How can JRE be enough for Tomcat to handle JSP files? Why not JDK?-Collection of common programming errors

Tomcat bundles a copy of the Eclipse compiler, rather than using the JDK javac compiler. It’s faster (or, at least, it used to be), has less restrictive licensing, and removes the dependency on the full JDK.

http://tomcat.apache.org/tomcat-5.5-doc/jasper-howto.html

The Eclipse JDT Java compiler is now used to perform JSP java source code compilation

This suggests that prior to Tomcat 5.5, the full JDK was required.