Java中存在Runnable、Callable、Future、FutureTask这几个与线程相关的类或者接口，在Java中也是比较重要的几个概念，我们通过下面的简单示例来了解一下它们的作用于区别。
Runnable
其中Runnable应该是我们最熟悉的接口，它只有一个run()函数，用于将耗时操作写在其中，该函数没有返回值。然后使用某个线程去执行该runnable即可实现多线程，Thread类在调用start()函数后就是执行的是Runnable的run()函数。Runnable的声明如下 :
public interface Runnable {/*** When an object implementing interface <code>Runnable</code> is used* to create a thread, starting the thread causes the object’s* <code>run</code> method to be called in that separately executing* thread.* <p>** @seejava.lang.Thread#run()*/public abstract void run();}Callable
Callable与Runnable的功能大致相似，Callable中有一个call()函数，，但是call()函数有返回值，而Runnable的run()函数不能将结果返回给客户程序。Callable的声明如下 :
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;}可以看到，这是一个泛型接口，call()函数返回的类型就是客户程序传递进来的V类型。Future
Executor就是Runnable和Callable的调度容器，Future就是对于具体的Runnable或者Callable任务的执行结果进行
)。Future声明如下 :
/*** @see FutureTask * @see Executor * @since 1.5 * @author Doug Lea * @param <V> The result type returned by this Future’s <tt>get</tt> 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 <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.**/boolean cancel(boolean mayInterruptIfRunning);/*** Returns <tt>true</tt> if this task was cancelled before it completed* normally.*/boolean isCancelled();/*** Returns <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*/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*/V get(long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException;}FutureTask
FutureTask则是一个RunnableFuture<V>，而RunnableFuture实现了Runnbale又实现了Futrue<V>这两个接口，
public class FutureTask<V> implements RunnableFuture<V>RunnableFuturepublic interface RunnableFuture<V> extends Runnable, Future<V> {/*** Sets this Future to the result of its computation* unless it has been cancelled.*/void run();}
另外它还可以包装Runnable和Callable<V>， 由构造函数注入依赖。
public FutureTask(Callable<V> callable) {if (callable == null)throw new NullPointerException();this.callable = callable;this.state = NEW;// ensure visibility of callable}public FutureTask(Runnable runnable, V result) {this.callable = Executors.callable(runnable, result);this.state = NEW;// ensure visibility of callable}可以看到，Runnable注入会被Executors.callable()函数转换为Callable类型，即FutureTask最终都是执行Callable类型的任务。该适配函数的实现如下 ： public static <T> Callable<T> callable(Runnable task, T result) {if (task == null)throw new NullPointerException();return new RunnableAdapter<T>(task, result);}RunnableAdapter适配器 /*** A callable that runs given task and returns given result*/static final class RunnableAdapter<T> implements Callable<T> {final Runnable task;final T result;RunnableAdapter(Runnable task, T result) {this.task = task;this.result = result;}public T call() {task.run();return result;}}
由于FutureTask实现了Runnable，因此它既可以通过Thread包装来直接执行，也可以提交给ExecuteService来执行。
并且还可以直接通过get()函数获取执行结果，该函数会阻塞，直到结果返回。因此FutureTask既是Future、
Runnable，又是包装了Callable( 如果是Runnable最终也会被转换为Callable )， 它是这两者的合体。
简单示例 package com.effective.java.concurrent.task;import java.util.concurrent.Callable;import java.util.concurrent.ExecutionException;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;import java.util.concurrent.FutureTask;/** * * @author mrsimple * */public class RunnableFutureTask {/** * ExecutorService */static ExecutorService mExecutor = Executors.newSingleThreadExecutor();/** * * @param args */public static void main(String[] args) {runnableDemo();futureDemo();}/** * runnable, 无返回值 */static void runnableDemo() {new Thread(new Runnable() {@Overridepublic void run() {System.out.println("runnable demo : " + fibc(20));}}).start();}/** * 其中Runnable实现的是void run()方法，无返回值；Callable实现的是 V * call()方法，并且可以返回执行结果。其中Runnable可以提交给Thread来包装下 * ，直接启动一个线程来执行，而Callable则一般都是提交给ExecuteService来执行。 */static void futureDemo() {try {/*** 提交runnable则没有返回值, future没有数据*/Future<?> result = mExecutor.submit(new Runnable() {@Overridepublic void run() {fibc(20);}});System.out.println("future result from runnable : " + result.get());/*** 提交Callable, 有返回值, future中能够获取返回值*/Future<Integer> result2 = mExecutor.submit(new Callable<Integer>() {@Overridepublic Integer call() throws Exception {return fibc(20);}});System.out.println("future result from callable : " + result2.get());/*** FutureTask则是一个RunnableFuture<V>，即实现了Runnbale又实现了Futrue<V>这两个接口，* 另外它还可以包装Runnable(实际上会转换为Callable)和Callable* <V>，所以一般来讲是一个符合体了，它可以通过Thread包装来直接执行，也可以提交给ExecuteService来执行* ，并且还可以通过v get()返回执行结果，在线程体没有执行完成的时候，主线程一直阻塞等待，执行完则直接返回结果。*/FutureTask<Integer> futureTask = new FutureTask<Integer>(new Callable<Integer>() {@Overridepublic Integer call() throws Exception {return fibc(20);}});// 提交futureTaskmExecutor.submit(futureTask) ;System.out.println("future result from futureTask : "+ futureTask.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}/** * 效率底下的斐波那契数列, 耗时的操作 * * @param num * @return */static int fibc(int num) {if (num == 0) {return 0;}if (num == 1) {return 1;}return fibc(num – 1) + fibc(num – 2);}}输出结果贪婪是最真实的贫穷，满足是最真实的财富
