Logging application behavior, whose are deployed on tomcat

You can log many events of application without changing its code. You can switch on Valves in tomcat configuration. Valve (org.apache.catalina.valves.Valve) is acting like filter on HttpRequest. It is executed just before application gets the request. There are some predefined Valves you can use by just uncommenting them in server.xml configuration in tomcat. You can also develop your own Valve and use them after adding in configuration and in shared libs in tomcat.
More information can be read in the article: http://www.tomcatexpert.com/blog/2011/11/11/finer-point-apache-tomcat-valves