How to pass session variables in JDBC url properly?

Try this:

jdbc.url=jdbc:mysql://xxxx.xx.xx.xx/dbName?sessionVariables=group_concat_max_len=204800

If you need to add more than one session parameter you can do it like this:

jdbc:mysql://localhost/database?sessionVariables=FOREIGN_KEY_CHECKS=0&sessionVariables=SQL_SAFE_UPDATES=0