Java线程之FutureTask

简述

FutureTask是Future接口的实现类，并提供了可取消的异步处理的功能，它包含了启动和取消（start and cancel）任务的方法，同时也包含了可以返回FutureTask状态（completed or cancelled）的方法。我们可以自定义一个Future任务，然后使用线程池执行器Java Thread Pool Executor 去异步执行任务。

FutureTask

从类图中可以看出，FutureTask实现了接口RunnableFuture，接口RunnableFuture又继承了Runnable和Future接口，所以它既可以作为Runnable被线程执行，又可以作为Future得到Callable的结果，因此FutureTask提供了两个构造方法：

public FutureTask(Callable<V> callable)
public FutureTask(Runnable runnable, V result)

使用事例

1.使用Callable+Future获取执行结果

package com.lkf.mulithread;

import java.util.concurrent.*;

public class CallableFutureExample {
    public static void main(String[] args) {
        //创建线程池
        ExecutorService executor = Executors.newCachedThreadPool();
        //实例化任务
        MyTask task = new MyTask();
        //提交任务
        Future<Integer> result = executor.submit(task);
        //关闭线程池
        executor.shutdown();

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

        System.out.println("主线程在执行任务");

        try {
            System.out.println("MyTask的返回值："+result.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        System.out.println("所有任务执行完毕");
    }

    static class MyTask implements Callable<Integer> {
        @Override
        public Integer call() throws Exception {
            System.out.println("子线程正在执行");
            Thread.sleep(5000);
            return 1000;
        }
    }
}

2.使用Callable+FutureTask获取执行结果

package com.lkf.mulithread;

import java.util.concurrent.*;

public class FutureTaskExample {
    public static class MyCallable implements Callable<String> {

        private long waitTime;

        public MyCallable(int timeInMillis){
            this.waitTime=timeInMillis;
        }
        @Override
        public String call() throws Exception {
            Thread.sleep(waitTime);
            //返回当前线程的名字
            return Thread.currentThread().getName();
        }

    }

    public static void main(String[] args) {
        MyCallable callable1 = new MyCallable(1000);
        MyCallable callable2 = new MyCallable(2000);

        FutureTask<String> futureTask1 = new FutureTask<String>(callable1);
        FutureTask<String> futureTask2 = new FutureTask<String>(callable2);

        ExecutorService executor = Executors.newFixedThreadPool(2);
        executor.execute(futureTask1);
        executor.execute(futureTask2);

        while (true) {
            try {
                if(futureTask1.isDone() && futureTask2.isDone()){
                    System.out.println("Done");
                    //shut down executor service
                    executor.shutdown();
                    return;
                }

                if(!futureTask1.isDone()){
                    //wait indefinitely for future task to complete
                    System.out.println("FutureTask1 output="+futureTask1.get());
                }

                System.out.println("Waiting for FutureTask2 to complete");
                String s = futureTask2.get(200L, TimeUnit.MILLISECONDS);
                if(s !=null){
                    System.out.println("FutureTask2 output="+s);
                }
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }catch(TimeoutException e){
                //do nothing
            }
        }

    }

}

输出结果：

FutureTask1 output=pool-1-thread-1
Waiting for FutureTask2 to complete
Waiting for FutureTask2 to complete
Waiting for FutureTask2 to complete
Waiting for FutureTask2 to complete
Waiting for FutureTask2 to complete
FutureTask2 output=pool-1-thread-2
Done