[Guava]ListenableFuture的使用

ListenableFuture定义

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

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<String> future = defaultAdapterExecutor.submit(new Task(string) {
        });

        ListenableFuture<String> listenInPoolThread = JdkFutureAdapters.listenInPoolThread(future);
        Futures.addCallback(listenInPoolThread, new FutureCallback<String>() {
            @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<String> {

        private final String data;

        public Task(String data) {
            this.data = 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