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。
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; }
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; }
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 <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);
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：
private Future<List> getDataFromRemoteByFuture() { return threadPool.submit(new Callable<List>() { @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。
