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 : 

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>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @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 :
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。 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.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return <tt>true</tt>.  Subsequent calls to {@link #isCancelled}
     * will always return <tt>true</tt> if this method returned <tt>true</tt>.
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return <tt>false</tt> if the task could not be cancelled,
     * typically because it has already completed normally;
     * <tt>true</tt> otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally.
     *
     * @return <tt>true</tt> if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns <tt>true</tt> if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * <tt>true</tt>.
     *
     * @return <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
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    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
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    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，

public class FutureTask<V> implements RunnableFuture<V>
RunnableFuture
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。

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 ：
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
/**
   * 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。

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);
  }

}

输出结果