Future和FutureTask的使用

java中提供了futuretask供异步任务使用。可以将一个任务提交给后台线程执行，当需要执行结果的时候调用task.get函数。

如果后台线程还没有执行完，那么task.get将会阻塞，直到后台线程返回。如果后台线程已经执行完，则直接返回结果。

package com.z;import java.util.Date;import java.util.concurrent.*;public class T {    public static void main(String[] args) {        fun();    }    public static void fun() {        ExecutorService executor = Executors.newSingleThreadExecutor();        FutureTask<String> task = new FutureTask<String>(new Callable<String>() {            @Override            public String call() throws Exception {                long b = new Date().getTime();                System.out.println("call begin " + b);                StringBuilder sb = new StringBuilder();                for (int i = 0; i < 100000; i++) {                    sb.append(i).append(",");                }                System.out.println("call end " + (new Date().getTime() - b));                return sb.toString();            }        });        executor.execute(task);        long begin = new Date().getTime();        System.out.println("begin" + begin);        try {            // System.out.println(task.get());            task.get();        } catch (InterruptedException e) {            e.printStackTrace();        } catch (ExecutionException e) {            e.printStackTrace();        }        System.out.println("end  " + (new Date().getTime() - begin));        begin = new Date().getTime();        System.out.println("begin" + begin);        try {            task.get();        } catch (InterruptedException e) {            e.printStackTrace();        } catch (ExecutionException e) {            e.printStackTrace();        }        System.out.println("end  " + (new Date().getTime() - begin));        executor.shutdown();    }}

demo的输出：

begin1409478497265
call begin 1409478497265
call end 30
end  31
begin1409478497296
end  0

从emo的代码可以看出，第一次调用.get函数耗时比较久。而第二次调用耗时为0。

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java中有Future和FutureTask这两个类，他们区别是什么呢？

Future是一个接口，代表可以取消的任务，并可以获得任务的执行结果

FutureTask 是基本的实现了Future和runnable接口
           实现runnable接口，说明可以把FutureTask实例传入到Thread中，在一个新的线程中执行。
           实现Future接口，说明可以从FutureTask中通过get取到任务的返回结果，也可以取消任务执行（通过interreput中断）

FutureTask is an implementation of Future. You can explicitly instantiate a FutureTask for a given Runnable or Callable. A FutureTask can be submitted to an Executor for execution as FutureTask implements Runnable. FutureTask can also be executed directly by calling its run method. Once a FutureTask enters the completed state, it stays in that state forever. If the task is completed. FutureTask constructors can accept either Callable or Runnable.