• 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 :

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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 :

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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，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 :

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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则是一个RunnableFuture，而RunnableFuture实现了Runnbale又实现了Futrue这两个接口，

     

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    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>

    另外它还可以包装Runnable和Callable， 由构造函数注入依赖。

     

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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 ：

     

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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适配器

     

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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。

     

    简单示例

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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, 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);
        }
     
    }</integer></integer></integer></v></v></v></integer></integer>

    输出结果
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  • 原文地址：https://www.cnblogs.com/caogang/p/4596761.html
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