用实际值替换字符串中的Shell环境变量

Question

I am reading a property file in Java.

Properties myProp = new Properties();
InputStream in = new FileInputStream(pathOfPropertyFile);
myProp.load(in);
in.close();

The values in the property file have references to Linux shell variables. For example, an entry in the property file might look like:

DATA_PATH=/data/${PROJECT}/${YEAR}${MONTH}${DAY}

I have to execute a shell script from java and so I have ProcessBuilder instance and also the environment variables ( envMap as given below):

List<String> command = new ArrayList<String>();
command.add(actualCommand);
command.add(param1);
command.add(param2);
ProcessBuilder processBuilder = new ProcessBuilder(command);
Map<String, String> envMap = processBuilder.environment();

The envMap has the environment variables I require along with over one hundred (> 100) other environment variables which I do not require.

I want to replace the ${USER} , ${PROJECT} ,etc., from the property-value string "/home/${USER}/${PROJECT}/data" with actual values from the shell.

I would consider iterating the Map as the last option(as the Map has between 100 and 200 elements to iterate) as it is not an efficient approach.

Please advise some approach that will fetch the environment variable enclosed by braces from the string, so that I can directly use the get() of the Map and replace. Or, any better approaches are most welcome.

Note: The reference offered ( Replace String values with value in Hash Map that made my question to look duplicate) is not the best fit in my case.

Answer 1

If you are open to using an external library, StrSubstitutor from apache-commons will do exactly what you want:

public static void main(String[] args) {
    String input = "DATA_PATH=/data/${PROJECT}/${YEAR}${MONTH}${DAY}";
    Map<String, String> env = new HashMap<>();
    env.put("PROJECT", "myProject");
    env.put("YEAR", "2017");
    env.put("MONTH", "7");
    env.put("DAY", "5");
    env.put("OTHER_VALUE", "someOtherValue");
    System.out.println(StrSubstitutor.replace(input, env));
}

Output:

DATA_PATH=/data/myProject/201775

It also has a method to directly replace system properties without the need for an explicit map.

(for a non-external-library approach see vefthym's answer )

Answer 2

I am not sure if it works, but I hope someone can edit to make it work, or at least you get the logic and make it work on your own:

command = command.replaceAll("\\$\\{(.*?)\\}", envMap.get("$1"));

Here, I am assuming that command is a String (not a List) and that all environment variables exist in your Map (otherwise you should check for null and handle this case as you wish).

A bit of an explanation:

this regex is looking for the pattern "${something}" and replaces it with envMap.get("something") . In this example, we use parentheses to mark "something" as a group, which can then be retracted as "$1" (since we have only one group, ie, only one set of parentheses). The question mark '?' is the non-greedy operator here meaning to stop at the smallest possible regex match (otherwise it would find a singe match for the first "${" until the last "}".