并发包的同步组件

1，概述

java并发包已经讲解了volatile，synchronized,CAS，AQS机制等。这些都是整个并发的基础，这篇主要讲一下并发包下的一些组件。

2，CountDownLatch

2.1 demo:

首先演示下这个组件的功能。主线程执行到await()方法后，就会同步等待2个线程执行，这里的2就是构造函数传入的。然后再执行下去。

public class CountDownLatchDemo {
    public static void main(String[] args) throws Exception{
        final CountDownLatch latch = new CountDownLatch(2);

        new Thread() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    System.out.println("线程1执行...");
                    latch.countDown();
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }
        }.start();

        new Thread() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    System.out.println("线程2执行...");
                    latch.countDown();
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }
        }.start();
        System.out.println("主线程即将执行await...");
        latch.await();
        System.out.println("主线程执行...");
    }
}

2.2 构造函数：

    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }
        //将传入的参数作为state的大小。这里的state即AQS下的state
        Sync(int count) {
            setState(count);
        }

2.3 await():

    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

这里需要看看tryAcquireShared()方法：

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

很明显，state是我们传入的2，不等于0，会进入方法doAcquireSharedInterruptibly():

    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) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

这个方法前面都分析过，直接画图了：

然后其他线程会执行countDown()方法，源码逻辑很简单，就是依次去减state的值。然后判断state是否等于0，如果state==0，则唤醒队列中main线程。可以发现，熟悉AQS原理之后，这些都是很简单的，所以AQS是并发的基础。

3，CyclicBarrier

3.1 demo:

public class CyclicBarrierDemo {

    public static void main(String[] args) {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(3, new Runnable() {
            @Override
            public void run() {
                System.out.println("召唤神龙。。。。。。。。。。。");
            }
        });

        for (int i = 1; i <= 3; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("收集第" + Thread.currentThread().getName() + "颗");
                    try {
                        cyclicBarrier.await();
                        //等待被唤醒,唤醒后的操作
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }, String.valueOf(i)).start();
        }
    }


}

3.2 构造函数：

    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }


    public CyclicBarrier(int parties) {
        this(parties, null);
    }

构造函数有两种，一种是带Runable,一种是不带的。

3.3 await()方法

    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();
        try {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

            int index = --count;
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        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();
        }
    }

 通过源码可以知道，await()方法就是充分利用ReentractLock和Condition来实现的。

具体流程说一下：

1，parties=3，线程1进来，将parties减1，变成2.获取锁成功，然后将自己入condition队列，释放锁，挂起

2，这时候可能线程2和线程3已经进入等待队列等着获取锁了。假设这时候线程2获取锁，将parties减1，变成1，然后将自己入condition队列，释放锁，挂起

3，线程3获取锁成功，将parties减1，变i成0.满足条件，会执行传入的Runable对象的run方法。然后将之前的在condition队列的线程执行signalAll()，也就是将其转换到等待队列，然后释放锁。

4，这时候等待队列的线程陆续获取锁，并释放，整个流程跑通。