What do you mean with "UserTask parameters" ? Are you referring to form properties? If so, you can access the submitted values via the process variables.
1. I want to access form properties for further processing depending on values set to them. 2. I am getting null for [ String username = (String) execution.getVariable("username"); ]. execution is of type DelegateExecution.
1. They are stored as process variables and can be accessed later on like regular variables 2. Check which values you have available in the database. Is 'username' the id of the form property?