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

2019年3月9日 239次阅读 来源: 曹刚

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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1 2 3 4 5 6 7 8 9 10 11 12 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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1 2 3 4 5 6 7 8 9 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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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 /** * @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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1 2 3 4 5 6 7 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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1 2 3 4 5 6 7 8 9 10 11 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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1 2 3 4 5 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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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 /**   * 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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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 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

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

 

 

    原文作者：曹刚
    原文地址: https://www.cnblogs.com/caogang/p/4596761.html
    本文转自网络文章，转载此文章仅为分享知识，如有侵权，请联系博主进行删除。
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