java - 为什么此代码中会出现 "No line found"异常？

Question

以下方法会导致未找到行异常:

private static String getRebaseAnswer(boolean isFirst, boolean isLast) {
    System.out.println("Would you like to (c)ontinue, (s)kip this commit, or"
            + " change this commit's (m)essage?");
    Scanner in = new Scanner(System.in);
    String answer;
    while (true) {
        answer = in.nextLine(); // <--- This Line
        if (answer.equals("c") || answer.equals("m")) {
            in.close();
            return answer;
        } else if (answer.equals("s") && !isFirst && !isLast) {
            in.close();
            return answer;
        } else {
            System.out.println("Would you like to (c)ontinue, (s)kip this commit, or"
                    + " change this commit's (m)essage?");
        }
    }
}

我正在这个方法中调用该方法:

...
String answer;
Scanner in;
currHead = branchHeads.get(arg);
while (toRebase != null) {
    System.out.println("Currently replaying:");
    toRebase.getNode().printInfo();
    answer = getRebaseAnswer(isFirst, toRebase.getParent() == null); // <--- This Line
...

是什么导致了这个错误？扫描仪不应该等待我输入一行然后再继续 getRebaseAnswer 方法吗？我的代码中的另一个方法与上述方法具有完全相同的结构，并且没有遇到任何问题。我已经检查了有关此问题的多个其他帖子，但他们的建议都与此问题无关或无法解决它。

此方法运行没有问题:

private static boolean handleDangerous() {
    System.out.println("Warning: The command you entered may alter the files in your"
            + " working directory. Uncommitted changes may be lost. Are you sure you"
            + " want to continue? (yes/no)");
    Scanner in = new Scanner(System.in);
    String answer;
    while (true) {
        answer = in.nextLine();
        if (answer.equals("yes")) {
            in.close();
            return true;
        } else if (answer.equals("no")) {
            in.close();
            return false;
        } else {
            System.out.println("Not a valid answer, please enter (yes/no).");
        }
    }
}

Answer 1

当您创建连接到 System.in 的扫描仪并将其关闭时，您也会关闭 System.in。因此，后续尝试从 System.in 读取数据将导致您观察到的异常。

避免这种情况的方法是只创建一次扫描仪，并且在程序完成之前永远不要关闭它。应将此扫描程序传递给需要从 System.in 读取的任何函数。