简述
在多线程中有时候我们希望一个线程执行完毕后可以返回一些值，在java5中引入了java.util.concurrent.Callable接口，它类似于Runnable接口，但是Callable可以有返回值。
Java Callable接口使用通用定义对象的返回类型，executor类提供了在线程池中执行Java调用的有用方法，由于可调用任务并行运行，所以我们必须等待返回的对象。
Future
实现接口Callable的任务返回的是java.util.concurrent.Future对象，使用Future对象我们可以获得返回值
#取消任务的运行
#如果任务已经完成或已经被取消或因某种原因不能被取消，当调用该方法时会失败
#如果任务还没启动，调用该方法后，该任务将永远不会执行
#如果任务已经启动，如果参数mayInterruptIfRunning为ture，将会中断任务，否则会允许任务执行完成
boolean cancel(boolean mayInterruptIfRunning);
#判断任务是否已经取消
boolean isCancelled();
#判断任务是否执行完成
boolean isDone();
#等待获取任务执行完成后的返回值
V get() throws InterruptedException, ExecutionException;
#在指定时间内等待任务完成并获取返回值，否则抛出超时异常
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
事例
下面是一个简单的Callable调用任务示例，它返回在一秒钟后执行任务的线程的名称，我们使用Executor框架来并行执行100个任务，并使用Java Future来获得提交的任务的结果。
package com.lkf.mulithread;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;
public class MyCallable implements Callable<String> {
@Override
public String call() throws Exception {
Thread.sleep(1000);
//返回当前线程名字
return Thread.currentThread().getName();
}
public static void main(String args[]){
//使用Executors创建一个固定大小的线程池，线程池大小为10
ExecutorService executor = Executors.newFixedThreadPool(10);
//创建一个列表来保存与Callable相关的Future对象
List<Future<String>> list = new ArrayList<Future<String>>();
//创建 MyCallable 实例
Callable<String> callable = new MyCallable();
for(int i=0; i< 100; i++){
//提交任务到线程池
Future<String> future = executor.submit(callable);
//将返回值放入集合
list.add(future);
}
for(Future<String> fut : list){
try {
//打印返回值，注意这时会有延时，因为Future.get()要等待任务完成
System.out.println(new Date()+ "::"+fut.get());
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
//关闭线程池
executor.shutdown();
}
}
当我们运行上边的代码，你会发现输出会有延时，那是因为Future get()方法需要等待任务执行完成。同时我们注意到，每次只有10个线程在执行任务。
输出结果：
Sun Mar 25 14:09:50 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:51 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:52 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:53 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:54 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:55 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:56 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:57 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:58 CST 2018::pool-1-thread-7
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-6
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-2
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-9
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-4
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-5
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-8
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-10
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-1
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-3
Sun Mar 25 14:09:59 CST 2018::pool-1-thread-6
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-2
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-9
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-7
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-4
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-5
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-10
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-8
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-3
Sun Mar 25 14:10:00 CST 2018::pool-1-thread-1