Java中的Callable、Future、FutureTask、Runnable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();

}

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;

}

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;

}

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，

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public class FutureTask<V> implements RunnableFuture<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();

}

另外它还可以包装Runnable和Callable<V>， 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

}

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);

}

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;

}

}

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);

}

}

输出结果