如何阻止 Java 扫描器接受输入

Question

所以，我正在开发一个从命令行运行的测验程序，测验有时间限制。我想做的是在用户时间结束时立即停止测验，即使他们正在回答问题。我正在使用 Java 的 Scanner 来获取用户的输入，所以我基本上想告诉 Scanner 对象终止，即使它正在接受输入。

现在，我知道我可以在事后追溯惩罚超时的用户，但我只是希望在超过时间限制后终止测验。例如，有没有办法用多线程来做到这一点？

Answer 1

A Java 扫描器正在使用阻塞操作。阻止它是不可能的。甚至不使用 Thread.interrupt();

但是您可以使用 BufferedLineReader 阅读并能够停止线程。这不是一个巧妙的解决方案，因为它需要暂停片刻（否则它会使用 100% CPU），但它确实有效。

public static class ConsoleInputReadTask {
    private final AtomicBoolean stop = new AtomicBoolean();

    public void stop() {
        stop.set(true);
    }

    public String requestInput() throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("ConsoleInputReadTask run() called.");
        String input;
        do {
            System.out.println("Please type something: ");
            try {
                // wait until we have data to complete a readLine()
                while (!br.ready() && !stop.get()) {
                    Thread.sleep(200);
                }
                input = br.readLine();
            } catch (InterruptedException e) {
                System.out.println("ConsoleInputReadTask() cancelled");
                return null;
            }
        } while ("".equals(input));
        System.out.println("Thank You for providing input!");
        return input;
    }
}

public static void main(String[] args) {
    final Thread scannerThread = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                String string = new ConsoleInputReadTask().requestInput();
                System.out.println("Input: " + string);
            }
            catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
    });
    scannerThread.start();

    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            scannerThread.interrupt();
        }
    }).start();
 }

Answer 2

我受够了看着人们谈论编程并提供不了解编程的低劣解决方案。最糟糕的是，谈论像 Java™ 这样灵活的语言是。

这是您的解决方案，各位：


    public static void main(String[] args) throws IOException
    {
        Scanner sc = new Scanner(new BufferedInputStream(System.in), "UTF-8");
        String item;
        
        while (true)
        {
            System.out.print("Give me some input:\t");
            if (!(item = sc.nextLine()).equals(""))
                System.out.println("\nYour Input was:\t" + item);
            else
                break;
        }
        
        System.out.println("Goodbye, see you again!");
    }

来自Java™编程大师（手握多年经验）8-|

示例输出：

Give me some input: Your

Your Input was: Your
Give me some input: Programming

Your Input was: Programming
Give me some input: Knowledge

Your Input was: Knowledge
Give me some input: Sucks

Your Input was: Sucks
Give me some input: !!!

Your Input was: !!!
Give me some input: In the next line I'll just type ENTER so that my input is recognized as empty ;-)

Your Input was: In the next line I'll just type ENTER so that my input is recognized as empty ;-)
Give me some input: 
Goodbye, see you again!

如何阻止 Java 扫描器接受输入

How to stop Java Scanner from accepting input

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