java中等待所有线程都执行结束

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。

package com.example.multithread;  
  
import java.util.concurrent.CountDownLatch;  
  
class Driver  
{  
    private static final int TOTAL_THREADS = 10;  
    private final CountDownLatch mStartSignal = new CountDownLatch(1);  
    private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS);  
  
    void main()  
    {  
        for (int i = 0; i < TOTAL_THREADS; i++)  
        {  
            new Thread(new Worker(mStartSignal, mDoneSignal, i)).start();  
        }  
        System.out.println("Main Thread Now:" + System.currentTimeMillis());  
        doPrepareWork();// 准备工作   
        mStartSignal.countDown();// 计数减一为0，工作线程真正启动具体操作   
        doSomethingElse();//做点自己的事情   
        try  
        {  
            mDoneSignal.await();// 等待所有工作线程结束   
        }  
        catch (InterruptedException e)  
        {  
            // TODO Auto-generated catch block   
            e.printStackTrace();  
        }  
        System.out.println("All workers have finished now.");  
        System.out.println("Main Thread Now:" + System.currentTimeMillis());  
    }  
  
    void doPrepareWork()  
    {  
        System.out.println("Ready,GO!");  
    }  
  
    void doSomethingElse()  
    {  
        for (int i = 0; i < 100000; i++)  
        {  
            ;// delay   
        }  
        System.out.println("Main Thread Do something else.");  
    }  
}  
  
class Worker implements Runnable  
{  
    private final CountDownLatch mStartSignal;  
    private final CountDownLatch mDoneSignal;  
    private final int mThreadIndex;  
  
    Worker(final CountDownLatch startSignal, final CountDownLatch doneSignal,  
            final int threadIndex)  
    {  
        this.mDoneSignal = doneSignal;  
        this.mStartSignal = startSignal;  
        this.mThreadIndex = threadIndex;  
    }  
  
    @Override  
    public void run()  
    {  
        // TODO Auto-generated method stub   
        try  
        {  
            mStartSignal.await();// 阻塞，等待mStartSignal计数为0运行后面的代码   
                                    // 所有的工作线程都在等待同一个启动的命令   
            doWork();// 具体操作   
            System.out.println("Thread " + mThreadIndex + " Done Now:"  
                    + System.currentTimeMillis());  
            mDoneSignal.countDown();// 完成以后计数减一   
        }  
        catch (InterruptedException e)  
        {  
            // TODO Auto-generated catch block   
            e.printStackTrace();  
        }  
    }  
  
    public void doWork()  
    {  
        for (int i = 0; i < 1000000; i++)  
        {  
            ;// 耗时操作   
        }  
        System.out.println("Thread " + mThreadIndex + ":do work");  
    }  
}  
  
public class CountDownLatchTest  
{  
    public static void main(String[] args)  
    {  
        // TODO Auto-generated method stub   
        new Driver().main();  
    }  
  
}

通过Executor启动线程：

class CountDownLatchDriver2  
{  
    private static final int TOTAL_THREADS = 10;  
    private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS);  
 

  
    void main()  
    {  
        System.out.println("Main Thread Now:" + System.currentTimeMillis());  
        doPrepareWork();// 准备工作   
  
        Executor executor = Executors.newFixedThreadPool(TOTAL_THREADS);  
        for (int i = 0; i < TOTAL_THREADS; i++)  
        {  
            // 通过内建的线程池维护创建的线程   
            executor.execute(new RunnableWorker(mDoneSignal, i));  
        }  
        doSomethingElse();// 做点自己的事情   
        try  
        {  
            mDoneSignal.await();// 等待所有工作线程结束   
        }  
        catch (InterruptedException e)  
        {  
            // TODO Auto-generated catch block   
            e.printStackTrace();  
        }  
        System.out.println("All workers have finished now.");  
        System.out.println("Main Thread Now:" + System.currentTimeMillis());  
    }  
  
    void doPrepareWork()  
    {  
        System.out.println("Ready,GO!");  
    }  
  
    void doSomethingElse()  
    {  
        for (int i = 0; i < 100000; i++)  
        {  
            ;// delay   
        }  
        System.out.println("Main Thread Do something else.");  
    }  
}  
  
class RunnableWorker implements Runnable  
{  
  
    private final CountDownLatch mDoneSignal;  
    private final int mThreadIndex;  
  
    RunnableWorker(final CountDownLatch doneSignal, final int threadIndex)  
    {  
        this.mDoneSignal = doneSignal;  
        this.mThreadIndex = threadIndex;  
    }  
  
    @Override  
    public void run()  
    {  
        // TODO Auto-generated method stub   
  
        doWork();// 具体操作   
        System.out.println("Thread " + mThreadIndex + " Done Now:"  
                + System.currentTimeMillis());  
        mDoneSignal.countDown();// 完成以后计数减一   
                                // 计数为0时，主线程接触阻塞，继续执行其他任务   
        try  
        {  
            // 可以继续做点其他的事情，与主线程无关了   
            Thread.sleep(5000);  
            System.out.println("Thread " + mThreadIndex  
                    + " Do something else after notifing main thread");  
  
        }  
        catch (InterruptedException e)  
        {  
            // TODO Auto-generated catch block   
            e.printStackTrace();  
        }  
  
    }  
  
    public void doWork()  
    {  
        for (int i = 0; i < 1000000; i++)  
        {  
            ;// 耗时操作   
        }  
        System.out.println("Thread " + mThreadIndex + ":do work");  
    }  
}

class CountDownLatchDriver2  
{  
    private static final int TOTAL_THREADS = 10;  
    private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS);  
 

  
    void main()  
    {  
        System.out.println("Main Thread Now:" + System.currentTimeMillis());  
        doPrepareWork();// 准备工作   
  
        Executor executor = Executors.newFixedThreadPool(TOTAL_THREADS);  
        for (int i = 0; i < TOTAL_THREADS; i++)  
        {  
            // 通过内建的线程池维护创建的线程   
            executor.execute(new RunnableWorker(mDoneSignal, i));  
        }  
        doSomethingElse();// 做点自己的事情   
        try  
        {  
            mDoneSignal.await();// 等待所有工作线程结束   
        }  
        catch (InterruptedException e)  
        {  
            // TODO Auto-generated catch block   
            e.printStackTrace();  
        }  
        System.out.println("All workers have finished now.");  
        System.out.println("Main Thread Now:" + System.currentTimeMillis());  
    }  
  
    void doPrepareWork()  
    {  
        System.out.println("Ready,GO!");  
    }  
  
    void doSomethingElse()  
    {  
        for (int i = 0; i < 100000; i++)  
        {  
            ;// delay   
        }  
        System.out.println("Main Thread Do something else.");  
    }  
}  
  
class RunnableWorker implements Runnable  
{  
  
    private final CountDownLatch mDoneSignal;  
    private final int mThreadIndex;  
  
    RunnableWorker(final CountDownLatch doneSignal, final int threadIndex)  
    {  
        this.mDoneSignal = doneSignal;  
        this.mThreadIndex = threadIndex;  
    }  
  
    @Override  
    public void run()  
    {  
        // TODO Auto-generated method stub   
  
        doWork();// 具体操作   
        System.out.println("Thread " + mThreadIndex + " Done Now:"  
                + System.currentTimeMillis());  
        mDoneSignal.countDown();// 完成以后计数减一   
                                // 计数为0时，主线程接触阻塞，继续执行其他任务   
        try  
        {  
            // 可以继续做点其他的事情，与主线程无关了   
            Thread.sleep(5000);  
            System.out.println("Thread " + mThreadIndex  
                    + " Do something else after notifing main thread");  
  
        }  
        catch (InterruptedException e)  
        {  
            // TODO Auto-generated catch block   
            e.printStackTrace();  
        }  
  
    }  
  
    public void doWork()  
    {  
        for (int i = 0; i < 1000000; i++)  
        {  
            ;// 耗时操作   
        }  
        System.out.println("Thread " + mThreadIndex + ":do work");  
    }  
}

输出：

Main Thread Now:1359959480786
Ready,GO!
Thread 0:do work
Thread 0 Done Now:1359959480808
Thread 1:do work
Thread 1 Done Now:1359959480811
Thread 2:do work
Thread 2 Done Now:1359959480813
Main Thread Do something else.
Thread 3:do work
Thread 3 Done Now:1359959480825
Thread 5:do work
Thread 5 Done Now:1359959480827
Thread 7:do work
Thread 7 Done Now:1359959480829
Thread 9:do work
Thread 9 Done Now:1359959480831
Thread 4:do work
Thread 4 Done Now:1359959480833
Thread 6:do work
Thread 6 Done Now:1359959480835
Thread 8:do work
Thread 8 Done Now:1359959480837
All workers have finished now.
Main Thread Now:1359959480838
Thread 0 Do something else after notifing main thread
Thread 1 Do something else after notifing main thread
Thread 2 Do something else after notifing main thread
Thread 3 Do something else after notifing main thread
Thread 9 Do something else after notifing main thread
Thread 7 Do something else after notifing main thread
Thread 5 Do something else after notifing main thread
Thread 4 Do something else after notifing main thread
Thread 6 Do something else after notifing main thread
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

@Test  
public void testThreadSync4() throws IOException {  
    final Vector<Integer> list = new Vector<Integer>();  
    Thread[] threads = new Thread[TEST_THREAD_COUNT];  
    final CyclicBarrier barrier = new CyclicBarrier(TEST_THREAD_COUNT,  
            new Runnable() {  
                public void run() {  
                    printSortedResult(list);  
                }  
            });  
    for (int i = 0; i < TEST_THREAD_COUNT; i++) {  
        final int num = i;  
        threads[i] = new Thread(new Runnable() {  
            public void run() {  
                try {  
                    Thread.sleep(random.nextInt(100));  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                }  
                list.add(num);  
                System.out.print(num + " add.\t");  
                try {  
                    barrier.await();  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                } catch (BrokenBarrierException e) {  
                    e.printStackTrace();  
                }  
            }  
        });  
        threads[i].start();  
    }  
    System.in.read();  
}

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 }

posted @ 2019-06-24 16:49 1456710017 阅读(790) 评论(0) 编辑收藏举报

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