Java中的Runnable、Callable、Future、FutureTask的区别和CompletionService的使用场景

2021-04-25
Java中的Runnable、Callable、Future、FutureTask的区别和CompletionService的使用场景

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 <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 :


@FunctionalInterface

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。


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);  

    }  


}

CompletionService

《Java并发编程实践》中关于CompletionService的描述如下:


如果向Executor提交了一组计算任务,并且希望在计算完成后获得结果,那么可以保留与每个任务关联的Future,然后反复使用get方法,同时将参数timeout指定为0,从而通过轮询来判断任务是否完成。这种方法虽然可行,但却有些繁琐。幸运的是,还有一种更好的方法:完成服务CompletionService。


CompleteService接口是为了方便多个任务执行时,可以方便得获取到执行任务的Future结果。接口内容如下:


public interface CompletionService<V> {

    Future<V> submit(Callable<V> task);

    Future<V> submit(Runnable task, V result);

    Future<V> take() throws InterruptedException;

    Future<V> poll();

    Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;

}

这五个方法分为两大方面。一个是对Callable和Runnable类型参数的任务提交,另一方面则是尝试对结果以不同的方式进行获取,take()方法一般是阻塞式的获取,后两者则更灵活。


通常来讲,CompleteService是要和Executor结合在一起使用的。


ExecutorCompletionService

在JDK中,ExecutorCompletionService是CompletionService接口的唯一实现类。

这个实现类主要做的事就是将执行完成的任务结果放到阻塞队列中,这样等待结果的线程,如果执行take()方法会得到结果并恢复执行。


ExecutorCompletionService有3个属性:


AbstractExecutorService类的对象aes

Executor类的对象executor

BlockingQueue<Future<V>>的completionQueue

通常,如果executor是AbstractExecutorService的一个实现,则将其赋值给aes属性,否则赋值为null。


在这个类中,executor负责执行任务,而aes则负责做适配处理,返回包装好任务的FutureTask对象。


这里面有一个对于实现功能很重要的内部类QueueingFuture,实现如下:


/**

  * FutureTask extension to enqueue upon completion

  */

 private class QueueingFuture extends FutureTask<Void> {

     QueueingFuture(RunnableFuture<V> task) {

         super(task, null);

         this.task = task;

     }

     protected void done() { completionQueue.add(task); }

     private final Future<V> task;

 }

QueueingFuture是FutureTask的一个子类,通过扩展该子类的done方法,可以实现当任务完成时,将结果放入到BlockingQueue中。


而通过使用BlockingQueue的take或poll方法,则可以得到结果。在BlockingQueue不存在元素时,这两个操作会阻塞,一旦有结果加入,则立即返回。


public Future<V> take() throws InterruptedException {  

    return completionQueue.take();  

}  


public Future<V> poll() {  

    return completionQueue.poll();  

}  

下面我们通过例子来体验下CompletionService的好处与使用场景。


首先定义一个实现了Callable接口的Task类:


private static class Task implements Callable<String>{  


        private volatile int i;  


        public Task(int i){  

            this.i = i;  

        }  


        @Override  

        public String call() throws Exception {  

            Thread.sleep(1000);  

            System.out.println(Thread.currentThread().getName());  

            return "任务 : " + i;  

        }  


    }

1)自己维护一个list保存submit的callable task所返回的Future对象。在主线程中遍历集合并调用Future的get()方法取到Task的返回值。如下代码所示:


public static void testFuture() throws InterruptedException, ExecutionException {  

        System.out.println("main Thread begin:");  

        ExecutorService executor = Executors.newCachedThreadPool();  

        List<Future<String>> result = new ArrayList<Future<String>>();  

        for (int i = 0;i<10;i++) {  

            Future<String> submit = executor.submit(new Task(i));  

            result.add(submit);  

        }  

        executor.shutdown();  

        for (int i = 0;i<10;i++) {//一个一个等待返回结果  

            System.out.println(result.get(i).get());  

        }  

        System.out.println("main Thread end:");  

    }

输出结果如下:


main Thread begin:

pool-1-thread-2

pool-1-thread-1

pool-1-thread-3

任务 : 0

任务 : 1

任务 : 2

pool-1-thread-10

pool-1-thread-6

pool-1-thread-9

pool-1-thread-8

pool-1-thread-7

pool-1-thread-4

pool-1-thread-5

任务 : 3

任务 : 4

任务 : 5

任务 : 6

任务 : 7

任务 : 8

任务 : 9

main Thread end:


从输出结果可以看出,我们只能一个一个阻塞的取出。这中间肯定会浪费一定的时间在等待上。如5返回了,但是前面的1-4都没有返回,那么5就得等1-4输出才能输出。


2)通过CompletionService包装ExecutorService,然后调用其take()方法去取Future对象。如下代码所示:


private static void testCompletionService() throws InterruptedException, ExecutionException {

        System.out.println("main Thread begin:");  

        ExecutorService executor = Executors.newCachedThreadPool();

        ExecutorCompletionService<String> completionService = new ExecutorCompletionService<>(executor);

        for(int i=0;i<10;i++){  

            completionService.submit(new Task(i));  

        }

        executor.shutdown();

        for(int i=0;i<10;i++){  

            System.out.println(completionService.take().get());  

        }

        System.out.println("main Thread end:");  

    }


输出结果如下:


main Thread begin:

pool-1-thread-9

pool-1-thread-7

pool-1-thread-5

pool-1-thread-6

pool-1-thread-3

pool-1-thread-4

任务 : 8

任务 : 5

任务 : 4

任务 : 6

pool-1-thread-2

pool-1-thread-10

pool-1-thread-1

pool-1-thread-8

任务 : 2

任务 : 3

任务 : 1

任务 : 9

任务 : 0

任务 : 7

main Thread end:


可以看出,结果的输出和线程的放入顺序无关系。每一个线程执行成功后,立刻就输出。如5返回了,不管前面的1-4有没有返回,5马上输出,不管它们加入线程池的顺序,从而节省时间。


总结

自己创建一个集合来保存Future存根并循环调用其返回结果的时候,主线程并不能保证首先获得的是最先完成任务的线程返回值。它只是按加入线程池的顺序返回。因为take方法是阻塞方法,后面的任务完成了,前面的任务却没有完成,主程序就那样等待在那儿,只到前面的完成了,它才知道原来后面的也完成了。

使用CompletionService来维护处理线程的返回结果时,主线程总是能够拿到最先完成的任务的返回值,而不管它们加入线程池的顺序。

CompletionService的实现是维护了一个保存Future的BlockingQueque。只有当这个Future的任务状态是结束的时候,才会加入到这个Queque中,take()方法其实就是Producer-Consumer中的Consumer。它会从Queue中取出Future对象,如果Queue是空的,就会阻塞在那里,直到有完成的Future对象加入到Queue中。也就是先完成的必定先被取出,这样就减少了不必要的等待时间。