Callable、Future和FutureTask

Callable

• Runnable没有返回值：public abstract void run();，Callable可以有返回值：V call() throws Exception;
• 可以抛出异常
• 方法不同，run()/call()
• 源码

public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

Future

• Future只是一个接口
• 能判断任务是否完成；能够中断任务；能够获取任务执行结果。
• 源码

public interface Future<V> {
	// 如果取消已经完成的任务会返回false；如果任务正在执行，若mayInterruptIfRunning设置为true，则返回true，若mayInterruptIfRunning设置为false，则返回false；如果任务还没有执行，则无论mayInterruptIfRunning为true还是false，肯定返回true。
    boolean cancel(boolean mayInterruptIfRunning);
	// 是否被取消成功
    boolean isCancelled();
    // 是否已经完成
    boolean isDone();
	// 获取执行结果，这个方法会产生阻塞，会一直等到任务执行完毕才返回
    V get() throws InterruptedException, ExecutionException;
	// 如果在指定时间内，还没获取到结果，就直接返回null
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

FutureTask

• FutureTask是Future接口的一个唯一实现类

• 继承关系

public interface RunnableFuture<V> extends Runnable, Future<V> {
    void run();
}

public class FutureTask<V> implements RunnableFuture<V> 

RunnableFuture继承了Runnable接口和Future接口，而FutureTask实现了RunnableFuture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的返回值。

• 构造方法

public FutureTask(Callable<V> callable) {
    if (callable == null)
        throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;      
}

public FutureTask(Runnable runnable, V result) {
    this.callable = Executors.callable(runnable, result);
    this.state = NEW;    
}

• Callable+Future

import java.util.concurrent.*;

class MyCallable {
    public static void main(String[] args) throws InterruptedException {
        ExecutorService executor = Executors.newCachedThreadPool();
        Task task = new Task();
        Future<Integer> result = executor.submit(task);
        executor.shutdown();

        TimeUnit.SECONDS.sleep(1);

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

        try {
            System.out.println("task运行结果" + result.get());
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }

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

class Task implements Callable<Integer> {
    @Override
    public Integer call() throws Exception {
        System.out.println("子线程在进行计算");
        TimeUnit.SECONDS.sleep(2);
        int sum = 0;
        for (int i = 0; i < 100; i++)
            sum += i;
        return sum;
    }
}

• Callable+FutureTask

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

class MyCallable {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // new Thread(new Runnable()).start();
        // new Thread(new FutureTask<V>()).start();
        // new Thread(new FutureTask<V>( Callable )).start();

        MyThread myThread = new MyThread();
        FutureTask<Integer> integerFutureTask = new FutureTask<>(myThread); // 适配类

        new Thread(integerFutureTask, "A").start();
        new Thread(integerFutureTask, "B").start(); // 结果会被缓存，只会打印一个call

        Integer o = integerFutureTask.get(); // 返回值   此方法可能会产生阻塞，最好放在最后一行
        System.out.println(o);
    }
}

class MyThread implements Callable<Integer> {
    @Override
    public Integer call() throws Exception {
        System.out.println("call()");
        return 7;
    }
}