Hi @eakbas and @Favonius Thanks for your answer.
I finally got a solution, maybe it is not the best but at least works for me.
As I mentioned before I used the flute library to implement the DocumentHandler class of the package org.w3c.sac package in order to analyze the css file.
So I implemented the 'property' method, this method has 3 parameter, the property name, an LexicalUnit object and a boolean indicating that the property has the important statement or not.
public void property(String property, LexicalUnit lexicalUnit, boolean important)
As I need the line number where a specific property is found, I made a search and I could see that the class that flute uses to implement the LexicalUnit interface holds the line number(it is LexicalUnitImp), so I used reflexion to make a casting from LexicalUnit interface to one LexicalUnitImp object.
Class<?> clazz = ClassUtils.getClass("org.w3c.flute.parser.LexicalUnitImpl");
Object lexicalObject = clazz.cast(lexicalUnit);
Integer line = (Integer)MethodUtils.invokeMethod(lexicalObject, "getLineNumber", null, null);
I did it in that way because the class LexicalUnitImpl is 'protected' and I cannot cast it in a traditional way.
class LexicalUnitImpl implements LexicalUnit
Note: The class ClassUtils and MethodUtils are part of the commons-beanutils apache library.