问题

我正在制作一个主要与管道一起使用的应用程序。在程序开始时，这是我如何获取所有输入的方式：

```java
Scanner scanner = new Scanner(System.in);
List<String> input = new ArrayList<>();

while (scanner.hasNextLine()) {
    input.add(scanner.nextLine());
}

scanner.close();

在与管道一起使用时，这个方法效果很好。例如，运行 ls | java -jar ... 会打印出从 ls 得到的所有行。然而，当我只运行 java -jar ...，没有输入/管道时，程序会冻结，什么也不会发生。即使我按下回车并不断敲击键盘，程序仍然卡住。我只能使用 ctrl-c 来停止程序。我认为这是因为没有内容可读，因此 Scanner 会等待直到有内容可读。如何在不冻结的情况下从 System.in 中读取所有行，或者是否有更好的方法来实现这一点？谢谢。

编辑：
将这个问题标记为重复的人显然没有阅读我的问题。我的问题是要检查 System.in 是否为空，以免冻结程序，而不是如何通过输入阶段。程序之所以冻结，是因为 System.in 中没有内容，所以 Scanner 会一直等待，但我希感知是否有内容在 System.in 中。

答案：
感谢 @QiuZhou 的答案。我修改了他的第二个示例，得到了这个代码：

List<String> input = new ArrayList<>();
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

while (reader.ready()) {
    String nextLine = reader.readLine();
    input.add(nextLine);
}

它运行得非常好。

<details>
<summary>英文:</summary>

I&#39;m making an application that is mainly used with pipes. At the start of the program, here is how I get all my input:

Scanner scanner = new Scanner(System.in);
List<String> input = new ArrayList<>();

while (scanner.hasNextLine()) {
input.add(scanner.nextLine());
}

scanner.close();

When used with pipes, this works well. For example, running ```ls | java -jar ...``` it prints out all the lines given from ```ls```. However, when I just run ```java -jar ...``` with no input/pipe, it freezes and nothing happens. Even when I press enter and mash my keyboard it's still stuck. I can only stop the program by using ctrl-c. I think this is because there is nothing to read so Scanner waits until there is something to read. How do I read all lines from ```System.in``` without freezing, or is there a better way to do this? Thanks.

Edit:
Whoever marked this as a duplicate clearly did not read my question. My question was to check if the ```System.in``` was empty to not freeze the program, not how to get past the input stage. It is freezing because there is nothing in ```System.in``` so Scanner is taking forever, but I want to check if there is nothing in ```System.in```.

Answer:
Thank you @QiuZhou for the answer. I modified his second example to get this:

List<String> input = new ArrayList<>();
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

while (reader.ready()) {
String nextLine = reader.readLine();
input.add(nextLine);
}

and it works like a charm.

</details>


# 答案1
**得分**: 4

因为你说了，"没有内容可读，所以扫描器会等待直到有内容可读"，所以它会冻结。但是你想要"从System.in读取所有行而不冻结"，这对我来说听起来有些矛盾，这不像从文件中读取，我们不知道使用你的程序的人会输入多少行，除非这个人告诉我们。我建议你定义一个特殊的字符作为`输入结束`的标志，例如`done`，当用户决定完成输入时，只需输入`done`以结束等待：

```java
        Scanner scanner = new Scanner(System.in);
        List<String> input = new ArrayList<>();

        System.out.println("请输入内容。输入'done'表示完成。");

        while (scanner.hasNextLine()) {
            String nextLine = scanner.nextLine();
            if (nextLine.equals("done")) break;
            input.add(nextLine);
        }

        scanner.close();

要在不被卡住的情况下检查System.in中是否有内容可读，你可以使用BufferedReader.ready()来实现，正如文档所说，"请注意，只有在流上的下次读取不会阻塞时，in.ready()才会返回true。"

        System.out.println("请输入内容。输入'done'表示完成。");

        List<String> input = new ArrayList<>();
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            if (reader.ready()) {
                String nextLine = reader.readLine();
                if (nextLine.equals("done")) break;
                input.add(nextLine);
            }
            //由于ready()不会阻塞，你可以在这里添加额外的逻辑，比如如果从输入中读取下一行需要太长时间，
            //你可以在这里直接结束循环
            // break;
        }

        Scanner scanner = new Scanner(System.in);
        List<String> input = new ArrayList<>();

        System.out.println("Please enter something. Enter 'done' when finished.");

        while (scanner.hasNextLine()) {
            String nextLine = scanner.nextLine();
            if(nextLine.equals("done"))break;
            input.add(nextLine);
        }

        scanner.close();

        System.out.println("Please enter something. Enter 'done' when finished.");

        List<String> input = new ArrayList<>();
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        while(true) {
            if (reader.ready()) {
                String nextLine = reader.readLine();
                if(nextLine.equals("done"))break;
                input.add(nextLine);
            }
            //since ready() will not block, you can add additional logic here, like if it
            //takes too long to read next line from input, you can just end here
            // break;
        }