Via command line

 $ java -Xdebug -agentlib:jdwp=transport=dt_socket,address=9999,server=y,suspend=n -jar start.jar

Via start.ini

 ## Enable remote debugging

 --exec

 -Xdebug

 -agentlib:jdwp=transport=dt_socket,address=9999,server=y,suspend=n