java中等待所有线程都执行结束

Posted 2020-09-20 鸭子船长

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转自：http://blog.csdn.net/liweisnake/article/details/12966761

今天看到一篇文章，是关于java中如何等待所有线程都执行结束，文章总结得很好，原文如下http://software.intel.com/zh-cn/blogs/2013/10/15/java-countdownlatchcyclicbarrier/?utm_campaign=CSDN&utm_source=intel.csdn.net&utm_medium=Link&utm_content=others-%20Java

看过之后在想java中有很大的灵活性，应该有更多的方式可以做这件事。

这个事情的场景是这样的：许多线程并行的计算一堆问题，然后每个计算存在一个队列，在主线程要等待所有计算结果完成后排序并展示出来。这样的问题其实很常见。

1. 使用join。这种方式其实并不是那么的优雅，将所有线程启动完之后还需要将所有线程都join，但是每次join都会阻塞，直到被join线程完成，很可能所有被阻塞线程已经完事了，主线程还在不断地join，貌似有点浪费，而且两个循环也不太好看。

 1 public void testThreadSync1() {  
 2   
 3     final Vector<Integer> list = new Vector<Integer>();  
 4     Thread[] threads = new Thread[TEST_THREAD_COUNT];  
 5     try {  
 6         for (int i = 0; i < TEST_THREAD_COUNT; i++) {  
 7             final int num = i;  
 8             threads[i] = new Thread(new Runnable() {  
 9                 public void run() {  
10                     try {  
11                         Thread.sleep(random.nextInt(100));  
12                     } catch (InterruptedException e) {  
13                         e.printStackTrace();  
14                     }  
15                     list.add(num);  
16                     System.out.print(num + " add.\t");  
17                 }  
18             });  
19             threads[i].start();  
20         }  
21         for (int i = 0; i < threads.length; i++) {  
22             threads[i].join();  
23             System.out.print(i + " end.\t");  
24         }  
25     } catch (InterruptedException ie) {  
26         ie.printStackTrace();  
27     }  
28     printSortedResult(list);  
29 }

1 9 add.  7 add.  3 add.  5 add.  4 add.  1 add.  0 add.  0 end.  1 end.  8 add.  2 add.  2 end.  3 end.  4 end.  5 end.  6 add.  6 end.  7 end.  8 end.  9 end.    
2 before sort  
3 9   7   3   5   4   1   0   8   2   6     
4 after sort  
5 0   1   2   3   4   5   6   7   8   9

2. 使用wait/notifyAll，这个方式其实跟上面是类似的，只是比较底层些吧(join实际上也是wait)。

 1 @Test  
 2 public void testThreadSync2() throws IOException, InterruptedException {  
 3     final Object waitObject = new Object();  
 4     final AtomicInteger count = new AtomicInteger(TEST_THREAD_COUNT);  
 5     final Vector<Integer> list = new Vector<Integer>();  
 6     Thread[] threads = new Thread[TEST_THREAD_COUNT];  
 7     for (int i = 0; i < TEST_THREAD_COUNT; i++) {  
 8         final int num = i;  
 9         threads[i] = new Thread(new Runnable() {  
10             public void run() {  
11                 try {  
12                     Thread.sleep(random.nextInt(100));  
13                 } catch (InterruptedException e) {  
14                     e.printStackTrace();  
15                 }  
16                 list.add(num);  
17                 System.out.print(num + " add.\t");  
18                 synchronized (waitObject) {  
19                     int cnt = count.decrementAndGet();  
20                     if (cnt == 0) {  
21                         waitObject.notifyAll();  
22                     }  
23                 }  
24             }  
25         });  
26         threads[i].start();  
27     }  
28     synchronized (waitObject) {  
29         while (count.get() != 0) {  
30             waitObject.wait();  
31         }  
32     }  
33     printSortedResult(list);  
34 }

3. 使用CountDownLatch，这其实是最优雅的写法了，每个线程完成后都去将计数器减一，最后完成时再来唤醒。

例1

 1 @Test  
 2 public void testThreadSync3() {  
 3     final Vector<Integer> list = new Vector<Integer>();  
 4     Thread[] threads = new Thread[TEST_THREAD_COUNT];  
 5     final CountDownLatch latch = new CountDownLatch(TEST_THREAD_COUNT);  
 6     for (int i = 0; i < TEST_THREAD_COUNT; i++) {  
 7         final int num = i;  
 8         threads[i] = new Thread(new Runnable() {  
 9             public void run() {  
10                 try {  
11                     Thread.sleep(random.nextInt(100));  
12                 } catch (InterruptedException e) {  
13                     e.printStackTrace();  
14                 }  
15                 list.add(num);  
16                 System.out.print(num + " add.\t");  
17                 latch.countDown();  
18             }  
19         });  
20         threads[i].start();  
21     }  
22     try {  
23         latch.await();  
24     } catch (InterruptedException e) {  
25         e.printStackTrace();  
26     }  
27     printSortedResult(list);  
28 }

例2

CountDownLatch 初始化设置count，即等待(await)count个线程或一个线程count次计数，通过工作线程来countDown计数减一，直到计数为0，await阻塞结束。

设置的count不可更改，如需要动态设置计数的线程数，可以使用CyclicBarrier.

下面的例子，所有的工作线程中准备就绪以后，并不是直接运行，而是等待主线程的信号后再执行具体的操作。

  1 package com.example.multithread;  
  2   
  3 import java.util.concurrent.CountDownLatch;  
  4   
  5 class Driver  
  6 {  
  7     private static final int TOTAL_THREADS = 10;  
  8     private final CountDownLatch mStartSignal = new CountDownLatch(1);  
  9     private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS);  
 10   
 11     void main()  
 12     {  
 13         for (int i = 0; i < TOTAL_THREADS; i++)  
 14         {  
 15             new Thread(new Worker(mStartSignal, mDoneSignal, i)).start();  
 16         }  
 17         System.out.println("Main Thread Now:" + System.currentTimeMillis());  
 18         doPrepareWork();// 准备工作   
 19         mStartSignal.countDown();// 计数减一为0，工作线程真正启动具体操作   
 20         doSomethingElse();//做点自己的事情   
 21         try  
 22         {  
 23             mDoneSignal.await();// 等待所有工作线程结束   
 24         }  
 25         catch (InterruptedException e)  
 26         {  
 27             // TODO Auto-generated catch block   
 28             e.printStackTrace();  
 29         }  
 30         System.out.println("All workers have finished now.");  
 31         System.out.println("Main Thread Now:" + System.currentTimeMillis());  
 32     }  
 33   
 34     void doPrepareWork()  
 35     {  
 36         System.out.println("Ready,GO!");  
 37     }  
 38   
 39     void doSomethingElse()  
 40     {  
 41         for (int i = 0; i < 100000; i++)  
 42         {  
 43             ;// delay   
 44         }  
 45         System.out.println("Main Thread Do something else.");  
 46     }  
 47 }  
 48   
 49 class Worker implements Runnable  
 50 {  
 51     private final CountDownLatch mStartSignal;  
 52     private final CountDownLatch mDoneSignal;  
 53     private final int mThreadIndex;  
 54   
 55     Worker(final CountDownLatch startSignal, final CountDownLatch doneSignal,  
 56             final int threadIndex)  
 57     {  
 58         this.mDoneSignal = doneSignal;  
 59         this.mStartSignal = startSignal;  
 60         this.mThreadIndex = threadIndex;  
 61     }  
 62   
 63     @Override  
 64     public void run()  
 65     {  
 66         // TODO Auto-generated method stub   
 67         try  
 68         {  
 69             mStartSignal.await();// 阻塞，等待mStartSignal计数为0运行后面的代码   
 70                                     // 所有的工作线程都在等待同一个启动的命令   
 71             doWork();// 具体操作   
 72             System.out.println("Thread " + mThreadIndex + " Done Now:"  
 73                     + System.currentTimeMillis());  
 74             mDoneSignal.countDown();// 完成以后计数减一   
 75         }  
 76         catch (InterruptedException e)  
 77         {  
 78             // TODO Auto-generated catch block   
 79             e.printStackTrace();  
 80         }  
 81     }  
 82   
 83     public void doWork()  
 84     {  
 85         for (int i = 0; i < 1000000; i++)  
 86         {  
 87             ;// 耗时操作   
 88         }  
 89         System.out.println("Thread " + mThreadIndex + ":do work");  
 90     }  
 91 }  
 92   
 93 public class CountDownLatchTest  
 94 {  
 95     public static void main(String[] args)  
 96     {  
 97         // TODO Auto-generated method stub   
 98         new Driver().main();  
 99     }  
100   
101 }

通过Executor启动线程：

 1 class CountDownLatchDriver2  
 2 {  
 3     private static final int TOTAL_THREADS = 10;  
 4     private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS);  
 5  
 6 
 7   
 8     void main()  
 9     {  
10         System.out.println("Main Thread Now:" + System.currentTimeMillis());  
11         doPrepareWork();// 准备工作   
12   
13         Executor executor = Executors.newFixedThreadPool(TOTAL_THREADS);  
14         for (int i = 0; i < TOTAL_THREADS; i++)  
15         {  
16             // 通过内建的线程池维护创建的线程   
17             executor.execute(new RunnableWorker(mDoneSignal, i));  
18         }  
19         doSomethingElse();// 做点自己的事情   
20         try  
21         {  
22             mDoneSignal.await();// 等待所有工作线程结束   
23         }  
24         catch (InterruptedException e)  
25         {  
26             // TODO Auto-generated catch block   
27             e.printStackTrace();  
28         }  
29         System.out.println("All workers have finished now.");  
30         System.out.println("Main Thread Now:" + System.currentTimeMillis());  
31     }  
32   
33     void doPrepareWork()  
34     {  
35         System.out.println("Ready,GO!");  
36     }  
37   
38     void doSomethingElse()  
39     {  
40         for (int i = 0; i < 100000; i++)  
41         {  
42             ;// delay   
43         }  
44         System.out.println("Main Thread Do something else.");  
45     }  
46 }  
47   
48 class RunnableWorker implements Runnable  
49 {  
50   
51     private final CountDownLatch mDoneSignal;  
52     private final int mThreadIndex;  
53   
54     RunnableWorker(final CountDownLatch doneSignal, final int threadIndex)  
55     {  
56         this.mDoneSignal = doneSignal;  
57         this.mThreadIndex = threadIndex;  
58     }  
59   
60     @Override  
61     public void run()  
62     {  
63         // TODO Auto-generated method stub   
64   
65         doWork();// 具体操作   
66         System.out.println("Thread " + mThreadIndex + " Done Now:"  
67                 + System.currentTimeMillis());  
68         mDoneSignal.countDown();// 完成以后计数减一   
69                                 // 计数为0时，主线程接触阻塞，继续执行其他任务   
70         try  
71         {  
72             // 可以继续做点其他的事情，与主线程无关了   
73             Thread.sleep(5000);  
74             System.out.println("Thread " + mThreadIndex  
75                     + " Do something else after notifing main thread");  
76   
77         }  
78         catch (InterruptedException e)  
79         {  
80             // TODO Auto-generated catch block   
81             e.printStackTrace();  
82         }  
83   
84     }  
85   
86     public void doWork()  
87     {  
88         for (int i = 0; i < 1000000; i++)  
89         {  
90             ;// 耗时操作   
91         }  
92         System.out.println("Thread " + mThreadIndex + ":do work");  
93     }  
94 }

输出：

 1 Main Thread Now:1359959480786
 2 Ready,GO!
 3 Thread 0:do work
 4 Thread 0 Done Now:1359959480808
 5 Thread 1:do work
 6 Thread 1 Done Now:1359959480811
 7 Thread 2:do work
 8 Thread 2 Done Now:1359959480813
 9 Main Thread Do something else.
10 Thread 3:do work
11 Thread 3 Done Now:1359959480825
12 Thread 5:do work
13 Thread 5 Done Now:1359959480827
14 Thread 7:do work
15 Thread 7 Done Now:1359959480829
16 Thread 9:do work
17 Thread 9 Done Now:1359959480831
18 Thread 4:do work
19 Thread 4 Done Now:1359959480833
20 Thread 6:do work
21 Thread 6 Done Now:1359959480835
22 Thread 8:do work
23 Thread 8 Done Now:1359959480837
24 All workers have finished now.
25 Main Thread Now:1359959480838
26 Thread 0 Do something else after notifing main thread
27 Thread 1 Do something else after notifing main thread
28 Thread 2 Do something else after notifing main thread
29 Thread 3 Do something else after notifing main thread
30 Thread 9 Do something else after notifing main thread
31 Thread 7 Do something else after notifing main thread
32 Thread 5 Do something else after notifing main thread
33 Thread 4 Do something else after notifing main thread
34 Thread 6 Do something else after notifing main thread
35 Thread 8 Do something else after notifing main thread

4. 使用CyclicBarrier。这里其实类似上面，这个berrier只是在等待完成后自动调用传入CyclicBarrier的Runnable。

例1

 1 @Test  
 2 public void testThreadSync4() throws IOException {  
 3     final Vector<Integer> list = new Vector<Integer>();  
 4     Thread[] threads = new Thread[TEST_THREAD_COUNT];  
 5     final CyclicBarrier barrier = new CyclicBarrier(TEST_THREAD_COUNT,  
 6             new Runnable() {  
 7                 public void run() {  
 8                     printSortedResult(list);  
 9                 }  
10             });  
11     for (int i = 0; i < TEST_THREAD_COUNT; i++) {  
12         final int num = i;  
13         threads[i] = new Thread(new Runnable() {  
14             public void run() {  
15                 try {  
16                     Thread.sleep(random.nextInt(100));  
17                 } catch (InterruptedException e) {  
18                     e.printStackTrace();  
19                 }  
20                 list.add(num);  
21                 System.out.print(num + " add.\t");  
22                 try {  
23                     barrier.await();  
24                 } catch (InterruptedException e) {  
25                     e.printStackTrace();  
26                 } catch (BrokenBarrierException e) {  
27                     e.printStackTrace();  
28                 }  
29             }  
30         });  
31         threads[i].start();  
32     }  
33     System.in.read();  
34 }

例2

 1 class WalkTarget  
 2 {  
 3     private final int mCount = 5;  
 4     private final CyclicBarrier mBarrier;  
 5     ExecutorService mExecutor;  
 6   
 7     class BarrierAction implements Runnable  
 8     {  
 9         @Override  
10         public void run()  
11         {  
12             // TODO Auto-generated method stub   
13             System.out.println("所有线程都已经完成任务,计数达到预设值");  
14             //mBarrier.reset();//恢复到初始化状态          
15               
16         }  
17     }  
18   
19     WalkTarget()  
20     {  
21         //初始化CyclicBarrier   
22         mBarrier = new CyclicBarrier(mCount, new BarrierAction());  
23         mExecutor = Executors.newFixedThreadPool(mCount);  
24   
25         for (int i = 0; i < mCount; i++)  
26         {  
27             //启动工作线程   
28             mExecutor.execute(new Walker(mBarrier, i));  
29         }  
30     }  
31 }  
32   
33 //工作线程   
34 class Walker implements Runnable  
35 {  
36     private final CyclicBarrier mBarrier;  
37     private final int mThreadIndex;  
38   
39     Walker(final CyclicBarrier barrier, final int threadIndex)  
40  
41 
42     {  
43         mBarrier = barrier;  
44         mThreadIndex = threadIndex;  
45     }  
46   
47     @Override  
48     public void run()  
49     {  
50         // TODO Auto-generated method stub   
51         System.out.println("Thread " + mThreadIndex + " is running...");  
52         // 执行任务   
53         try  
54         {  
55             TimeUnit.MILLISECONDS.sleep(5000);  
56             // do task   
57         }  
58         catch (InterruptedException e)  
59         {  
60             // TODO Auto-generated catch block   
61             e.printStackTrace();  
62         }  
63   
64         // 完成任务以后，等待其他线程完成任务   
65         try  
66         {  
67             mBarrier.await();  
68         }  
69         catch (InterruptedException e)  
70         {  
71             // TODO Auto-generated catch block   
72             e.printStackTrace();  
73         }  
74         catch (BrokenBarrierException e)  
75         {  
76             // TODO Auto-generated catch block   
77             e.printStackTrace();  
78         }  
79         // 其他线程任务都完成以后，阻塞解除，可以继续接下来的任务   
80         System.out.println("Thread " + mThreadIndex + " do something else");  
81     }  
82   
83 }  
84   
85 public class CountDownLatchTest  
86 {  
87     public static void main(String[] args)  
88     {  
89         // TODO Auto-generated method stub   
90         //new CountDownLatchDriver2().main();   
91         new WalkTarget();  
92     }  
93   
94 }

输出(注意，只有所有的线程barrier.await之后才能继续执行其他的操作）：

Thread 0 is running... Thread 2 is running... Thread 3 is running... Thread 1 is running... Thread 4 is running... 所有线程都已经完成任务,计数达到预设值 Thread 4 do something else Thread 0 do something else Thread 2 do something else Thread 3 do something else Thread 1 do something else

5、

CountDownLatch和CyclicBarrier简单比较：

	CountDownLatch	CyclicBarrier
软件包	java.util.concurrent	java.util.concurrent
适用情景	主线程等待多个工作线程结束	多个线程之间互相等待，直到所有线程达到一个障碍点(Barrier point)
主要方法	CountDownLatch(int count) （主线程调用）初始化计数 CountDownLatch.await （主线程调用）阻塞，直到等待计数为0解除阻塞 CountDownLatch.countDown 计数减一（工作线程调用）	CyclicBarrier(int parties, Runnable barrierAction) //初始化参与者数量和障碍点执行Action，Action可选。由主线程初始化 CyclicBarrier.await() //由参与者调用阻塞，直到所有线程达到屏障点
等待结束	各线程之间不再互相影响，可以继续做自己的事情。不再执行下一个目标工作。	在屏障点达到后，允许所有线程继续执行，达到下一个目标。可以重复使用CyclicBarrier
异常		如果其中一个线程由于中断，错误，或超时导致永久离开屏障点，其他线程也将抛出异常。
其他		如果BarrierAction不依赖于任何Party中的所有线程，那么在任何party中的一个线程被释放的时候，可以直接运行这个Action。 If(barrier.await()==2) { //do action }

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