1 概述
Future代表异步计算返回的结果，提供了检查是否结束、等待结束以及获取计算结果的方法。Executor框架使用Runnable作为其任务表示形式，但它不能返回一个值。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。
/**
* A task that returns a result and may throw an exception.
* Implementors define a single method with no arguments called
* <tt>call</tt>.
*
* <p>The <tt>Callable</tt> interface is similar to {@link
* java.lang.Runnable}, in that both are designed for classes whose
* instances are potentially executed by another thread. A
* <tt>Runnable</tt>, however, does not return a result and cannot
* throw a checked exception.
*
* <p> The {@link Executors} class contains utility methods to
* convert from other common forms to <tt>Callable</tt> classes.
*
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> the result type of method <tt>call</tt>
*/
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。
public interface ExecutorService extends Executor {
/**
* 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);
/**
* Executes the given tasks, returning a list of Futures holding
* their status and results when all complete.
* {@link Future#isDone} is <tt>true</tt> for each
* element of the returned list.
* Note that a <em>completed</em> task could have
* terminated either normally or by throwing an exception.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* @param tasks the collection of tasks
* @return A list of Futures representing the tasks, in the same
* sequential order as produced by the iterator for the
* given task list, each of which has completed.
* @throws InterruptedException if interrupted while waiting, in
* which case unfinished tasks are cancelled.
* @throws NullPointerException if tasks or any of its elements are <tt>null</tt>
* @throws RejectedExecutionException if any task cannot be
* scheduled for execution
*/
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException;
/**
* Executes the given tasks, returning a list of Futures holding
* their status and results
* when all complete or the timeout expires, whichever happens first.
* {@link Future#isDone} is <tt>true</tt> for each
* element of the returned list.
* Upon return, tasks that have not completed are cancelled.
* Note that a <em>completed</em> task could have
* terminated either normally or by throwing an exception.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* @param tasks the collection of tasks
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return a list of Futures representing the tasks, in the same
* sequential order as produced by the iterator for the
* given task list. If the operation did not time out,
* each task will have completed. If it did time out, some
* of these tasks will not have completed.
* @throws InterruptedException if interrupted while waiting, in
* which case unfinished tasks are cancelled
* @throws NullPointerException if tasks, any of its elements, or
* unit are <tt>null</tt>
* @throws RejectedExecutionException if any task cannot be scheduled
* for execution
*/
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Executes the given tasks, returning the result
* of one that has completed successfully (i.e., without throwing
* an exception), if any do. Upon normal or exceptional return,
* tasks that have not completed are cancelled.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* @param tasks the collection of tasks
* @return the result returned by one of the tasks
* @throws InterruptedException if interrupted while waiting
* @throws NullPointerException if tasks or any element task
* subject to execution is <tt>null</tt>
* @throws IllegalArgumentException if tasks is empty
* @throws ExecutionException if no task successfully completes
* @throws RejectedExecutionException if tasks cannot be scheduled
* for execution
*/
<T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException;
/**
* Executes the given tasks, returning the result
* of one that has completed successfully (i.e., without throwing
* an exception), if any do before the given timeout elapses.
* Upon normal or exceptional return, tasks that have not
* completed are cancelled.
* The results of this method are undefined if the given
* collection is modified while this operation is in progress.
*
* @param tasks the collection of tasks
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the result returned by one of the tasks.
* @throws InterruptedException if interrupted while waiting
* @throws NullPointerException if tasks, or unit, or any element
* task subject to execution is <tt>null</tt>
* @throws TimeoutException if the given timeout elapses before
* any task successfully completes
* @throws ExecutionException if no task successfully completes
* @throws RejectedExecutionException if tasks cannot be scheduled
* for execution
*/
<T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
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。
Java本身没有提供Promise方式实现，在Scala以及Netty中都有Promise模式的实现，可以让用户控制future的完成状态（成功、失败等）以及future的返回值，参考文档中是一个自定义Promise的简单实现。
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