多线程获取返回值 - Future and CompletionService and Callable

public class CompletionServiceTest {

	public static void main(String[] args) {
		CompletionServiceTest test = new CompletionServiceTest();
		try {
			test.getValueByFuture();
			test.getValueByCompletionService();
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}

	public void getValueByFuture() throws InterruptedException, ExecutionException {
		ExecutorService threadPool = Executors.newSingleThreadExecutor();
		Future<String> future = threadPool.submit(new ExeCall());
		
		System.out.println(future.get());//返回值是一个随机数
		threadPool.shutdown();
	}

	public void getValueByCompletionService() throws InterruptedException, ExecutionException  {
		int threadPoolSize = 10;
		ExecutorService threadPool = Executors.newFixedThreadPool(threadPoolSize);
		CompletionService<String> completionService = new ExecutorCompletionService<String>(threadPool);
		
		for (int i = 1; i <= threadPoolSize; i++) {
			Future<String> future = completionService.submit(new ExeCall());
			System.out.println(future.get());//返回值是一个随机数
		}
		
		//也可以等所有线程执行完再获取线程的返回值
		for (int i = 1; i <= threadPoolSize; i++) {
			System.out.println(completionService.take().get());
		}
		
		threadPool.shutdown();
	}
	
	class ExeCall implements Callable<String> {
		@Override
		public String call() throws Exception {
			Random ra = new Random();
			int num = ra.nextInt(100);
			return "Get a random number : " + num;
		}

	}
}

1.Future模式可以这样来描述：
     我有一个任务，提交给了Future，Future替我完成这个任务。期间我自己可以去做任何想做的事情。一段时间之后，我就便可以从Future那儿取出结果。

2.Future接口
     Future 表示异步计算的结果。
     Future接口提供方法来检测任务是否被执行完，等待任务执行完获得结果。也可以设置任务执行的超时时间。
     如果需要设定代码执行的最长时间，即超时，可以用Java线程池ExecutorService类配合Future接口来实现。
 public void getValueByFuture() throws InterruptedException, ExecutionException {
      FutureTask<String> future = new FutureTask<String>(new ExeCall()); 
      ExecutorService threadPool = Executors.newSingleThreadExecutor();
      threadPool.execute(future);
 
      try {
          String result = future.get(3000,TimeUnit.MILLISECONDS);//取得结果，同时设置超时执行时间为3秒
          System.out.println(result);
      } catch (TimeoutException e) {
          future.cancel(true);
      } finally {
          threadPool.shutdown();
      }
  }

3.Future的实现类
     Future的实现类有java.util.concurrent.FutureTask<V>，FutureTask类同时又实现了Runnable接口，所以可以直接提交给Thread、Executor执行。

     public void getValueByFuture() throws InterruptedException, ExecutionException {
          FutureTask<String> future = new FutureTask<String>(new ExeCall()); 
          new Thread(future).start();
          System.out.println(future.get());
     }

     或

     public void getValueByFuture() throws InterruptedException, ExecutionException {
          FutureTask<String> future = new FutureTask<String>(new ExeCall()); 
          ExecutorService threadPool = Executors.newSingleThreadExecutor();
          threadPool.execute(future);
          System.out.println(future.get());
          threadPool.shutdown();
     }