We wanted to implement a class in JRuby that implements a Java interface, to fulfill a dependency injection contract. This is seemingly (unfortunately) a much more difficult task than it sounds. So, here's our simple Java interface that defines one public method, "indexObject": 
public interface ScriptIndexer {
    public void indexObject(RepositoryProfile profile, 
        String pid, 
        SolrInputDocument doc);
}
And here's what we really wish just worked (in JRuby 1.7.0): 
java_package 'org.fcrepo.indexer.solr'
class DemoRubySolrIndexer
  include org.fcrepo.indexer.solr.ScriptIndexer
  
  def indexObject a,b,c
    true
  end  
end
Unfortunately, here's what that compiles to (using jrubyc): 
public class DemoRubySolrIndexer extends RubyObject  {
    private static final Ruby __ruby__ = Ruby.getGlobalRuntime();
    private static final RubyClass __metaclass__;

    static {
        String source = new StringBuilder("require 'java'\n" +
            "require 'rubygems'\n" +
            "require 'active_fedora'\n" +
            "\n" +
            "java_package 'org.fcrepo.indexer.solr'\n" +
            "class DemoRubySolrIndexer\n" +
            "  include org.fcrepo.indexer.solr.ScriptIndexer\n" +
            "  \n" +
That's not a ScriptIndexer instance! In the end, we gave in and ended up using the spring-lang dynamic language support (for jruby, groovy and beanshell only): 
  <lang:jruby id="rubyScriptClass" 
              script-interfaces="org.fcrepo.indexer.solr.ScriptIndexer" 
              script-source="classpath:demo_ruby_solr_indexer.rb" />
I assume, under the hood, this makes a similar proxy class, but does so a little smarter. It's annoying we have to tie ourselves so tightly to Spring to get this to work, without writing some unnecessary Proxy classes ourselves.