I wrote this simple extension to the Java Properties class:
import java.io.Serializable;
import java.util.Properties;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/**
* Allows properties to contain expansions of the form ${propertyName}. This
* class makes no attempt to detect circular references, so be careful.
*/
public class ExpandingProperties extends Properties implements PropertySource {
private static final long serialVersionUID = 259782782423517925L;
private final Expander expander = new Expander();
@Override
public String getProperty(String key) {
return expander.expand(super.getProperty(key), this);
}
}
class Expander implements Serializable {
private static final long serialVersionUID = -2229337918353092460L;
private final Pattern pattern = Pattern.compile("\\$\\{([^}]+)\\}");
/**
* Expands variables of the form "${variableName}" within the
* specified string, using the property source to lookup the
* relevant value.
*/
public String expand(final String s, final PropertySource propertySource) {
if (s == null) {
return null;
}
final StringBuffer sb = new StringBuffer();
final Matcher matcher = pattern.matcher(s);
while (matcher.find()) {
final String variableName = matcher.group(1);
final String value = propertySource.getProperty(variableName);
if (value == null) {
throw new RuntimeException("No property found for: " + variableName);
}
matcher.appendReplacement(sb, value.replace("$", "\\$"));
}
matcher.appendTail(sb);
return sb.toString();
}
}
interface PropertySource {
String getProperty(String key);
}
Example usage:
public static void main(String[] args) {
Properties properties = new ExpandingProperties();
properties.put("myVar", "myLongExpression");
properties.put("foo", "${myVar}_1");
properties.put("bar", "${foo}_abc");
System.out.println(properties.getProperty("bar"));
}
Prints:
myLongExpression_1_abc
As ExpandingProperties is an extension of Properties it inherits all the load...() methods for loading values from property files.
An alternative is EProperties which does a similar thing to the above code, but goes even further and allows you to nest property files etc. I found it overkill for what I needed.
