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Java中Runnable和Callable有何不同

Java中Runnable和Callable有何不同?针对这个问题,这篇文章详细介绍了相对应的分析和解答,希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

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Java中的Runnable,Callable,Future,FutureTask的比较

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 :

@FunctionalInterface
public interface Runnable {
  /**
   * When an object implementing interface Runnable is used
   * to create a thread, starting the thread causes the object's
   * run method to be called in that separately executing
   * thread.
   * 

* The general contract of the method run 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 :

@FunctionalInterface
public interface Callable {
  /**
   * 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:

* @see FutureTask
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param  The result type returned by this Future's {@code get} method
 */
public interface Future {

  /**
   * 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 {@code cancel} is called,
   * this task should never run. If the task has already started,
   * then the {@code mayInterruptIfRunning} parameter determines
   * whether the thread executing this task should be interrupted in
   * an attempt to stop the task.
   *
   * 

After this method returns, subsequent calls to {@link #isDone} will * always return {@code true}. Subsequent calls to {@link #isCancelled} * will always return {@code true} if this method returned {@code true}. * * @param mayInterruptIfRunning {@code true} if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete * @return {@code false} if the task could not be cancelled, * typically because it has already completed normally; * {@code true} otherwise */ boolean cancel(boolean mayInterruptIfRunning); /** * Returns {@code true} if this task was cancelled before it completed * normally. * * @return {@code true} if this task was cancelled before it completed */ boolean isCancelled(); /** * Returns {@code true} if this task completed. * * Completion may be due to normal termination, an exception, or * cancellation -- in all of these cases, this method will return * {@code true}. * * @return {@code true} 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 implements RunnableFuture {
......
}

RunnableFuture

/**
 * A {@link Future} that is {@link Runnable}. Successful execution of
 * the {@code run} method causes completion of the {@code Future}
 * and allows access to its results.
 * @see FutureTask
 * @see Executor
 * @since 1.6
 * @author Doug Lea
 * @param  The result type returned by this Future's {@code get} method
 */
public interface RunnableFuture extends Runnable, Future {
  /**
   * Sets this Future to the result of its computation
   * unless it has been cancelled.
   */
  void run();
}

另外FutureTask还可以包装Runnable和Callable< V>, 由构造函数注入依赖。

/**
   * Creates a {@code FutureTask} that will, upon running, execute the
   * given {@code Callable}.
   *
   * @param callable the callable task
   * @throws NullPointerException if the callable is null
   */
  public FutureTask(Callable callable) {
    if (callable == null)
      throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;    // ensure visibility of callable
  }

  /**
   * Creates a {@code FutureTask} that will, upon running, execute the
   * given {@code Runnable}, and arrange that {@code get} will return the
   * given result on successful completion.
   *
   * @param runnable the runnable task
   * @param result the result to return on successful completion. If
   * you don't need a particular result, consider using
   * constructions of the form:
   * {@code Future<?> f = new FutureTask(runnable, null)}
   * @throws NullPointerException if the runnable is null
   */
  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 :

/**
   * Returns a {@link Callable} object that, when
   * called, runs the given task and returns the given result. This
   * can be useful when applying methods requiring a
   * {@code Callable} to an otherwise resultless action.
   * @param task the task to run
   * @param result the result to return
   * @param  the type of the result
   * @return a callable object
   * @throws NullPointerException if task null
   */
  public static  Callable callable(Runnable task, T result) {
    if (task == null)
      throw new NullPointerException();
    return new RunnableAdapter(task, result);
  }

RunnableAdapter适配器

/**
   * A callable that runs given task and returns given result
   */
  static final class RunnableAdapter implements Callable {
    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。

完整示例:

package com.stay4it.rx;

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;

public class FutureTest {

  public static class Task implements Runnable {

    @Override
    public void run() {
      // TODO Auto-generated method stub
      System.out.println("run");
    }

  }
  public static class Task2 implements Callable {

    @Override
    public Integer call() throws Exception {
      System.out.println("call");
      return fibc(30);
    }

  }

   /** 
   * runnable, 无返回值 
   */ 
  public static void testRunnable(){
    ExecutorService executorService = Executors.newCachedThreadPool();

    Future future = (Future) executorService.submit(new Task());
    try {
      System.out.println(future.get());
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (ExecutionException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    executorService.shutdown();
  }

  /** 
   * Callable, 有返回值 
   */ 
  public static void testCallable(){
    ExecutorService executorService = Executors.newCachedThreadPool();

    Future future = (Future) executorService.submit(new Task2());
    try {
      System.out.println(future.get());
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (ExecutionException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    executorService.shutdown();
  }

   /** 
   * FutureTask则是一个RunnableFuture,即实现了Runnbale又实现了Futrue这两个接口, 
   * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable 
   * ,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。 
   */ 
  public static void testFutureTask(){
    ExecutorService executorService = Executors.newCachedThreadPool();
    FutureTask futureTask = new FutureTask(new Task2());

    executorService.submit(futureTask);
    try {
      System.out.println(futureTask.get());
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (ExecutionException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    executorService.shutdown();
  }

   /** 
   * FutureTask则是一个RunnableFuture,即实现了Runnbale又实现了Futrue这两个接口, 
   * 另外它还可以包装Runnable(实际上会转换为Callable)和Callable 
   * ,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。 
   */ 
  public static void testFutureTask2(){
    ExecutorService executorService = Executors.newCachedThreadPool();
    FutureTask futureTask = new FutureTask(new Runnable() {

      @Override
      public void run() {
        // TODO Auto-generated method stub
        System.out.println("testFutureTask2 run");
      }
    },fibc(30));

    executorService.submit(futureTask);
    try {
      System.out.println(futureTask.get());
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (ExecutionException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

    executorService.shutdown();
  }



  public static void main(String[] args) {

    testCallable();

  }

  /** 
   * 效率低下的斐波那契数列, 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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