As you probably know, UberJar is a special distribution of application - a standalone JAR.
You can easily start UberJar as follows:
java -jar app.jar
But what if there is a problem that cannot be reproduced in a standard Tomcat and occurs only in UberJar? You should definitely debug it!
In order to enable debugging options start app.jar with these options:
java -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005 -jar app.jar
Java will wait and start application only after you attach debugger.
Then create a new debug configuration with type Remote in Intellij Idea:
Port should be 5005, the same as in Java options.
Debug it!