java多线程之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。

  

java多线程之Future和FutureTask
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;
}
java多线程之Future和FutureTask

 

java多线程之Future和FutureTask
public interface Future<V> {

    /**
     * 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 <tt>cancel</tt> is called,
     * this task should never run.  If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return <tt>true</tt>.  Subsequent calls to {@link #isCancelled}
     * will always return <tt>true</tt> if this method returned <tt>true</tt>.
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return <tt>false</tt> if the task could not be cancelled,
     * typically because it has already completed normally;
     * <tt>true</tt> otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

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

    /**
     * Returns <tt>true</tt> if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * <tt>true</tt>.
     *
     * @return <tt>true</tt> 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;
}
java多线程之Future和FutureTask

 

  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。

   

java多线程之Future和FutureTask
    /**
     * Submits a value-returning task for execution and returns a
     * Future representing the pending results of the task. The
     * Future‘s <tt>get</tt> method will return the task‘s result upon
     * successful completion.
     *
     * <p>
     * If you would like to immediately block waiting
     * for a task, you can use constructions of the form
     * <tt>result = exec.submit(aCallable).get();</tt>
     *
     * <p> 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
     */
    <T> Future<T> submit(Callable<T> task);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future‘s <tt>get</tt> 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
     */
    <T> Future<T> submit(Runnable task, T result);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future‘s <tt>get</tt> method will
     * return <tt>null</tt> upon <em>successful</em> 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);
java多线程之Future和FutureTask

  另外ThreadPoolExecutor中的newTaskFor(Callable<T> 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:

java多线程之Future和FutureTask
private Future<List> getDataFromRemoteByFuture() {

        return threadPool.submit(new Callable<List>() {
            @Override
            public List call() throws Exception {
                return getDataFromRemote();
            }
        });
    }
java多线程之Future和FutureTask

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:

    

java多线程之Future和FutureTask
    FutureTask<List> futureTask = new FutureTask<List>(new Callable<List>() {
            @Override
            public List call() throws Exception {
                return getDataFromRemote();
            }
        });

        threadPool.submit(futureTask);


        futureTask.get();
java多线程之Future和FutureTask

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。

    

java多线程之Future和FutureTask,布布扣,bubuko.com

java多线程之Future和FutureTask

上一篇:python中判断语句用两个or连接的奇葩


下一篇:《Linux命令行与shell脚本编程大全》第二十五章 创建与数据库、web及电子邮件相关的脚本