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 : 

[java] view plain copy print ? 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();  }   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 :

[java] view plain copy print ? 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;  }   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

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 :

[java] view plain copy print ? /** * @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;  }   /** * @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，

[java] view plain copy print ? public class FutureTask<V> implements RunnableFuture<V>   public class FutureTask<V> implements RunnableFuture<V> RunnableFuture

[java] view plain copy print ? public interface RunnableFuture<V> extends Runnable, Future<V> {      /**      * Sets this Future to the result of its computation      * unless it has been cancelled.      */      void run();  }   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。

[java] view plain copy print ? 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 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 ：

[java] view plain copy print ? public static <T> Callable<T> callable(Runnable task, T result) {      if (task == null)          throw new NullPointerException();      return new RunnableAdapter<T>(task, result);  }   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

[java] view plain copy print ? /**  * 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;      }  }   /** * 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

[java] view plain copy print ?  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);      }    }   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