0100 Java中的Runnable、Callable、Future、FutureTask的区别与示例【进阶】

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();}

allable

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);  }}

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