I've followed instructions at [1] and generated Eclipse projects to browse Jenkins source. Unfortunately, when I imported those projects into Eclipse, it turned out that jenkins-core and maven-plugin contain errors preventing them from compiling (and hence parsing) properly.

When I clicked on the very first error it turned out that Eclipse complained about @Override annotation on the method implementing an interface. According to [2] this was NOT allowed in Java 5 and was first added in Java 6. I've used instructions on the same page to fix project settings for jenkins-core and maven-plugin to be Java 6 compliant at the source level, and all the errors were gone.

Given that according to [1], JDK 6 is required to build Jenkins, and some Java 6 features are used in the code anyway, I don't see why generated projects try to be Java 5 compliant.

[1] https://wiki.jenkins-ci.org/display/JENKINS/Building+Jenkins
[2] http://stackoverflow.com/questions/8220786/java-eclipse-override-error