Java:多线程等待所有线程结束(CountDownLatch/CyclicBarrier) .

官方链接：

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/CountDownLatch.html

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/CyclicBarrier.html

多线程设计过程中，经常会遇到需要等待其它线程结束以后再做其他事情的情况，比如多线程下载文件，每个线程都会下载文件的一部分，在所有线程结束以后，需要将各部分再次拼接成一个完整的文件。

有几种方案：

1.在主线程中设置一自定义全局计数标志，在工作线程完成时，计数减一。主线程侦测该标志是否为0，一旦为0，表示所有工作线程已经完成。

2.使用Java标准的类CountDownLatch来完成这项工作，原理是一样的，计数。

CountDownLatch

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");
}
}

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

CyclicBarrier

使用CyclickBarrier的例子：

class WalkTarget
{
private final int mCount = 5;

private final CyclicBarrier mBarrier;

ExecutorService mExecutor;
class BarrierAction implements Runnable

{
@Override

public void run()

{
// TODO Auto-generated method stub

System.out.println("所有线程都已经完成任务,计数达到预设值");

//mBarrier.reset();//恢复到初始化状态

}
}
WalkTarget()
{
//初始化CyclicBarrier

mBarrier = new CyclicBarrier(mCount, new BarrierAction());

mExecutor = Executors.newFixedThreadPool(mCount);
for (int i = 0; i < mCount; i++)

{
//启动工作线程

mExecutor.execute(new Walker(mBarrier, i));

}
}
}
//工作线程

class Walker implements Runnable

{
private final CyclicBarrier mBarrier;

private final int mThreadIndex;

Walker(final CyclicBarrier barrier, final int threadIndex)

{
mBarrier = barrier;
mThreadIndex = threadIndex;
}
@Override
public void run()
{
// TODO Auto-generated method stub
System.out.println("Thread " + mThreadIndex + " is running...");
// 执行任务
try
{
TimeUnit.MILLISECONDS.sleep(5000);
// do task
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
// 完成任务以后，等待其他线程完成任务
try
{
mBarrier.await();
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
catch (BrokenBarrierException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
// 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
System.out.println("Thread " + mThreadIndex + " do something else");
}
}
public class CountDownLatchTest
{
public static void main(String[] args)
{
// TODO Auto-generated method stub
//new CountDownLatchDriver2().main();
new WalkTarget();
}
}

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

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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Java:多线程等待所有线程结束(CountDownLatch/CyclicBarrier) .

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