[Guava]ListenableFuture的使用 [Guava]ListenableFuture的使用

ListenableFuture定义介绍ListenableFuture之前先介绍下Future,
Future是Java 5增加的，代表一个异步计算的结果，提供检查结果是否计算完成的方法，等待它的完成并且检索计算结果，只能计算完成时，才能获取到计算的结果，必要时阻塞，直到准备好为止。
虽然Future以及相关使用方法提供了异步执行任务的能力，但是对于结果的获取却是很不方便，只能通过阻塞或者轮询的方式得到任务的结果。阻塞的方式显然和我们的异步编程的初衷相违背，轮询的方式又会耗费无谓的CPU资源，而且也不能及时地得到计算结果，为什么不能用观察者设计模式当计算结果完成及时通知监听者呢？
【[Guava]ListenableFuture的使用】guava的ListenableFuture 和jdk8的CompletableFuture 都增加了监听器来监听计算的结果，后面再讲CompletableFuture，今天重点在ListenableFuture。
ListenableFuture继承了Future，在此基础上增加了下面方法

void addListener(Runnable listener, Executor executor);

是为了在运算（多线程执行）完成的时候进行调用
特性增加了适配jdk的Future接口（com.google.common.util.concurrent.JdkFutureAdapters#listenInPoolThread）
提供静态工具类Futures，丰富对Future的操作
作用例子

public class ListenableFutureLearn {private static final int processors = Runtime.getRuntime().availableProcessors(); private static final ThreadFactory threadFactory = new ThreadFactoryBuilder() .setDaemon(true) .setNameFormat("ListenableFutureAdapter-thread-%d") .build(); private static final ExecutorService defaultAdapterExecutor = Executors.newFixedThreadPool(processors,threadFactory); public static void main(String[] args) {String string = "test"; Future future = defaultAdapterExecutor.submit(new Task(string) { }); ListenableFuture listenInPoolThread = JdkFutureAdapters.listenInPoolThread(future); Futures.addCallback(listenInPoolThread, new FutureCallback() { @Override public void onSuccess(String result) { System.out.printf("success", result); }@Override public void onFailure(Throwable t) { System.err.printf("failure"); } }); }private static class Task implements Callable {private final String data; public Task(String data) { this.data = https://www.it610.com/article/data; }@Override public String call() throws Exception { try { return"result_Success" + data; } catch (Exception e) {return "result_Failure" + data; }} } }

源码分析 AbstractFuture 实现了ListenableFuture接口，实现了addListener()方法

public void addListener(Runnable listener, Executor executor) { // 校验 checkNotNull(listener, "Runnable was null."); checkNotNull(executor, "Executor was null."); Listener oldHead = listeners; if (oldHead != Listener.TOMBSTONE) { Listener newNode = new Listener(listener, executor); do { newNode.next = oldHead; if (ATOMIC_HELPER.casListeners(this, oldHead, newNode)) { return; } oldHead = listeners; // re-read } while (oldHead != Listener.TOMBSTONE); } // If we get here then the Listener TOMBSTONE was set, which means the future is done, call // the listener. executeListener(listener, executor); }

参考文献 [http://colobu.com/2016/02/29/Java-CompletableFuture/]