学步园

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   1、Future取得的结果类型和Callable返回的结果类型必须一致，这是通过泛型来实现的。
   2、Callable要采用ExecutorSevice的submit方法提交，返回的future对象可以取消任务。
   3、CompletionService用于提交一组Callable任务，其take方法返回已完成的一个Callable任务对应的Future对象。
         (1) 好比我同时种了几块地的麦子，然后就等待收割。收割时，则是那块先成熟了，则先去收割哪块麦子。
  

   例子程序:

package edu.java5.threadpool;

import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class CallableAndFuture {

	public static void main(String[] args) {
         ExecutorService threadPool = Executors.newSingleThreadExecutor();
         /*
          * threadPool的execute(Runnable)方法调用不能返回结果，submit(Callable)方法调用可以返回结果，
          * 返回的结果封装在Future对象中
          */
         Future<String> future = threadPool.submit(new Callable<String>() {
			@Override
			public String call() throws Exception {
				Thread.sleep(2000);
				return "hello";
			}
		 });
         System.out.println("等待结果...");
         //通过调用future.get()取得结果
		 try {
			//future.get()下面的代码须取得后才能被执行
			System.out.println("取得结果:"+future.get());
			System.out.println("已经取得!");
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
		
		/**
		 * CompletionService用于提交一组Callable任务，其take方法返回已完成的一个Callable任务对应的Future对象。 
		 **/
		ExecutorService threadPool2 = Executors.newFixedThreadPool(10);
		CompletionService<Integer> completionService = new ExecutorCompletionService<Integer>(threadPool2);
		for (int i = 0; i < 10; i++) {
			final int seq = i;
			completionService.submit(new Callable<Integer>() {
				@Override
				public Integer call() throws Exception {
					Thread.sleep(new Random().nextInt(6000));
					System.out.println(Thread.currentThread().getName()+" execute the Callable and return "+seq);
					return seq;
				}
			});
		}
		for (int i = 0; i < 10; i++) {
			try {
				System.err.println(completionService.take().get());
			} catch (InterruptedException e) {
				e.printStackTrace();
			} catch (ExecutionException e) {
				e.printStackTrace();
			}
		}
		System.out.println("全部取完！");
	}

}

输出:

等待结果...
取得结果:hello
已经取得!
pool-2-thread-6 execute the Callable and return 5
5
pool-2-thread-8 execute the Callable and return 7
7
pool-2-thread-9 execute the Callable and return 8
8
pool-2-thread-10 execute the Callable and return 9
9
pool-2-thread-2 execute the Callable and return 1
1
pool-2-thread-5 execute the Callable and return 4
4
pool-2-thread-1 execute the Callable and return 0
0
pool-2-thread-7 execute the Callable and return 6
6
pool-2-thread-3 execute the Callable and return 2
2
pool-2-thread-4 execute the Callable and return 3
3
全部取完！