rjb-1.3.0 increase jar's handling. for example, you'd like to use foo.jar, bar.jar. you can code as:
Rjb::add_classpath(File.expand_path('foo.jar'))
Rjb::add_classpath(File.expand_path('bar.jar')) Rjb::load ...
instead of
Rjb::load("#{File.expand_path('foo.jar')}#{File::PATH_SEPARATOR}#{File.expand_path('bar.jar')}")

Later, you need add baz.jar then,
Rjb::add_jar(File.expand_path('baz.jar'))
Then Rjb create new ClassLoader for newly add jar (baz.jar).
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OS X 10.6 update 3 deprecated its JDK. So after update was applied, JVM name was changed. So I correctly set default JVM name. It was made Jeff's great helps.
But at the update was applied, some important JDK's header files may be deleted by the update. That's 1.6.0/Headers.
After that, building Gem may fail. You must restore 1.6.0/Headers directory if that happened.