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。