Java并发编程之CyclicBarrier的使用

Java并发编程之CyclicBarrier的使用

 

先看javadoc对这个类的说明：A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point. CyclicBarriers are useful in programs involving a fixed sized party of threads that must occasionally wait for each other. The barrier is called cyclic because it can be re-used after the waiting threads are released.

什么时候用这个类？当有多个线程完成类似的任务，如果需要每个线程在执行任务中的某个地方停下来，等待其它线程也到达相同的等待点(其实不同的等待点也行)，那这个类就完美解决了这个需求，比如(后面有代码)：有多个跑步的人，各自跑的速度不一样，但是大家需要在中途某个点聚一聚，然后在一起出发。

 

以下是CyclicBarrier的public方法：

 

 

关于两个参数的构造方法：

 也就是说如果用了这个方法，那么当等待在await()方法上的线程数 = getParties() 时，该runnable会被最后一个等待的线程去跑。

 

 

关于reset()方法:

调用该方法时，若有线程等待在await方法上，则这些线程都会收到一个BrokenBarrierException异常。

 

关于isBroken()方法:

只要任何一个等待在await()方法上的线程非正常地从await()上返回，那么isBroken()就返回true。

 

示例：

说明：两个人AAA、BBB，分别以1m/s、2m/s速度跑，若跑到6m处时还有人未到达6m处，则等待所有人都到达时才出发。跑完10m结束

 package cyclicbarrier;

 import java.util.concurrent.BrokenBarrierException;
 import java.util.concurrent.CyclicBarrier;

 public class RunningMan implements Runnable{

     private int speed; // 1 m/s
     private int waitAfterMeters; // m
     private int totalMeters; // m
     private String name;

     private CyclicBarrier barrier;

     public RunningMan(int speed, int waitAfterMeters, int totalMeters) {
         this.speed = speed;
         this.waitAfterMeters = waitAfterMeters;
         this.totalMeters = totalMeters;
     }

     public RunningMan barrier(CyclicBarrier barrier) {
         this.barrier = barrier;
         return this;
     }

     public RunningMan name(String name) {
         this.name = name;
         return this;
     }

     private boolean needToWait() {
         return barrier != null;
     }

     private long time() {
         return System.currentTimeMillis() / 1000;
     }

     @Override
     public void run() {
         System.out.println(time() + ": runningman " + name + " start running.");
         int ranMiles = 0;// 跑了多少m
         while (ranMiles < totalMeters) {
             // 跑1s
             try {
                 Thread.sleep(1000 * 1);
             } catch (InterruptedException e) {
                 // 当前线程被中断了，那就不跑了
                 Thread.currentThread().interrupt();
                 break;
             }
             //计数
             ranMiles += speed;
             //是否到达指定等待点
             if (ranMiles == waitAfterMeters) {
                 if (!needToWait()) { //没有barrier
                     continue;
                 }

                 // 是否是最后一个到达指定等待点的
                 if (barrier.getNumberWaiting() + 1 == barrier.getParties()) {
                     System.out.println(time() + ": runningman " + name + " is the last one reached at " + waitAfterMeters);
                 } else {
                     System.out.println(time() + ": runningman " + name + " reached at " + waitAfterMeters + ", waiting for others");
                 }
                 // 等待, 直到barrier.getNumberWaiting() == barrier.getParties()时返回
                 try {
                     barrier.await();
                 } catch (InterruptedException e) {
                     // 当前线程被中断了，那就不跑了
                     Thread.currentThread().interrupt();
                     break;
                 } catch (BrokenBarrierException e) {
                     System.err.println("someone reset the barrier state.");
                     break;
                 }
             } else {
                 System.out.println(time() + ": runningman " + name + " reached at " + ranMiles);
             }
         }
         System.out.println(time() + ": runningman " + name + " done running.");
     }

     public static void main(String[] args) {
         CyclicBarrier barrier = new CyclicBarrier(2);
         RunningMan rm1 = new RunningMan(1, 6, 10).name("AAA").barrier(barrier);
         RunningMan rm2 = new RunningMan(2, 6, 10).name("BBB").barrier(barrier);
         Thread t1 = new Thread(rm1);
         Thread t2 = new Thread(rm2);
         t1.start();
         t2.start();
     }
 }

 结果：