@Prathap: Make sure that while you created your process definition via web-based editor, you have defined a list of variables. Just open the process definition editor, then click the arrow button on right side of editor to show the popup allowing to edit metadata (including list of variables) of your process
Having played with JBPM for a while, i figured out that fetching variables in this way is not a good manner. We had better configure the process variable <--> task params bindings in process designer tool :)