How to load and parse (ant style) properties file in Java?

Question

How to load property file into Property object in Java and get property values parsed (${x} gets replaced as it is done for ant properties)? For example using this property file:

foo=1
bar=${foo}.0

I need to get bar property as 1.0 instead ${foo}.0. Is there a easy way for this?

EDIT: Alex's solution works for simple scenarios. In my case I had to solve another issue leading to this question: Pulling values from a Java Properties file in order?.

Resulting sample code for loading and parsing properties:

import java.util.*;
import java.io.*;
import org.apache.commons.text.StringSubstitutor;

public class Prop {

    Properties          parsedProperties   = null;

    public static Properties parseProperties(String filename) {
        // inner helper class keeping order of properties:
        class LinkedProperties extends Properties {

            private static final long serialVersionUID = 1L;
            private final HashSet<Object> keys = new LinkedHashSet<Object>();

            public LinkedProperties() {
            }

            public Iterable<Object> orderedKeys() {
                return Collections.list(keys());
            }

            public Enumeration<Object> keys() {
                return Collections.<Object>enumeration(keys);
            }

            public Object put(Object key, Object value) {
                keys.add(key);
                return super.put(key, value);
            }
        }

        LinkedProperties result = new LinkedProperties();
        try (InputStream input  = new FileInputStream(filename)) {

            result.load(input);

            @SuppressWarnings("unchecked")
            StringSubstitutor sub = new StringSubstitutor((Map) result);

            for (Object k : result.orderedKeys()) {
                result.setProperty((String)k, sub.replace(result.getProperty((String)k)));
            }
        } catch (IOException ex) { ex.printStackTrace(); }

        return ((Properties)result);
    }

    public static void main(String[] args) {
        Prop app = new Prop();

        // test - write sample properties file:
        try {
            PrintWriter writer = new PrintWriter(new FileWriter("config.properties"));
            writer.println("foo=1");
            writer.println("bar=1.${foo}");
            writer.println("baz=${bar}.0");
            writer.println("xxx=V.${baz}");
            writer.close();
        } catch (IOException ex) { ex.printStackTrace(); }

        // read and parse properties:
        app.parsedProperties = parseProperties("config.properties");

        // debug print:
        for (Object k : app.parsedProperties.keySet()) {
            System.out.println((String)k + " = " + app.parsedProperties.getProperty((String)k));
        }
    }
}

Answer 1

You may use StringSubstitutor from Apache Commons Text, its Maven dependency is pretty modest (~200K):

<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-text -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-text</artifactId>
    <version>1.8</version>
</dependency>

Code example:

// init sample properties
Properties p = new Properties();
p.setProperty("foo", "${baz}.${baz}");
p.setProperty("bar", "${foo}.0");
p.setProperty("baz", "5");

Properties resolved = parseProperties(p);

System.out.println("resolved: " + resolved);
/////

public static Properties parseProperties(Properties orig) {
    Properties result = new Properties();
    StringSubstitutor sub = new StringSubstitutor((Map) orig);
    orig.entrySet().forEach(e -> result.put(e.getKey(), sub.replace(e.getValue())));
    return result;
}

Output:

resolved: {bar=5.5.0, foo=5.5, baz=5}