Java并发编程之Callable, Runnable, Future, FutureTask
Java中存在Callable, Runnable, Future, FutureTask这几个与线程相关的类或接口, 下面来了解一下它们的作用和区别.
一.Callable和Runnable
Callable和Runnable类似, 实现Callable和Runnable接口的类都是可以被其他线程运行的任务, Callable和Runnable主要有以下几点区别:
(1). Callable中声明的方法是call(), Runnable中声明的方法是run().
(2). Callable任务执行后可返回值, Runnable任务执行后没有返回值.
(3). call()方法可以抛出异常, run()方法不能抛出异常.
(4). 运行Callable任务可以拿到一个Future对象.
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;
}
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();
}
二. Future对象
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。可以通过get()方法获取执行结果, get()方法会阻塞，直到任务返回结果
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.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
* will always return <tt>true</tt> if this method returned <tt>true</tt>.
*
* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return <tt>false</tt> if the task could not be cancelled,
* typically because it has already completed normally;
* <tt>true</tt> otherwise
*/
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.
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* <tt>true</tt>.
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its 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.
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
下面依次解释每个方法的作用:
- cancel方法用来取消任务，如果取消任务成功则返回true，如果取消任务失败则返回false。参数 mayInterruptIfRunning表示是否允许取消正在执行却没有执行完毕的任务，如果设置true，则表示可以取消正在执行过程中的任务。如 果任务已经完成，则无论mayInterruptIfRunning为true还是false，此方法肯定返回false，即如果取消已经完成的任务会返 回false；如果任务正在执行，若mayInterruptIfRunning设置为true，则返回true，若 mayInterruptIfRunning设置为false，则返回false；如果任务还没有执行，则无论 mayInterruptIfRunning为true还是false，肯定返回true。
- isCancelled方法表示任务是否被取消成功，如果在任务正常完成前被取消成功，则返回 true。
- isDone方法表示任务是否已经完成，若任务完成，则返回true；
- get()方法用来获取执行结果，这个方法会产生阻塞，会一直等到任务执行完毕才返回；
- get(long timeout, TimeUnit unit)用来获取执行结果，如果在指定时间内，还没获取到结果，就直接返回null。
也就是说Future提供了三种功能：
1）判断任务是否完成；
2）能够中断任务；
3）能够获取任务执行结果。
因为Future只是一个接口，所以是无法直接用来创建对象使用的，因此就有了下面的FutureTask。
三. FutureTask
我们先来看一下FutureTask的实现：
public class FutureTask<V> implements RunnableFuture<V> {
}
FutureTask类实现了RunnableFuture接口，我们看一下RunnableFuture接口的实现：
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
可以看出RunnableFuture继承了Runnable接口和Future接口，而FutureTask实现了RunnableFuture接口。所以它既可以作为Runnable被线程执行，又可以作为Future得到Callable的返回值。
FutureTask提供了2个构造器：
public FutureTask(Callable<V> callable) {
}
public FutureTask(Runnable runnable, V result) {
}
事实上，FutureTask是Future接口的一个唯一实现类
简单示例:
import java.util.concurrent.*;
/**
* Created by xinfengyao on 16-2-17.
*/
public class RunnableFutureTest {
static ExecutorService executorService = Executors.newCachedThreadPool();
public static void main(String[] args) throws ExecutionException, InterruptedException {
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() throws ExecutionException, InterruptedException {
/**
* 提交runnable则没有返回值, future没有数据
*/
Future<?> result = executorService.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 = executorService.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>，既实现了Runnable又实现了Future<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复合体了，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
executorService.submit(futureTask);
System.out.println("future result from futureTask: " + futureTask.get());
}
static int fibc(int n) {
if (n==0 || n==1) {
return n;
}
return fibc(n-1) + fibc(n-2);
}
}
运行结果:
runnable demo: 6765 future result from runnable: null future result from callable: 6765 future result from futureTask: 6765