Any attempts to load org.jruby.demo.IRBApplet fail due to various security related issues. The most critical one is NPE from LateLoadingLibrary (used when 'readline' is requested). Also, when the NPE is resolved, IRBApplet is not very useful and dies when users try to open files and perform some other basic things in the IRB. The proposed patch closes most common issues, and when applied, JIRB is actually useful and working fine. Note: I tested that "ant test" passes and executed rubinius spec to see that there are no major problems with the patch.