栅栏CyclicBarrier

最新推荐文章于 2024-12-04 08:27:09 发布

huangzebiao007

最新推荐文章于 2024-12-04 08:27:09 发布

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分类专栏： java并发编程

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本文通过一个具体的示例，展示了如何使用 Java 中的 CyclicBarrier 类来同步多个线程，确保它们达到某个公共的同步点后再一并继续执行。这个例子模拟了四位游客同时到达公园的情景。

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（设定的需要相互等待的线程数）时，栅栏将打开。

publicclass BarrierVisitors
{
    publicstaticvoid main(String[] args){
        final String[] visitors =new String[] {"甲","乙","丙","丁" };
        finalint visitorNum = visitors.length;
        final SimpleDateFormat sdf =new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        //设置栅栏，四个游客全部到达后一起进入公园
        final CyclicBarrier arrived =new CyclicBarrier(visitorNum,new Runnable() {
            @Override
            publicvoid run() {
                Date arrivedDate =new Date(System.currentTimeMillis());
                System.out.println(sdf.format(arrivedDate) +" all Arrived!");
            }
        });
       
        for(int i = 0; i < visitorNum; i++){
            new Thread(new BarrierVisitor(visitors[i], sdf,
                            arrived)).start();
            try{
                Thread.sleep(1000);
            }catch(InterruptedException e){
                //TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }  
}
class BarrierVisitorimplements Runnable{
    final CyclicBarrier  arrived;
    final String          name;
    final SimpleDateFormatsdf;
    public BarrierVisitor(final String name, SimpleDateFormat sdf,
                    final CyclicBarrier arrived){
        this.name = name;
        this.arrived = arrived;
        this.sdf = sdf;
    }
    @Override
    publicvoid run(){
        try{           
            //各自从家里出发，模拟从家出发到公园门口
            Date startDate = new Date(System.currentTimeMillis());
            System.out.println(sdf.format(startDate) +" " + this.name
                            + " begin going to the park!");
            Thread.sleep((long) (Math.random() * 10000));
        }catch(Throwable e){
            e.printStackTrace();
        }finally{
            //到达公园
            Date arrivedDate = new Date(System.currentTimeMillis());
            System.out.println(sdf.format(arrivedDate) +" " + this.name
                            + " arrived the park!");
            try{
                arrived.await();
            }catch(InterruptedException e){
                e.printStackTrace();
            }catch(BrokenBarrierException e){
                e.printStackTrace();
            }
        }
    }  
}

输出结果：

2013-10-09 10:38:03 甲 begin going to the park!

2013-10-09 10:38:04 乙 begin going to the park!

2013-10-09 10:38:05 丙 begin going to the park!

2013-10-09 10:38:06 丁 begin going to the park!

2013-10-09 10:38:09 乙 arrived the park!

2013-10-09 10:38:09 丙 arrived the park!

2013-10-09 10:38:12 甲 arrived the park!

2013-10-09 10:38:14 丁 arrived the park!

2013-10-09 10:38:14 all Arrived!

当4个人都到达公园时，都调用了一次await()方法，使栅栏的计数器变为4，等于visitorNum，所以栅栏将打开，执行内部 Runnable里的run方法。