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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> * * @see java.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;}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
取消、查询是否完成、获取结果、设置结果操作。get方法会阻塞，直到任务返回结果(Future简介)。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>
public 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 }
public static <T> Callable<T> callable(Runnable task, T result) { if (task == null) throw new NullPointerException(); return new RunnableAdapter<T>(task, result); }
/** * 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() { @Override public 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() { @Override public 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>() { @Override public 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>() { @Override public Integer call() throws Exception { return fibc(20); } }); // 提交futureTask mExecutor.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
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