1 Future接口的缺点
通过Future接口的get()方法可以获取类Callable的返回值,但是此方法最大缺点是阻塞的,因此在并发环境下,效率比较低。Google公司提供的开源Guava库提供了有效的处理异步Future的问题。
2 addListener()方法
Guava包的ListenableFuture实现了Future接口, 其源码如下:
public interface ListenableFuture<V> extends Future<V> {
void addListener(Runnable listener, Executor executor);
}
addListener(Runnable listener, Executor executor)方法是异步的,它注册了一个监听器,当Callable类执行结束,后会进入Runnable接口的实现类中。
public class MyCallable implements Callable<String> {
private String name;
public MyCallable(String name) {
this.name = name;
}
@Override
public String call() throws Exception{
Thread.sleep(5000);
return name;
}
}
public class ListenableFutureApp {
public static void main(String[] args) {
/*
* 方法1:采用 ListenableFuture类自带的addListener(Runnable listener, Executor executor)方法注册回调函数(不推荐)
*/
ExecutorService executor = Executors.newFixedThreadPool(1);
ListeningExecutorService service = MoreExecutors.listeningDecorator(executor);
MyCallable callable = new MyCallable("JayLai");
ListenableFuture<String> listenableFuture = service.submit(callable);
final long time1 = System.currentTimeMillis();
//异步方法
//注册监听,当Callable类者中call()方法执行结束,后会进入new Runnable() { }中
listenableFuture.addListener(new Runnable() {
@Override
public void run() {
try {
System.out.println("执行结果:" + listenableFuture.get());
long time2 = System.currentTimeMillis();
System.out.println("执行时间为:" + (time2 - time1) + "毫秒");
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}, executor); //注册回调函数
System.out.println("=======main method end======");
}
}
运行结果:
3 addCallback()方法
Guava包还提供了更加便利的注册监听方法,使用Futures类中的
public static <V> void addCallback(ListenableFuture<V> future,
FutureCallback<? super V> callback) {
addCallback(future, callback, directExecutor());
}
来注册回调方法,其中FutureCallback类的源码如下
public interface FutureCallback<V> {
void onSuccess(@Nullable V result);
void onFailure(Throwable t);
}
它具体2个方法,当Callable类成功执行结束时候,则进入onSuccess()方法中;若失败,如遇到异常,则进入onFailure()方法。
public class ListenableFutureApp {
public static void main(String[] args) {
/*
* 方法2:采用addCallback注册回调函数(推荐此法)
*/
ExecutorService executorService = Executors.newSingleThreadExecutor();
MyCallable callable = new MyCallable("JayLai");
Future<String> future = executorService.submit(callable);
ListenableFuture<String> listenableFuture = JdkFutureAdapters.listenInPoolThread(future);
long time1 = System.currentTimeMillis();
//注册回调函数
Futures.addCallback(listenableFuture, new FutureCallback<String>() { //异步执行
@Override
public void onSuccess(String result) {
System.out.println("执行结果:" + result);
long time2 = System.currentTimeMillis();
System.out.println("执行时间为:" + (time2 - time1) + "毫秒");
}
@Override
public void onFailure(Throwable t) {
System.out.println("Error!!!");
}
});
System.out.println("=====main method end !====");
}
}
运行结果:
4 参考文献
[1] 高洪岩, Java多线程编程核心技术. 2015.
[2] http://ifeve.com/google-guava-listenablefuture/
[3] http://tool.oschina.net/apidocs/apidoc?api=guava