展开文本中的环境变量 [英] Expand environment variables in text

问题描述

我正在尝试编写一个函数来执行Java中环境变量的替换.因此，如果我有一个看起来像这样的字符串:

I'm trying to write a function to perform substitutions of environment variables in java. So if I had a string that looked like this:

用户$ {USERNAME}的APPDATA路径为$ {APPDATA}.

User ${USERNAME}'s APPDATA path is ${APPDATA}.

我希望结果是:

用户msmith的APPDATA路径为C:\ Users \ msmith \ AppData \ Roaming.

User msmith's APPDATA path is C:\Users\msmith\AppData\Roaming.

到目前为止，我坏了的实现看起来像这样:

So far my broken implementation looks like this:

public static String expandEnvVars(String text) {        
    Map<String, String> envMap = System.getenv();
    String pattern = "\\$\\{([A-Za-z0-9]+)\\}";
    Pattern expr = Pattern.compile(pattern);
    Matcher matcher = expr.matcher(text);
    if (matcher.matches()) {
        for (int i = 1; i <= matcher.groupCount(); i++) {
            String envValue = envMap.get(matcher.group(i).toUpperCase());
            if (envValue == null) {
                envValue = "";
            } else {
                envValue = envValue.replace("\\", "\\\\");
            }
            Pattern subexpr = Pattern.compile("\\$\\{" + matcher.group(i) + "\\}");
            text = subexpr.matcher(text).replaceAll(envValue);
        }
    }
    return text;
}

使用以上示例文本， matcher.matches()返回false.但是，如果我的示例文本为 $ {APPDATA} ，则可以使用.

Using the above sample text, matcher.matches() returns false. However if my sample text, is ${APPDATA} it works.

任何人都可以帮忙吗?

推荐答案

您不想使用

You don't want to use matches(). Matches will try to match the entire input string.

尝试根据图案匹配整个区域.

Attempts to match the entire region against the pattern.

您想要的是 while(matcher.find()){.这将匹配您的模式的每个实例.请查看 的文档> find() .

What you want is while(matcher.find()) {. That will match each instance of your pattern. Check out the documentation for find().

在每次匹配中，第0组将是整个匹配的字符串( $ {appdata} )，而第1组将是 appdata 部分.

Within each match, group 0 will be the entire matched string (${appdata}) and group 1 will be the appdata part.

您的最终结果应类似于:

Your end result should look something like:

String pattern = "\\$\\{([A-Za-z0-9]+)\\}";
Pattern expr = Pattern.compile(pattern);
Matcher matcher = expr.matcher(text);
while (matcher.find()) {
    String envValue = envMap.get(matcher.group(1).toUpperCase());
    if (envValue == null) {
        envValue = "";
    } else {
        envValue = envValue.replace("\\", "\\\\");
    }
    Pattern subexpr = Pattern.compile(Pattern.quote(matcher.group(0)));
    text = subexpr.matcher(text).replaceAll(envValue);
}

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