+"线程执行完毕");
            cyclicBarrier.await();
        }
    }
}

原理

   public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock(); //获取锁
        try {
            final Generation g = generation;
            if (g.broken) //确认当前的 genration的barrier是否失效
                throw new BrokenBarrierException();
            //线程是否中断 
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
            int index = --count;
            //如果到达了屏障 需要去释放被阻塞的线程
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        //如果 CylicBarrier回调不为空 直接触发回调
                        command.run();
                    ranAction = true;
                    //进入下一个屏障周期
                    nextGeneration();
                    return 0;
                } finally {
                     //回调任务失败
                    if (!ranAction)
                        breakBarrier();
                }
            }
            // loop until tripped, broken, interrupted, or timed out
            //自旋 等待到达屏障点或线程中断 等待超时等
            for (;;) {
                try {
                    //是否有超时时间 如果没有直接阻塞
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        //采用超时等待机制
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    if (g == generation && ! g.broken) {
                        breakBarrier();
                        throw ie;
                    } else {
                        // We're about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        Thread.currentThread().interrupt();
                    }
                }
                if (g.broken)
                    throw new BrokenBarrierException();
                if (g != generation)
                    return index;
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }