java - Windows - 在Java程序中使用set和echo

Question

我想在 Java 中设置并回显 Windows 变量:

public static void main(String[] args) throws IOException
{

    Runtime rt = Runtime.getRuntime();
    String[] cmd = { "cmd.exe", "/c", "set HOSTNAME=%COMPUTERNAME% "
            + "&& echo %HOSTNAME%" };
    Process proc = rt.exec(cmd);

    BufferedReader stdInput = new BufferedReader(
            new InputStreamReader(proc.getInputStream()));

    BufferedReader stdError = new BufferedReader(
            new InputStreamReader(proc.getErrorStream()));

    System.out.println("Output:\n");
    String s = null;
    while ((s = stdInput.readLine()) != null)
    {
        System.out.println(s);
    }

    System.out.println("Error (if any):\n");
    while ((s = stdError.readLine()) != null)
    {
        System.out.println(s);
    }
}

我希望程序会打印出我的计算机主机名，否则我会将此值用于其他目的。但输出是这样的:

Output:

%HOSTNAME%
Error (if any):

如何获取在命令 set HOSTNAME=%COMPUTERNAME% 中设置的值

Answer 1

这与 Java 无关，因为这是 cmd 解析命令的方式。整个命令将立即被解析以进行变量扩展。在解析命令时，变量尚不可用，因此它将被批处理文件中的任何内容替换，或在命令行中保持原样

您需要使用delayed expansion并使用 !!

打印变量

cmd.exe /V:ON /c set HOSTNAME=%COMPUTERNAME% && echo !HOSTNAME!

/V:ON用于启用延迟扩展

CMD [/A | /U] [/Q] [/D] [/E:ON | /E:OFF] [/F:ON | /F:OFF] [/V:ON | /V:OFF]
    [[/S] [/C | /K] string]
...
/V:ON   Enable delayed environment variable expansion using ! as the
        delimiter. For example, /V:ON would allow !var! to expand the
        variable var at execution time.  The var syntax expands variables
        at input time, which is quite a different thing when inside of a FOR
        loop.

在批处理文件中，可以通过 setlocal EnableDelayedExpansion 启用它

但是为此目的，只需 cmd.exe/V:ON/c echo %COMPUTERNAME% 就足够了。但这仍然不是有效的方法。 There are better ways to get hostname in Java

Map<String, String> env = System.getenv();
if (env.containsKey("COMPUTERNAME"))
    return env.get("COMPUTERNAME");

或

InetAddress.getLocalHost().getHostName()