Java多线程面试题：假如有Thread1、Thread2、Thread3、Thread4四条线程分别统计C、D、E、F四个盘的大小，所有线程都统计完毕交给Thread5线程去做汇总，应当如何实现？

利用java.util.concurrent包下的CountDownLatch（减数器）或CyclicBarrier（循环栅栏）可以实现此类问题

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1. 利用CountDownLatch 的代码实现

public class test1{
public static void main(String[] args) throws Exception {
        //创建一个能容纳4个线程的减数器
        final CountDownLatch countDownLatch= new CountDownLatch(4);
        Runnable runC= new Runnable() {
            
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("统计C盘");
                    countDownLatch.countDown();//单任务，把计数器减1
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        };
        Runnable runD= new Runnable() {
            
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("统计D盘");
                    countDownLatch.countDown();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        };
        Runnable runE= new Runnable() {
            
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("统计E盘");
                    countDownLatch.countDown();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        };
        Runnable runF= new Runnable() {
            
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("统计F盘");
                    countDownLatch.countDown();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        };
        
        ExecutorService service= Executors.newFixedThreadPool(4);
        service.submit(runC);
        service.submit(runD);
        service.submit(runE);
        service.submit(runF);
        
        countDownLatch.await();//主线程，即第5线程等待
        System.out.println("合计C,D,E,F");
        service.shutdown();
}

}

　　

2. 利用CyclicBarrier的代码实现

public class test2{
public static void main(String[] args) {
        Runnable barrierAction= new Runnable() {
            @Override
            public void run() {
                System.out.println("统计C,D,E,F盘");
            }
        };
        
        final CyclicBarrier cyclicBarrier= new CyclicBarrier(4, barrierAction );
 
       Runnable runC= new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("C盘");
                    cyclicBarrier.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
                
            }
        };

        Runnable runD= new Runnable() {   
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("D盘");
                    cyclicBarrier.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
                
            }
        };
 
       Runnable runE= new Runnable() {           
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("E盘");
                    cyclicBarrier.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
                
            }
        };
 
       Runnable runF= new Runnable() {            
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("F盘");
                    cyclicBarrier.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
                
            }
        };
        
        ExecutorService service= Executors .newFixedThreadPool(4);
        service.submit(runC);
        service.submit(runD);
        service.submit(runE);
        service.submit(runF);
        service.shutdown();
}
}