Java中的Runnable、Callable、Future、FutureTask的区别

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 :

@FunctionalInterfacepublic 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 :

@FunctionalInterfacepublic 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。

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 <V> The result type returned by this Future's {@code get} 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 {@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.     *     * <p>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<V> implements RunnableFuture<V> {......}

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 <V> The result type returned by this Future's {@code get} method */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<V> 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<Void>(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 <T> the type of the result     * @return a callable object     * @throws NullPointerException if task null     */    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。

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<Integer> {        @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<String> future = (Future<String>) 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<Integer> future = (Future<Integer>) 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<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。      */      public static void testFutureTask(){        ExecutorService executorService = Executors.newCachedThreadPool();        FutureTask<Integer> futureTask = new FutureTask<Integer>(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<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。      */      public static void testFutureTask2(){        ExecutorService executorService = Executors.newCachedThreadPool();        FutureTask<Integer> futureTask = new FutureTask<Integer>(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);      }  }

学习Java的同学注意了！！！ 
学习过程中遇到什么问题或者想获取学习资源的话，欢迎加入Java学习交流群，群号码：286945438 我们一起学Java！