Java基础--CountDownLatch

CountDownLatch是线程同步辅助类，它允许一个或多个线程wait直到countdown被调用使count为0。

CountDownLatch是在java1.5被引入，存在于java.util.concurrent包下。

常用于

1:用于在一个线程中等待N个线程完成

2:让N个子线程同时开始run

方法：

CountDownLatch(int count) 　　构造器，初始化次数

void await() throws InterruptedException　　使一个线程wait，直到count为0

boolean await(long timeout, TimeUnit unit) 　　使一个线程wait，直到count为0或等待时间timeout

void countDown() 　　count减一

long getCount() 　　　　返回当前count

String toString() 　　 返回当前对象及当前count

 

例子：

-----------------

class Driver {
    void main() throws InterruptedException {
        CountDownLatch startSignal = new CountDownLatch(1); // 使线程同时开始
        CountDownLatch doneSignal = new CountDownLatch(10); // 全部结束

        // create and start threads
        for (int i = 0; i < 10; ++i) {
            new Thread(new Worker(startSignal, doneSignal)).start();
        }
        
        // let all threads proceed
        startSignal.countDown();

        // wait for all to finish
        doneSignal.await();
    }
}

class Worker implements Runnable {
    private final CountDownLatch startSignal;
    private final CountDownLatch doneSignal;

    Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
        this.startSignal = startSignal;
        this.doneSignal = doneSignal;
    }

    public void run() {
        try {
            //wait the start signal
            startSignal.await();
            doWork();
            doneSignal.countDown();
        } catch (InterruptedException ex) {
        }
    }

    void doWork() {
    }
}

----------------- 

实现：

AQS静态内部类实现的同步控制

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;
            }
        }
    }

end