也是網上看的一道題目：關於假如有Thread1、Thread2、Thread3、Thread4四條執行緒分別統計C、D、E、F四個盤的大小，所有執行緒都統計完畢交給Thread5執行緒去做彙總，應當如何實現？

蒐集整理了網上朋友提供的方法，主要有：

1. 多執行緒都是Thread或者Runnable(後者比較多)，但是，這兩種都是沒返回值的，所以我們需要使用callable(有返回值的多執行緒)和future(獲得執行緒的返回值)來實現了。

public class TestThread {  
   public static void main(String[] args) {  
       ThreadCount tc = null;  
       ExecutorService es = Executors.newCachedThreadPool();//執行緒池  
       CompletionService<Integer> cs = new ExecutorCompletionService<Integer>(es);  
       for(int i=0;i<4;i++){  
           tc = new ThreadCount(i+1);  
           cs.submit(tc);  
       }  
         
       // 新增結束，及時shutdown，不然主執行緒不會結束  
       es.shutdown();  
         
       int total = 0;  
       for(int i=0;i<4;i++){  
           try {  
               total+=cs.take().get();  
           } catch (InterruptedException e) {  
               e.printStackTrace();  
           } catch (ExecutionException e) {  
               e.printStackTrace();  
           }  
       }  
         
       System.out.println("總大小 = "+total);  
   }  
}  
 
class ThreadCount implements Callable<Integer>{  
   private int type;  
   ThreadCount(int type){  
       this.type = type;  
   }  
   @Override  
   public Integer call() throws Exception {  
       if(type==1){  
           System.out.println("C盤統計大小");  
           return 1;  
       }else if(type==2){   
           System.out.println("D盤統計大小");  
           return 2;  
       }else if(type==3){  
           System.out.println("E盤統計大小");  
           return 3;  
       }else if(type==4){  
           System.out.println("F盤統計大小");  
           return 4;  
       }  
       return null;  
   }  
}  
 

輸出結果：

D盤統計大小
C盤統計大小
E盤統計大小
F盤統計大小
總大小 = 10

分析：四個執行緒實現的是Collable介面，執行完成後返回的值會存放與佇列中，用cs.take.get()獲取返回值獲取佇列中的值

2.用Java.util.concurrent下的方法解決

用CountDownLatch : 一個執行緒(或者多個)， 等待另外N個執行緒完成某個事情之後才能執行

CountDownLatch 是計數器, 執行緒完成一個就記一個, 就像 報數一樣, 只不過是遞減的.

public class CountDownLatchDemo {      
    final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");      
    public static void main(String[] args) throws InterruptedException {      
        CountDownLatch latch=new CountDownLatch(4);//兩個工人的協作      
        Worker worker1=new Worker("C", 1000, latch);      
        Worker worker2=new Worker("D", 1000, latch);  
        Worker worker3=new Worker("E", 1000, latch);    
        Worker worker4=new Worker("F", 1000, latch);    
        worker1.start();//      
        worker2.start();//   
        worker3.start();//     
        worker4.start();//     
        latch.await();//等待所有工人完成工作      
        System.out.println("all work done at "+sdf.format(new Date()));  
        System.out.println(Worker.count);  
    }      
          
          
    static class Worker extends Thread{  
    	static int count = 0;
        String workerName;       
        int workTime;      
        CountDownLatch latch;      
        public Worker(String workerName ,int workTime ,CountDownLatch latch){      
             this.workerName=workerName;      
             this.workTime=workTime;      
             this.latch=latch;      
        }      
        public void run(){  
        	synchronized (Worker.class) {  
    		System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));  
    		int random = new Random().nextInt(5)+1;
    		System.out.println("Worker "+workerName+" size = "+random);      
    		count = count + random;
    		System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));      
    		latch.countDown();//工人完成工作，計數器減一      
        	}
        }      
    }      
         
}  
 

輸出結果：

Worker C do work begin at 2018-11-08 00:23:34
Worker C size = 5
Worker C do work complete at 2018-11-08 00:23:34
Worker F do work begin at 2018-11-08 00:23:34
Worker F size = 4
Worker F do work complete at 2018-11-08 00:23:34
Worker E do work begin at 2018-11-08 00:23:34
Worker E size = 1
Worker E do work complete at 2018-11-08 00:23:34
Worker D do work begin at 2018-11-08 00:23:34
Worker D size = 4
Worker D do work complete at 2018-11-08 00:23:34
all work done at 2018-11-08 00:23:34
14
 

分析：count中存放的是當前統計完成後的總大小，當4個盤的統計執行緒都完成以後，這個時候就可以執行執行緒5，這個與上面的區別是前面的執行緒執行完後沒有返回值，只能確定執行緒執行完成了，但卻沒有返回值。彙總統計沒有放線上程5中，前面4個統計執行緒執行完成以後，彙總統計結果已經出來了。