Java中的Runnable、Callable、Future、FutureTask的区别与示例

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 :

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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(); }</p>

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 :

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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; }</v>

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

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 :

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/** * @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; }</v></v>

FutureTask

FutureTask则是一个RunnableFuture，而RunnableFuture实现了Runnbale又实现了Futrue这两个接口，

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1	public class FutureTask<v> implements RunnableFuture<v></v></v>

RunnableFuture

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public interface RunnableFuture<v> extends Runnable, Future<v> {     /**      * Sets this Future to the result of its computation      * unless it has been cancelled.      */     void run(); }</v></v>

另外它还可以包装Runnable和Callable， 由构造函数注入依赖。

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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 }</v>

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 ：

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public static <t> Callable<t> callable(Runnable task, T result) {     if (task == null)         throw new NullPointerException();     return new RunnableAdapter<t>(task, result); }</t></t></t>

RunnableAdapter适配器

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/**  * 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;     } }</t></t>

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

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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);     }   }</integer></integer></integer></v></v></v></integer></integer>

输出结果