Java中的Runnable、Callable、Future、FutureTask的区别

本文转载自：http://blog.csdn.net/bboyfeiyu/article/details/24851847

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 :

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 :

 /**
 * @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，

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
     }

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() {

             @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);
     }

 }