Future和FutureTask

Executor框架使用Runnable 作为其基本的任务表示形式。Runnable是一种有局限性的抽象，然后可以写入日志，或者共享的数据结构，但是他不能返回一个值。

许多任务实际上都是存在延迟计算的：执行数据库查询，从网络上获取资源，或者某个复杂耗时的计算。对于这种任务，Callable是一个更好的抽象，他能返回一个值，并可能抛出一个异常。Future表示一个任务的周期，并提供了相应的方法来判断是否已经完成或者取消，以及获取任务的结果和取消任务。

public interface Callable {
    /**
     * 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;
}
public interface Future {

    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when cancel is called,
     * this task should never run.  If the task has already started,
     * then the mayInterruptIfRunning parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * 
After this method returns, subsequent calls to {@link #isDone} will
     * always return true.  Subsequent calls to {@link #isCancelled}
     * will always return true if this method returned true.
     *
     * @param mayInterruptIfRunning true if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return false if the task could not be cancelled,
     * typically because it has already completed normally;
     * true otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns true if this task was cancelled before it completed
     * normally.
     *
     * @return true if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns true if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * true.
     *
     * @return true if this task completed
     */
    boolean isDone();

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

可以通过多种方法来创建一个Future来描述任务。ExecutorService中的submit方法接受一个Runnable或者Callable，然后返回一个Future来获得任务的执行结果或者取消任务。

/**
    * Submits a value-returning task for execution and returns a
    * Future representing the pending results of the task. The
    * Future's get method will return the task's result upon
    * successful completion.
    *
    * 

    * If you would like to immediately block waiting
    * for a task, you can use constructions of the form
    * result = exec.submit(aCallable).get();
    *
    * 
 Note: The {@link Executors} class includes a set of methods
    * that can convert some other common closure-like objects,
    * for example, {@link java.security.PrivilegedAction} to
    * {@link Callable} form so they can be submitted.
    *
    * @param task the task to submit
    * @return a Future representing pending completion of the task
    * @throws RejectedExecutionException if the task cannot be
    *       scheduled for execution
    * @throws NullPointerException if the task is null
    */
     Future submit(Callable task);

    /**
    * Submits a Runnable task for execution and returns a Future
    * representing that task. The Future's get method will
    * return the given result upon successful completion.
    *
    * @param task the task to submit
    * @param result the result to return
    * @return a Future representing pending completion of the task
    * @throws RejectedExecutionException if the task cannot be
    *       scheduled for execution
    * @throws NullPointerException if the task is null
    */
     Future submit(Runnable task, T result);

    /**
    * Submits a Runnable task for execution and returns a Future
    * representing that task. The Future's get method will
    * return null upon successful completion.
    *
    * @param task the task to submit
    * @return a Future representing pending completion of the task
    * @throws RejectedExecutionException if the task cannot be
    *       scheduled for execution
    * @throws NullPointerException if the task is null
    */
    Future submit(Runnable task);

另外ThreadPoolExecutor中的newTaskFor(Callable task) 可以返回一个FutureTask。

假设我们通过一个方法从远程获取一些计算结果，假设方法是 List getDataFromRemote()，如果采用同步的方法，代码大概是 List data = getDataFromRemote(),我们将一直等待getDataFromRemote返回，然后才能继续后面的工作，这个函数是从远程获取计算结果的，如果需要很长时间，后面的代码又和这个数据没有什么关系的话，阻塞在那里就会浪费很多时间。我们有什么办法可以改进呢？？？

能够想到的办法是调用函数后，立即返回，然后继续执行，等需要用数据的时候，再取或者等待这个数据。具体实现有两种方式，一个是用Future，另一个是回调。

Future<List> future = getDataFromRemoteByFuture();
        //do something....
        List data = future.get();

可以看到我们返回的是一个Future对象，然后接着自己的处理后面通过future.get()来获得我们想要的值。也就是说在执行getDataFromRemoteByFuture的时候，就已经启动了对远程计算结果的获取，同时自己的线程还继续执行不阻塞。知道获取时候再拿数据就可以。看一下getDataFromRemoteByFuture的实现：

private Future getDataFromRemoteByFuture() {

        return threadPool.submit(new Callable() {
            @Override
            public List call() throws Exception {
                return getDataFromRemote();
            }
        });
    }

我们在这个方法中调用getDataFromRemote方法，并且用到了线程池。把任务加入线程池之后，理解返回Future对象。Future的get方法，还可以传入一个超时参数，用来设置等待时间，不会一直等下去。

也可以利用FutureTask来获取结果：

FutureTask<List> futureTask = new FutureTask<List>(new Callable<List>() {
        @Override
        public List call() throws Exception {
        return getDataFromRemote();
        }
    });

    threadPool.submit(futureTask);


    futureTask.get();

FutureTask是一个具体的实现类，ThreadPoolExecutor的submit方法返回的就是一个Future的实现，这个实现就是FutureTask的一个具体实例，FutureTask帮助实现了具体的任务执行，以及和Future接口中的get方法的关联。FutureTask除了帮助ThreadPool很好的实现了对加入线程池任务的Future支持外，也为我们提供了很大的便利，使得我们自己也可以实现支持Future的任务调度。

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