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limestreetlab
Asked: 2024-06-25 03:54:28 +0800 CST

Callable 如何从预定义的 void 回调返回值？

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我正在使用一个数据相关的 API 接口，它有一个关键的void回调函数，该函数会自动调用来标记某些 IO 操作的结束。我想创建类Callable<String>并使用Future<String> result。

我苦苦思索如何让 Callable 返回字符串。String returnResult(){return this.result}在内部创建一个函数来调用不行。

请指教。

它看起来像：

public class MyCallable implements someAPIWrapper, Callable<String> {
    
    String result;

    @Override
    public void endOfJobCallback() { //predefined API callback marking end of work
      /*
      usually read the data and write to a file, but not my case.
      how to return this.result string from here?
      */
    }

    @Override
    public String call() throws Exception {
      //some logic stuff
      //make API call to request a bunch of data
      //inside a loop to listen to incoming messages, receiving and appending to the *result* variable
      //end of all messages signalled by the ending callback, stop loop and return result var
    }

}

class Main {
    public static void main(String[] args){
      MyCallable callable = new MyCallable();
      ExecutorService executor = Executors.newFixedThreadPool(2);
      Future<String> future = executor.submit(callable);
      String result = future.get(); //blocking until result ready
    }
}
java

2 个回答

  1. Best Answer

    您可以使用（原子）布尔值来知道何时停止循环：

    public class MyCallable implements someAPIWrapper, Callable<String> {
    
    String result;
    private final AtomicBoolean complete = new AtomicBoolean();

    @Override
    public void endOfJobCallback() { //predefined API callback marking end of work
      complete.set(true); //set Boolean to true atomically so that the other method will know you have been called
    }

    @Override
    public String call() throws Exception {
      while (!complete.get()) {
        //Do your stuff
      }
      //if you reach this the callback was called
      return result;
    }

}

    注意：由于您没有添加任何代码，因此不清楚您究竟是如何收听消息的。您可能希望采纳该想法并将其改编为实际代码，具体取决于您是否进行简短轮询、是否拥有 webhook 等

    • 2

  2. CompletableFuture您可以通过在 中添加 来使其工作MyCallable，然后在 中完成它endOfJobCallback()。

    public class MyCallable implements someAPIWrapper, Callable<String> {
    
    String result;
    CompletableFuture<String> future = new CompletableFuture<>();

    @Override
    public void endOfJobCallback() { //predefined API callback marking end of work
      future.complete(result);
    }
}

    然后在你的main()：

    MyCallable callable = new MyCallable();
// do stuff with the API…

String result = callable.future.join(); //blocking until result ready

    （当然这需要确保endOfJobCallback()在 100％ 的情况下被调用，否则您可能需要实现一些错误处理，或者您可能想要使用get(timeout, unit)而不是join()）

    • 2

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