Future接口源碼解讀：
此接口有5個方法
boolean cancel(boolean mayInterruptIfRunning);
此方法目的是想讓其實現類實現以下功能：
嘗試取消一個任務的執行。
當任務已經執行結束後，或已經被取消執行後，或者因爲其他原因不能被取消任務執行時，返回false。
當cancel調用時，如果任務還未開始執行，那麼任務將會直接取消並永遠不會執行，返回true。
參數 boolean mayInterruptIfRunning設爲true時，正在執行的任務將會被中斷。
參數 boolean mayInterruptIfRunning設爲false時，正在執行的任務將會被允許執行完成。
boolean isCancelled();
如果方法已經被結束了在正常執行完成之前，放回true。
boolean isDone();
如果一個任務已經被執行完成，則返回true。
這種完成可以是正常的執行結束，或拋出異常結束，或被取消結束。
V get() throws InterruptedException, ExecutionException;
此方法用於一個可以放回結果的任務，去接收此任務的返回結果，get方法在調用並獲取到結果之前會等待。
當任務被取消時，調用get會拋出 CancellationException 異常。
當任務計算出錯時，調用get會拋出 ExecutionException 異常。
當任務被中斷時，調用get會拋出 CancellationException 異常。
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
相當於給執行的任務線程一個等待時間，當大於某一個等待時間後，就不再等待任務返回的結果，直接拋出TimeoutException異常。
第一個參數是 等待時間的長度。
第二個參數是 時間長度的單位。 通常通過 TimeUnit.**（工具類）獲取對應時間單位。
其他拋出的異常和上面無參的get方法一致。
補充知識：
關於中斷：
Future接口源碼：
package java.util.concurrent;
/**
* A {@code Future} represents the result of an asynchronous
* computation. Methods are provided to check if the computation is
* complete, to wait for its completion, and to retrieve the result of
* the computation. The result can only be retrieved using method
* {@code get} when the computation has completed, blocking if
* necessary until it is ready. Cancellation is performed by the
* {@code cancel} method. Additional methods are provided to
* determine if the task completed normally or was cancelled. Once a
* computation has completed, the computation cannot be cancelled.
* If you would like to use a {@code Future} for the sake
* of cancellability but not provide a usable result, you can
* declare types of the form {@code Future<?>} and
* return {@code null} as a result of the underlying task.
*
* <p>
* <b>Sample Usage</b> (Note that the following classes are all
* made-up.)
* <pre> {@code
* interface ArchiveSearcher { String search(String target); }
* class App {
* ExecutorService executor = ...
* ArchiveSearcher searcher = ...
* void showSearch(final String target)
* throws InterruptedException {
* Future<String> future
* = executor.submit(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* displayOtherThings(); // do other things while searching
* try {
* displayText(future.get()); // use future
* } catch (ExecutionException ex) { cleanup(); return; }
* }
* }}</pre>
*
* The {@link FutureTask} class is an implementation of {@code Future} that
* implements {@code Runnable}, and so may be executed by an {@code Executor}.
* For example, the above construction with {@code submit} could be replaced by:
* <pre> {@code
* FutureTask<String> future =
* new FutureTask<String>(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* executor.execute(future);}</pre>
*
* <p>Memory consistency effects: Actions taken by the asynchronous computation
* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
* actions following the corresponding {@code Future.get()} in another thread.
*
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
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 {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} 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 {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code 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
* {@code true}.
*
* @return {@code 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;
}