Java中的Runnable、Callable、Future、FutureTask的区别

本文转载自：http://blog.csdn.net/bboyfeiyu/article/details/24851847

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>
     *
     * @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 :

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 <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;
}

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>

 

 

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 ：

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。

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);
    }

}