java.util.concurrent.Future学习

JDK API的定义:
    A Future represents the result of an asynchronous computation. Methods are provided to check if the computation is complete, to wait for its completion, and to retrieve the result of the computation. The result can only be retrieved using method get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the cancel method. Additional methods are provided to determine if the task completed normally or was cancelled. Once a computation has completed, the computation cannot be cancelled. If you would like to use a Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future<?> and return null as a result of the underlying task.

相应的中文为：
Future 表示异步计算的结果。它提供了检查计算是否完成的方法，以等待计算的完成，并获取计算的结果。计算完成后只能使用 get 方法来获取结果，如有必要，计算完成前可以阻塞此方法。取消则由cancel 方法来执行。还提供了其他方法，以确定任务是正常完成还是被取消了。一旦计算完成，就不能再取消计算。如果为了可取消性而使用 Future 但又不提供可用的结果，则可以声明Future<?> 形式类型、并返回 null 作为底层任务的结果。

举例：
 

import java.util.concurrent.Callable;  
import java.util.concurrent.ExecutionException;  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.Executors;  
import java.util.concurrent.Future;  
  
public class TestFutureTask {  
    public static void main(String[] args) throws InterruptedException,  
            ExecutionException {  
        final ExecutorService exec = Executors.newFixedThreadPool(5);  
        Callable<String> call = new Callable<String>() {  
            public String call() throws Exception {  
                Thread.sleep(1000 * 3);//休眠指定的时间，此处表示该操作比较耗时  
                return "Other less important but longtime things.";  
            }  
        };  
        Future<String> task = exec.submit(call);  
        //重要的事情  
        Thread.sleep(1000 * 3);  
        System.out.println("Let's do important things.");  
        //不重要的事情  
        String obj = task.get();  
        System.out.println(obj);  
        //关闭线程池  
        exec.shutdown();  
    }  
}