JUC thread pool (1): FutureTask use

Future 表示了一个任务的生命周期,is a cancelable asynchronous operation,It's done at some point in the future,并提供对其结果的访问.
 
Many asynchronous task classes in the concurrent package inherit from Future,其中最典型的就是 FutureTask.

一. FutureTask简介

FutureTask常用来封装 Callable 和 Runnable ,也可以作为一个任务提交到线程池中执行.

FutureThe thread safety hasCAS保证.

FutureTask实现了Future的基础功能,如获取任务执行结果（get）和取消任务（cancel）等.If the task has not completed, it will block when the task execution result is obtained.
 

二. 使用案例

通过FutureTask封装Callable实现,The state of the thread can be obtained at runtime（是否运行完）、也可以取消任务,The result can be obtained when the run is complete,There are two ways to run a thread：

• 线程池运行futureTask
• Thread线程运行futureTask

public class CallDemo {
    

    public static void main(String[] args) {
    


        /** * * 方式1 : 线程池运行futureTask（futureTask包装的callable接口,futureTaskThe execution result can be obtained）. * * FutureTask + ExecutorService: * * ExecutorService executor = Executors.newCachedThreadPool(); * Task task = new Task(); * FutureTask<Integer> futureTask = new FutureTask<Integer>(task); * executor.submit(futureTask); * executor.shutdown(); */

        /** * 方式2 : 线程运行futureTask * FutureTask + Thread */

        // 2. 新建FutureTask,需要一个实现了Callable接口的类的实例作为构造函数参数
        FutureTask<Integer> futureTask = new FutureTask<Integer>(new MyTask());
        // 3. 新建Thread对象并启动
        Thread thread = new Thread(futureTask);
        thread.setName("Task thread");
        thread.start();

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

        System.out.println("Thread [" + Thread.currentThread().getName() + "] is running");

        // 4. 调用isDone()判断任务是否结束
        if (!futureTask.isDone()) {
    
            System.out.println("Task is not done");
            try {
    
                Thread.sleep(2000);
            } catch (InterruptedException e) {
    
                e.printStackTrace();
            }
        }
        int result = 0;
        try {
    
            // 5. 调用get()方法获取任务结果,如果任务没有执行完成则阻塞等待
            result = futureTask.get();
        } catch (Exception e) {
    
            e.printStackTrace();
        }

        System.out.println("result is " + result);

    }

    // 1. 继承Callable接口,实现call()方法,泛型参数为要返回的类型
    static class MyTask implements Callable<Integer> {
    

        @Override
        public Integer call() throws Exception {
    
            System.out.println("Thread [" + Thread.currentThread().getName() + "] is running");
            int result = 0;
            for (int i = 0; i < 100; ++i) {
    
                result += i;
            }

            Thread.sleep(3000);
            return result;
        }
    }
}

三. FutureTask原理

FutureTask的类关系

FutureTask实现了RunnableFuture接口,RunnableFuture继承了Runnable和Future接口,所以FutureTask既能作为Runnable被Thread使用,也可以作为 Future 用来得到计算结果.

参考：

https://pdai.tech/md/java/thread/java-thread-x-juc-executor-FutureTask.html