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解读java同步类CountDownLatch(一)
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Tags:解读 java 同步 CountDownLatch

同步辅助类:

    CountDownLatch是一个同步辅助类,在jdk5中引入,它允许一个或多个线程等待其他线程操作完成之后才执行。    

实现原理 :

    CountDownLatch是通过计数器的方式来实现,计数器的初始值为线程的数量。每当一个线程完成了自己的任务之后,就会对计数器减1,当计数器的值为0时,表示所有线程完成了任务,此时等待在闭锁上的线程才继续执行,从而达到等待其他线程完成任务之后才继续执行的目的。

 

CountDownLatch主要方法:

    CountDownLatch具体是通过同步器来实现的,使用AQS状态来表示计数:

    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

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

        int getCount() {
            return getState();
        }

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

        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;
            }
        }
    }

    private final Sync sync;

  

1、构造函数 

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

通过传入一个数值来创建一个CountDownLatch,数值表示线程可以从等待状态恢复,countDown方法必须被调用的次数

 

2、countDown方法

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

线程调用此方法对count进行减1。当count本来就为0,此方法不做任何操作,当count比0大,调用此方法进行减1,当new count为0,释放所有等待当线程。

 

3、await方法

(1)不带参数

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

调用此方法时,当count为0,直接返回true,当count比0大,线程会一直等待,直到count的值变为0,或者线程被中断(interepted,此时会抛出中断异常)。

(2)带参数

    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

调用此方法时,当count为0,直接返回true,当count比0大,线程会等待一段时间,等待时间内如果count的值变为0,返回true;当超出等待时间,返回false;或者等待时间内线程被中断,此时会抛出中断异常。 

 

CountDownLatch实践

司机和工人,工人必须等到司机来了才能装货上车,司机必须得等到所有工人把货物装上车了之后才能把车开走。

工人类:

public class Worker implements Runnable {

    private String workerCode;

    private CountDownLatch startLatch;
    private CountDownLatch latch;

    Worker(CountDownLatch startLatch, CountDownLatch latch, String workerCode) {
        this.startLatch = startLatch;
        this.latch = latch;
        this.workerCode = workerCode;
    }

    public void run() {
        try {
            startLatch.await();
            doWork();
            latch.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void doWork() {
        System.out.println("Worker " + workerCode + " is loading goods...");
    }
}

司机类:

public class Driver {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch startLatch = new CountDownLatch(1);
        CountDownLatch latch = new CountDownLatch(10);
        ExecutorService executor = Executors.newFixedThreadPool(10);

        for(int i=0; i<10; i++) {
            executor.execute(new Worker(startLatch, latch, "worker" + i));
        }

        System.out.println("Driver is here.");

        startLatch.countDown();

        System.out.println("Workers get to work.");

        latch.await();

        System.out.println("Driver is ready to go.");

        executor.shutdown();
    }
}  

运行结果:

Driver is here.
Workers get to work.
Worker worker0 is loading goods...
Worker worker1 is loading goods...
Worker worker2 is loading goods...
  
		

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解读java同步类CountDownLatch(一) https://www.cppentry.com/bencandy.php?fid=76&id=153842

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