JSP 2.0 vs JSP 2.1
Jetty does not implement JSP directly. Instead it uses a servlet to provide this functionality required by the servle specification. As part of Java EE 5 (the next version of J2EE), the companion of 2.5 servlet specification is the 2.1 JSP specification.
While all Java EE 5 technologies formally depend on java 5 (jre 1.5), this is a soft dependency for servlets and Jetty 6 will compile and run on jre 1.4. However, for JSP the dependency of java 5 is a hard dependency and jre 1.5 must be used to run the server with JSP 2.1
Thus Jetty 6 supports both JSP 2.0 and JSP 2.1 so that jre 1.4 users may be supported.