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You can find quite a few examples about working with Java classes at:
https://github.com/jruby/jruby/wiki/CallingJavaFromJRuby
It states, that you should use java_import instead of import due to the JRUBY-3171 bug.
Also include_class is or will be deprecated (JRUBY-3797) in favor of java_import.
Currently java_import is the recommended way to import a Java class.