FX Task，返回一个值总是返回null [英] FX Task, returning a value always returns null

问题描述

我正在尝试从FX任务返回一个字符串。

I am trying to return a String from a FX Task.

预期输出：true，testABC。

Expected output: true,testABC.

实际输出：true，null。

Real output: true,null.

对任务的调用：

    public static void main(String[] args) {
    ExecutorService pool = Executors.newCachedThreadPool();
    Future<String> my_String = (Future<String>) pool.submit(new my_task());

    try {
        Thread.sleep(500);
        System.out.println(my_String.isDone());
        System.out.println(my_String.get());//gettings the String from the future
    } catch (InterruptedException | ExecutionException ex) {
        Logger.getLogger(Return_test.class.getName()).log(Level.SEVERE, null, ex);
    }}

任务：

public class my_task extends Task<String>{

@Override
protected String call() throws Exception {
    String tempString = "testABC";
    return tempString;
}}

推荐答案

任务 实现 Runnable ，但不是 Callable 。所以当你调用 pool.submit（myTask）时，你正在调用重载形式的 ExecutorService.submit（...）获取 Runnable 。当然，一般来说， Runnable s不会返回值，因此从该版本的<返回 Future code>提交（...），正如Aerus已经指出的那样，只需从返回 null get（）方法（它无法从 Runnable 获取值）。

Task implements Runnable, but not Callable. So when you call pool.submit(myTask), you are calling the overloaded form of ExecutorService.submit(...) taking a Runnable. In general, of course, Runnables do not return values, so the Future that is returned from that version of submit(...), as Aerus has already pointed out, just returns null from its get() method (it cannot get a value from the Runnable).

然而，任务也直接扩展 FutureTask ，所以您可以直接使用您的任务来获得结果。只需

However, Task also extends FutureTask directly, so you can use your task directly to get the result. Just do

Task<String> myTask = new my_task(); // Please use standard naming conventions, i.e. new MyTask();
pool.submit(myTask);

try {
    // sleep is unnecessary, get() will block until ready anyway
    System.out.println(myTask.get());
} catch (InterruptedException | ExecutionException ex) {
    Logger.getLogger(Return_test.class.getName()).log(Level.SEVERE, null, ex);
}

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