How to debug Java memory errors?

Tomcat runs a Java Struts application that has some memory errors. Sometimes it becomes slow and accumulates all of Tomcat's memory until it works.

I know how to find and fix “ordinary code errors” using tests, debugging, etc., but I don’t know how to deal with memory errors (how can I reproduce?) How can I check? code where the memory error most often occurs?).

In one question: where to start? Thanks

EDIT:

Photo sent by IT department (I do not have direct access to the production application)