使用事件侦听器作为Java 8 Stream源

快速正脏

是否可以将传统事件侦听器重构为Java 8 Stream,以便侦听器事件成为流源？

很长的故事

客户端提交任意作业,然后侦听结果：

Client client = new JobClient()
client.addTaskListener(this)
client.submitJobAsync( new MultiTaskJob())  //returns void,important (see below)

public void onTaskResult(TaskResult result){
  if(result.isLastResult())
    aggregateJobResults(result)
  else
    processResult(result)
}

问题

对于提交的任何作业,客户端收到n个结果,但它不知道它将收到多少结果(它使用isLastResult()来确定何时停止和聚合).

目标

我想将侦听器重构为“供应商”或类似的东西,以便onTaskResult()是流源：

Supplier<TaskResult> taskResultSupplier = 
    () -> Stream.of( .. )   //onTaskResult() feeds this
            .map(result -> {
               if(result.isLastResult())
                 //logic here
             });

像这样的东西;如果我能在没有客户知道会有多少结果的情况下做到这一点,我就是金色的;现在,submitJobAsync()返回void,我想保持这种方式,但我也对选项持开放态度……

备择方案

在Tomasz Nurkiewicz在CompletableFutures上阅读类似的情况后,假设对客户进行了微小的更改,则存在备用选项：

List<CompletableFuture<TaskResult>> taskFutures = 
  client.submitJobAsync( new MultiTaskJob())

在这里,客户获得CompletableFutures< TaskResult>的列表,因此我们需要在完成时收集期货的结果：

//processes all task result futures 
List<TaskResult> = taskFutures.stream()
  .map(taskResult ->
     taskResult.thenApply(this::processResult))
  .collect(Collectors.<TaskResult>toList());

本文还说明了使用CompletableFuture.allOf(..)来执行最终处理,但只有在所有期货完成之后(它非常光滑);这就是我的情况下会发生聚合的地方.没有代码可以在这里显示,虽然这篇文章很好地解释了它(我总共有n00b的流,但如果我得到它工作,我会发布代码：-D)

解决方法

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

/**
 * Created for https://stackoverflow.com/q/27670421/1266906.
 */
public class AsyncToStream {
    public static void main(String[] args) {

        System.out.println("Unbuffered Test:");
        AsyncTaskResultIterator<TaskResult> taskListener1 = new AsyncTaskResultIterator<>();
        new TaskResultGenerator(taskListener1,5).start();
        taskListener1.unbufferedStream().forEach(System.out::println);

        System.out.println("Buffered Test:");
        AsyncTaskResultIterator<TaskResult> taskListener2 = new AsyncTaskResultIterator<>();
        new TaskResultGenerator(taskListener2,5).start();
        taskListener2.bufferedStream().forEach(System.out::println);
    }

    /**
     * This class wraps a sequence of TaskResults into an iterator upto the first TaskResult where {@code }isLastResult()} returns {@code true}
     */
    public static class AsyncTaskResultIterator<T extends TaskResult> implements Iterator<T>,TaskListener<T> {

        /**
         * This acts as an asynchronous buffer so we can easily wait for the next TaskResult
         */
        private final BlockingQueue<T> blockingQueue;
        /**
         * Becomes {@code true} once {@code TaskResult.isLastResult()} is received
         */
        private       boolean          ended;

        public AsyncTaskResultIterator() {
            blockingQueue = new LinkedBlockingQueue<>();
        }

        /**
         * Waits on a new TaskResult and returns it as long as the previous TaskResult did not specify {@code isLastResult()}. Afterwards no more elements can be retrieved.
         */
        @Override
        public T next() {
            if (ended) {
                throw new NoSuchElementException();
            } else {
                try {
                    T next = blockingQueue.take();
                    ended = next.isLastResult();
                    return next;
                } catch (InterruptedException e) {
                    throw new IllegalStateException("Could not retrieve next value",e);
                }
            }
        }

        @Override
        public boolean hasNext() {
            return !ended;
        }

        /**
         * Enqueue another TaskResult for retrieval
         */
        @Override
        public void onTaskResult(T result) {
            if (ended) {
                throw new IllegalStateException("Already received a TaskResult with isLastResult() == true");
            }
            try {
                blockingQueue.put(result);
            } catch (InterruptedException e) {
                throw new IllegalStateException("Could not enqueue next value",e);
            }
        }

        /**
         * Builds a Stream that acts upon the results just when they become available
         */
        public Stream<T> unbufferedStream() {
            Spliterator<T> spliterator = Spliterators.spliteratorUnknownSize(this,0);
            return StreamSupport.stream(spliterator,false);
        }

        /**
         * Buffers all results and builds a Stream around the results
         */
        public Stream<T> bufferedStream() {
            Stream.Builder<T> builder = Stream.builder();
            this.forEachRemaining(builder);
            return builder.build();
        }
    }

    public static class TaskResultImpl implements TaskResult {
        private boolean lastResult;
        private String  name;

        public TaskResultImpl(boolean lastResult,String name) {
            this.lastResult = lastResult;
            this.name = name;
        }

        @Override
        public String toString() {
            return "TaskResultImpl{" +
                    "lastResult=" + lastResult +
                    ",name='" + name + ''' +
                    '}';
        }

        @Override
        public boolean isLastResult() {
            return lastResult;
        }
    }

    public static interface TaskListener<T extends TaskResult> {
        public void onTaskResult(T result);
    }

    public static interface TaskResult {
        boolean isLastResult();
    }

    private static class TaskResultGenerator extends Thread {
        private final TaskListener<TaskResult> taskListener;
        private final int                      count;

        public TaskResultGenerator(TaskListener<TaskResult> taskListener,int count) {
            this.taskListener = taskListener;
            this.count = count;
        }

        @Override
        public void run() {
            try {
                for (int i = 1; i < count; i++) {
                    Thread.sleep(200);
                    taskListener.onTaskResult(new TaskResultImpl(false,String.valueOf(i)));
                }
                Thread.sleep(200);
                taskListener.onTaskResult(new TaskResultImpl(true,String.valueOf(count)));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

你没有提供你的TaskResult和TaskListener定义所以我自己编造了. AsyncTaskResultIterator仅适用于单个TaskResult-sequences.如果没有提供具有isLastResult()== true的TaskResult next(),那么无缓冲的Stream和缓冲的Stream生成将无休止地等待.

（编辑：安卓应用网）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!