Java多线程简单实现取消和进度

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多线程 java

Java中简单实现多线程调度时的可取消和显示进度

一个简单的多线程调度实现，统一开始，为了使得所有线程统一开始，类似运动员在听到发令枪时一起进行，使用了CountDownLatch进行控制。

CountDownLatch beginLatch = new CountDownLatch(1);
CountDownLatch endLatch = new CountDownLatch(personCount);

主线程建立线程池，并进行调度，由于要在最后进行汇总结果，使用了FutureTask

List<FutureTask<String>> futureTaskList = new ArrayList<FutureTask<String>>();
for (int i = 0; i < personCount; i++) {
	futureTaskList.add(new FutureTask<String>(new ExecuteCallable(beginLatch, endLatch，i)));
}

ExecutorService execService = Executors.newFixedThreadPool(threadCount);
	for (FutureTask<String> futureTask : futureTaskList) {
		execService.execute(futureTask);
	}
beginLatch.countDown();

这样所有线程就会统一开始执行，执行完成后，汇总结果，并关闭线程池。

endLatch.await();
System.out.println("--------------");
for (FutureTask<String> futureTask : futureTaskList) {
	System.out.println(futureTask.get());
}
execService.shutdown();

对于每个线程的执行，都需要共享变量beginLatch和endLatch，各线程代码：

public class ExecuteCallable implements Callable<String> {

	private int id;
	private CountDownLatch beginLatch;
	private CountDownLatch endLatch;

	public ExecuteCallable(CountDownLatch beginLatch, CountDownLatch endLatch,
			Exchanger<Integer> exchanger, int id,
			ConcurrentTaskExecutor concurrentTaskExecutor) {
		this.beginLatch = beginLatch;
		this.endLatch = endLatch;
		this.id = id;
	}

	@Override
	public String call() throws Exception {
		beginLatch.await();
		long millis = (long) (Math.random() * 10 * 1000);
		String result = String.format("Player :%s arrived, use %s millis", id, millis);
		Thread.sleep(millis);
		System.out.println(result);
		endLatch.countDown();
		return result;
	}

}

每个线程在开始等待发令枪(beginLatch)，随机等待一段时间（模拟执行时间），最后通知endLatch减一（执行完毕通知），并返回结果。

到这里只是一个简单的实现，我们并不能在主线程中实时了解各线程的执行情况，除非到了所有线程执行完毕(endLatch解除阻塞状态)。这时候我们使用Exchanger机制来进行线程之间数据的交换，在每个线程执行完成后，将其完成的数据量传给主线程进行刷新（模拟进度条工作）。

主线程ConcurrentTaskExecutor类中：

Exchanger<Integer> exchanger = new Exchanger<Integer>();
beginLatch.countDown();

Integer totalResult = Integer.valueOf(0);
for (int i = 0; i < personCount; i++) {
	Integer partialResult = exchanger.exchange(Integer.valueOf(0));
        if(partialResult != 0){
	        totalResult = totalResult + partialResult;
		System.out.println(String.format("Progress: %s/%s", totalResult, personCount));
	}
}

endLatch.await();

线程类ExecuteCallable构造函数加入exchanger

@Override
public String call() throws Exception {
        beginLatch.await();
	long millis = (long) (Math.random() * 10 * 1000);
	String result = String.format("Player :%s arrived, use %s millis", id, millis);
	Thread.sleep(millis);
	System.out.println(result);
	exchanger.exchange(1);
	endLatch.countDown();
	return result;
}

在执行完成进行数据交换，返回本次执行进度给主线程（当前默认设置成1，可修改），主线程在所有线程执行完成前，endLatch.await()必定是阻塞状态的，这样主线程就能实时拿到子线程执行完成的进度数据。

下面我们再加入一个可以取消的功能，加入系统随机在某个时间点进行取消操作，那么开始执行的线程是无法进行实时响应了，只能等待当前操作执行完毕；如果线程还没有开始执行，那么就取消其行为。

更改的ExecuteCallable执行方法如下：

@Override
public String call() throws Exception {
	beginLatch.await();
	if(concurrentTaskExecutor.isCanceled()){
		endLatch.countDown();
		exchanger.exchange(0);
		return String.format("Player :%s is given up", id);
	}
	long millis = (long) (Math.random() * 10 * 1000);
	String result = String.format("Player :%s arrived, use %s millis", id, millis);
	Thread.sleep(millis);
	System.out.println(result);
	exchanger.exchange(1);
	endLatch.countDown();
	return result;
}

其中concurrentTaskExecutor类中加入一个类型为boolean的canceled变量，注意这个变量必须是volatile的，以便能够在线程间共享数据，并且该变量的setter和getter方法也是原子性的。

我们的取消操作不能放在主线程中操作，需要额外建立一个线程，并且这个线程也不能通过线程池进行调度，新建的InterruptRunnable类：

public class InterruptRunnable implements Runnable {

	private CountDownLatch beginLatch;
	private ConcurrentTaskExecutor concurrentTaskExecutor;

	public InterruptRunnable(ConcurrentTaskExecutor currConcurrentTaskExecutor, CountDownLatch beginLatch) {
		this.beginLatch = beginLatch;
		this.concurrentTaskExecutor = currConcurrentTaskExecutor;
	}

	@Override
	public void run() {
		try {
			beginLatch.await();
			long millis = (long) (Math.random() * 10 * 1000);
			System.out.println(String.format("System need sleep %s millis", millis));
			Thread.sleep(millis);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		concurrentTaskExecutor.setCanceled(true);
	}

}

更改后的ConcurrentTaskExecutor，在执行发令前，先让该中断线程启动，以便一起等待开始命令：

new Thread(new InterruptRunnable(this, beginLatch)).start();

beginLatch.countDown();

最后执行结果（取决于中断线程的随机时间长短）：

System need sleep 2920 millis
Player :4 arrived, use 917 millis
Progress: 1/10
Player :5 arrived, use 1076 millis
Progress: 2/10
Player :3 arrived, use 2718 millis
Progress: 3/10
Player :1 arrived, use 4013 millis
Progress: 4/10
Player :0 arrived, use 8541 millis
Progress: 5/10
Player :2 arrived, use 8570 millis
Progress: 6/10
Player :6 arrived, use 7261 millis
Progress: 7/10
Player :7 arrived, use 7015 millis
Progress: 8/10
--------------
Player :0 arrived, use 8541 millis
Player :1 arrived, use 4013 millis
Player :2 arrived, use 8570 millis
Player :3 arrived, use 2718 millis
Player :4 arrived, use 917 millis
Player :5 arrived, use 1076 millis
Player :6 arrived, use 7261 millis
Player :7 arrived, use 7015 millis
Player :8 is given up
Player :9 is given up

最后，附上最终的程序代码

ConcurrentTaskExecutor：

public class ConcurrentTaskExecutor {

	private volatile boolean canceled = false;

	public void executeTask() throws Exception {
		int personCount = 10;
		int threadCount = 5;

		CountDownLatch beginLatch = new CountDownLatch(1);
		CountDownLatch endLatch = new CountDownLatch(personCount);
		Exchanger<Integer> exchanger = new Exchanger<Integer>();

		List<FutureTask<String>> futureTaskList = new ArrayList<FutureTask<String>>();
		for (int i = 0; i < personCount; i++) {
			futureTaskList.add(new FutureTask<String>(new ExecuteCallable(beginLatch, endLatch, exchanger, i, this)));
		}

		ExecutorService execService = Executors.newFixedThreadPool(threadCount);
		for (FutureTask<String> futureTask : futureTaskList) {
			execService.execute(futureTask);
		}
		
		new Thread(new InterruptRunnable(this, beginLatch)).start();

		beginLatch.countDown();

		Integer totalResult = Integer.valueOf(0);
		for (int i = 0; i < personCount; i++) {
			Integer partialResult = exchanger.exchange(Integer.valueOf(0));
			if(partialResult != 0){
				totalResult = totalResult + partialResult;
				System.out.println(String.format("Progress: %s/%s", totalResult, personCount));
			}
		}

		endLatch.await();
		System.out.println("--------------");
		for (FutureTask<String> futureTask : futureTaskList) {
			System.out.println(futureTask.get());
		}
		execService.shutdown();
	}

	public boolean isCanceled() {
		return canceled;
	}
	
	public void setCanceled(boolean canceled){
		this.canceled = canceled;
	}

	public static void main(String[] args) throws Exception {
		ConcurrentTaskExecutor executor = new ConcurrentTaskExecutor();
		executor.executeTask();
	}

}

ExecuteCallable

public class ExecuteCallable implements Callable<String> {

	private int id;
	private CountDownLatch beginLatch;
	private CountDownLatch endLatch;
	private Exchanger<Integer> exchanger;
	private ConcurrentTaskExecutor concurrentTaskExecutor;

	public ExecuteCallable(CountDownLatch beginLatch, CountDownLatch endLatch,
			Exchanger<Integer> exchanger, int id,
			ConcurrentTaskExecutor concurrentTaskExecutor) {
		this.beginLatch = beginLatch;
		this.endLatch = endLatch;
		this.exchanger = exchanger;
		this.id = id;
		this.concurrentTaskExecutor = concurrentTaskExecutor;
	}

	@Override
	public String call() throws Exception {
		beginLatch.await();
		if(concurrentTaskExecutor.isCanceled()){
			endLatch.countDown();
			exchanger.exchange(0);
			return String.format("Player :%s is given up", id);
		}
		long millis = (long) (Math.random() * 10 * 1000);
		String result = String.format("Player :%s arrived, use %s millis", id, millis);
		Thread.sleep(millis);
		System.out.println(result);
		exchanger.exchange(1);
		endLatch.countDown();
		return result;
	}

}

InterruptRunnable

public class InterruptRunnable implements Runnable {

	private CountDownLatch beginLatch;
	private ConcurrentTaskExecutor concurrentTaskExecutor;

	public InterruptRunnable(ConcurrentTaskExecutor currConcurrentTaskExecutor, CountDownLatch beginLatch) {
		this.beginLatch = beginLatch;
		this.concurrentTaskExecutor = currConcurrentTaskExecutor;
	}

	@Override
	public void run() {
		try {
			beginLatch.await();
			long millis = (long) (Math.random() * 10 * 1000);
			System.out.println(String.format("System need sleep %s millis", millis));
			Thread.sleep(millis);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		concurrentTaskExecutor.setCanceled(true);
	}

}

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2013-03-25 10:39
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