JAVA栅栏和闭锁的区别

最近在看java并发相关的书籍，有一些同步的知识是平常不常用但是觉得会很有用的东西，这里参考别人的文章和自己的理解，将闭锁和栅栏的用途与区别简单描述一下。

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CountDownLatch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 Runnable 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。

示例：引用自http://aaron-han.iteye.com/blog/1591755

1. 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();
        }
    }

}

    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();  
            }  
        }  

    }  

2. 继续，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();
        }
    }
}

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();
        }
    }
}

http://blog.csdn.net/lmc_wy/article/details/7866863 转载