闭锁:一个同步辅助类,在完成一组正在其他线程中执行的操作之前,它允许一个或多个线程一直等待。即,一组线程等待某一事件发生,事件没有发生前,所有线程将阻塞等待;而事件发生后,所有线程将开始执行;闭锁最初处于封闭状态,当事件发生后闭锁将被打开,一旦打开,闭锁将永远处于打开状态。

  闭锁唯一的构造方法CountDownLatch(int count),当在闭锁上调用countDown()方法时,闭锁的计数器将减1,当闭锁计数器为0时,闭锁将打开,所有线程将通过闭锁开始执行。

  栅栏:一个同步辅助类,它允许一组线程互相等待,直到到达某个公共屏障点。利用栅栏,可以使线程相互等待,直到所有线程都到达某一点,然后栅栏将打开,所有线程将通过栅栏继续执行。CyclicBarrier支持一个可选的  参数,当线程通过栅栏时,runnable对象将被调用。构造函数CyclicBarrier(int parties, Runnable barrierAction),当线程在CyclicBarrier对象上调用await()方法时,栅栏的计数器将增加1,当计数器为parties时,栅栏将打开。

  区别:闭锁用于所有线程等待一个外部事件的发生;栅栏则是所有线程相互等待,直到所有线程都到达某一点时才打开栅栏,然后线程可以继续执行。

闭锁示例:

  有五个人,一个裁判。这五个人同时跑,裁判开始计时,五个人都到终点了,裁判喊停,然后统计这五个人从开始跑到最后一个撞线用了多长时间。

import java.util.concurrent.CountDownLatch;

public class Race {

    public static void main(String[] args) {
final int num = 5;
final CountDownLatch begin = new CountDownLatch(1);
final CountDownLatch end = new CountDownLatch(num);
for (int i = 0; i < num; i++) {
new Thread(new AWorker(i, begin, end)).start();
} // judge prepare...
try {
Thread.sleep((long) (Math.random() * 5000));
} catch (InterruptedException e1) {
e1.printStackTrace();
} System.out.println("judge say : run !");
begin.countDown();
long startTime = System.currentTimeMillis(); try {
end.await();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
long endTime = System.currentTimeMillis();
System.out.println("judge say : all arrived !");
System.out.println("spend time: " + (endTime - startTime));
} }
}
class AWorker implements Runnable {
final CountDownLatch begin;
final CountDownLatch end;
final int id; public AWorker(final int id, final CountDownLatch begin,
final CountDownLatch end) {
this.id = id;
this.begin = begin;
this.end = end;
} @Override
public void run() {
try {
System.out.println(this.id + " ready !");
begin.await();
// run...
Thread.sleep((long) (Math.random() * 10000));
} catch (Throwable e) {
e.printStackTrace();
} finally {
System.out.println(this.id + " arrived !");
end.countDown();
}
} }

栅栏示例:

  还是这五个人(这五个人真无聊..),这次没裁判。规定五个人只要都跑到终点了,大家可以喝啤酒。但是,只要有一个人没到终点,就不能喝。 这里也没有要求大家要同时起跑(当然也可以,加latch)。

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier; public class Beer { public static void main(String[] args) {
final int count = 5;
final CyclicBarrier barrier = new CyclicBarrier(count, new Runnable() {
@Override
public void run() {
System.out.println("drink beer!");
}
});
// they do not have to start at the same time...
for (int i = 0; i < count; i++) {
new Thread(new Worker(i, barrier)).start();
}
} } class Worker implements Runnable {
final int id;
final CyclicBarrier barrier; public Worker(final int id, final CyclicBarrier barrier) {
this.id = id;
this.barrier = barrier;
} @Override
public void run() {
try {
System.out.println(this.id + "starts to run !");
Thread.sleep((long) (Math.random() * 10000));
System.out.println(this.id + "arrived !");
this.barrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
05-27 09:06