Java中的Runnable、Callable、Future、FutureTask的區別與示例

Java中存在Runnable、Callable、Future、FutureTask這幾個與線程相關的類或者接口，在Java中也是比較重要的幾個概念，我們通過下面的簡單示例來了解一下它們的作用於區別。

Runnable

其中Runnable應該是我們最熟悉的接口，它只有一個run()函數，用於將耗時操作寫在其中，該函數沒有返回值。然后使用某個線程去執行該runnable即可實現多線程，Thread類在調用start()函數后就是執行的是Runnable的run()函數。Runnable的聲明如下 :

 

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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與Runnable的功能大致相似，Callable中有一個call()函數，但是call()函數有返回值，而Runnable的run()函數不能將結果返回給客戶程序。Callable的聲明如下 :

 

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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，這是一個泛型接口，call()函數返回的類型就是客戶程序傳遞進來的V類型。

 

 

Future

Executor就是Runnable和Callable的調度容器，Future就是對於具體的Runnable或者Callable任務的執行結果進行

取消、查詢是否完成、獲取結果、設置結果操作。get方法會阻塞，直到任務返回結果(Future簡介)。Future聲明如下 :

 

 

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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則是一個RunnableFuture，而RunnableFuture實現了Runnbale又實現了Futrue這兩個接口，

 

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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>

 

另外它還可以包裝Runnable和Callable， 由構造函數注入依賴。

 

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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注入會被Executors.callable()函數轉換為Callable類型，即FutureTask最終都是執行Callable類型的任務。該適配函數的實現如下 ：

 

 

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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適配器

 

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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>

 

由於FutureTask實現了Runnable，因此它既可以通過Thread包裝來直接執行，也可以提交給ExecuteService來執行。

並且還可以直接通過get()函數獲取執行結果，該函數會阻塞，直到結果返回。因此FutureTask既是Future、

Runnable，又是包裝了Callable( 如果是Runnable最終也會被轉換為Callable )， 它是這兩者的合體。

 

簡單示例

 

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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, 無返回值       */      static void runnableDemo() {            new Thread( new Runnable() {                @Override              public void run() {                  System.out.println( "runnable demo : " + fibc( 20 ));              }          }).start();      }        /**       * 其中Runnable實現的是void run()方法，無返回值；Callable實現的是 V       * call()方法，並且可以返回執行結果。其中Runnable可以提交給Thread來包裝下       * ，直接啟動一個線程來執行，而Callable則一般都是提交給ExecuteService來執行。       */      static void futureDemo() {          try {              /**               * 提交runnable則沒有返回值, future沒有數據               */              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中能夠獲取返回值               */              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則是一個RunnableFuture<v>，即實現了Runnbale又實現了Futrue<v>這兩個接口，               * 另外它還可以包裝Runnable(實際上會轉換為Callable)和Callable               * <v>，所以一般來講是一個符合體了，它可以通過Thread包裝來直接執行，也可以提交給ExecuteService來執行               * ，並且還可以通過v get()返回執行結果，在線程體沒有執行完成的時候，主線程一直阻塞等待，執行完則直接返回結果。               */              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數列, 耗時的操作       *       * @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>

 

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