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

巴特西

Java基础--CountDownLatch

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