CyclicBarrier,CountDownLatch 的使用以及实现原理

 CyclicBarrier 可以使多个线程达到同一状态

package com.sqh.TestLimiter;

import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.CyclicBarrier;

public class TestLimiter {
    public static void main(String[] args) {
        CyclicBarrier barrier = new CyclicBarrier(10);
        for (int i=0;i<10;i++){
            new Thread(() -> {
                try {
                    System.out.println("开始集结");
                    barrier.await();//等待10个线程同时开启
                    System.out.println("集结完毕");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }).start();

        }
    }

}

执行结果：

开始集结
开始集结
开始集结
开始集结
开始集结
开始集结
开始集结
开始集结
开始集结
开始集结
集结完毕
集结完毕
集结完毕
集结完毕
集结完毕
集结完毕
集结完毕
集结完毕
集结完毕
集结完毕

它维护了一个count值

private int count;

await方法会阻塞直到所有线程都到达 

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

每次执行await count都会减1  

int index = --count;

 

    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();
        }
    }

使用Lock保证线程安全 使用自旋的方式让线程阻塞一段时间

 

可以用它写一个接口压力测试 让多个线程一起访问接口

 

【CountDownLatch】 

下面的代码实现了一个让三个线程顺序执行的例子。

package com.sqh.TestLimiter;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchTest {
    public static void main(String[] args) {
        CountDownLatch countDownLatch1 = new CountDownLatch(1);
        CountDownLatch countDownLatch2 = new CountDownLatch(1);
        CountDownLatch countDownLatch3 = new CountDownLatch(1);

        Thread  thread1 = new Thread(new MyTask(countDownLatch1,null,"1"));
        Thread  thread2 = new Thread(new MyTask(countDownLatch2,countDownLatch1,"2"));
        Thread  thread3 = new Thread(new MyTask(countDownLatch3,countDownLatch2,"3"));

        thread1.start();
        thread2.start();
        thread3.start();
    }



}

class  MyTask implements Runnable{
    CountDownLatch countDownLatchOut;
    CountDownLatch countDownLatch;
    String threadName;
    public MyTask(CountDownLatch countDownLatch,CountDownLatch countDownLatchOut,String threadName) {
        this.countDownLatch = countDownLatch;
        this.countDownLatchOut = countDownLatchOut;
        this.threadName = threadName;
    }

    @Override
    public void run() {
        try {
            if (null != countDownLatchOut ) {
                countDownLatchOut.await();
            }
            System.out.println("do my task"+threadName);
            countDownLatch.countDown();
        }catch (Exception e){
            e.printStackTrace();
        }

    }
}

CountDownLatch底层是基于AQS 实现的

public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

它实际是基于

 Sync(int count) {
    setState(count);
 }

上面是他的构造函数 那么他的CountDown方法是如何实现的呢

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

在Sync中

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

tryReleaseShared 是由子类实现的，在CountDownLatch 中实现如下 它就是AQS中的state 减 1 ，使用cas的方式原子的将它更新。

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }

在AQS中 

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

tryAcquireShared 方法也是子类实现的

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

只有getState() == 0  返回1 的时候大于 0  acquireSharedInterruptibly 方法返回

    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);
        }
    }

自适应的自选操作