I am trying to write an IzPack installer which is able to detect a previously installed version of the application. After finding out that it doesn't really support this feature, I think the only way to do this is through writing a Java class and calling it from IzPack.

Tim Williscroft has previously suggested this method:

 <condition type="java" id="jbossEnv">
             <java> 
                 <class>au.com.codarra.ela.installer.JBossChecker</class
                 <field>hasJBossEnv</field>
             </java>
             <returnvalue type="boolean">true</returnvalue>
 </condition>

However, is it possible to call <java> from somewhere other than a <condition> tag? I want to be able to call it from a <variable> tag.
Also, is it possible to call a method from a Java class instead of reading a value of a field? I want to be able to pass parameters to my class.

Thank you!