Runnable、Callable、Future、FutureTask

接口的不同

public interface Runnable {
    public abstract void run();
}

public interface Callable {
    V call() throws Exception;
}

Callable的代码也非常简单，不同的是它是一个泛型接口，call()函数返回的类型就是创建Callable传进来的V类型。

Future

public interface Future {

    boolean cancel(boolean mayInterruptIfRunning);

    boolean isCancelled();

    boolean isDone();

    V get() throws InterruptedException, ExecutionException;

    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

Future是一个接口，定义了Future对于具体的Runnable或者Callable任务的执行结果进行取消、查询任务是否被取消，查询是否完成、获取结果。

Future基本用法：

class MyCallable implements Callable{
    @Override
    public String call() throws Exception {
        System.out.println("做一些耗时的任务...");
        Thread.sleep(5000);
        return "OK";
    }
}

public class FutureSimpleDemo {
    public static void main(String[] args) throws InterruptedException, ExecutionException {

        ExecutorService executorService = Executors.newCachedThreadPool();
       //主要是线程池返回的就是这个类型，做了包装。用submit
        Future future = executorService.submit(new MyCallable());
        System.out.println("dosomething...");
        //阻塞等待结果返回
        System.out.println("得到异步任务返回结果：" + future.get());
        System.out.println("Completed!");
    }
}

Future引用对象指向的实际是FutureTask。

FutureTask

FutureTask的父类是RunnableFuture，而RunnableFuture继承了Runnbale和Futrue这两个接口

public class FutureTask implements RunnableFuture
public interface RunnableFuture extends Runnable, Future

FutureTask构造方法

public FutureTask(Callable callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
}

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

FutureTask最终都是执行Callable类型的任务。
如果构造函数参数是Runnable，会被Executors.callable方法转换为Callable类型。

看看Executors.callable方法代码

public static  Callable callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter(task, result);
}

看看RunnableAdapter方法代码

/**
     * A callable that runs given task and returns given result
     */
    static final class RunnableAdapter implements Callable {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }

RunnableAdapter是FutureTask的一个静态内部类并且实现了Callable，也就是说RunnableAdapter是Callable子类。
call方法实现代码是，执行Runnable的run方法，并返回构造FutureTask传入result参数。

FutureTask基本用法

public class CallableAndFuture {
    public static void main(String[] args) {
        Callable callable = new Callable() {
            public Integer call() throws Exception {
                return new Random().nextInt(100);
            }
        };
        FutureTask future = new FutureTask(callable);
        new Thread(future).start();
        try {
            Thread.sleep(3000);// 可能做一些事情
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

FutureTask实现了两个接口，Runnable和Future，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，那么这个组合的使用有什么好处呢？假设有一个很费时逻辑需要计算并且返回这个值，同时这个值不是马上需要，那么就可以使用这个组合，用另一个线程去计算返回值，而当前线程在使用这个返回值之前可以做其它的操作，等到需要这个返回值时，再通过Future得到！

Ref:
http://www.jianshu.com/p/cf12d4244171