Test of become_java! and having java code call a ruby object.

become_java_test.rb

require 'jruby/core_ext'

class MyObserver
  include Java::JavaUtil::Observer

  def update(observable, arg)
    puts "UPDATE! #{observable}"
  end
end
MyObserver.become_java! false

name = MyObserver.java_class.to_s

### Pretend a third-party Java module is doing the following:

puts "loading #{name}"
klass = Java::JavaLang::Thread.currentThread.getContextClassLoader.loadClass name
inst = klass.newInstance
puts "have instance: #{inst}"

inst.update('(invoked from JRuby)', nil)

observable = Java::JavaUtil::Observable.new
observable.addObserver(inst)
observable.setChanged
observable.notifyObservers

results

jruby -v test.rb
jruby 1.7.2 (1.9.3p327) 2013-01-04 302c706 on Java HotSpot(TM) 64-Bit Server VM 1.6.0_35-b10-428-10M3811 [darwin-x86_64]
loading rubyobj.MyObserver
have instance: #<MyObserver:0x1a0283e>
UPDATE! (invoked from JRuby)
Observable.java:142:in `notifyObservers': java.lang.AbstractMethodError: rubyobj.MyObserver.update(Ljava/util/Observable;Ljava/lang/Object;)V
  from Observable.java:98:in `notifyObservers'
	from NativeMethodAccessorImpl.java:-2:in `invoke0'
	from NativeMethodAccessorImpl.java:39:in `invoke'
	from DelegatingMethodAccessorImpl.java:25:in `invoke'
	from Method.java:597:in `invoke'
	from JavaMethod.java:440:in `invokeDirectWithExceptionHandling'
	from JavaMethod.java:304:in `invokeDirect'
	from InstanceMethodInvoker.java:52:in `call'
	from CachingCallSite.java:306:in `cacheAndCall'
	from CachingCallSite.java:136:in `call'
	from test.rb:23:in `__file__'
	from test.rb:-1:in `load'
	from Ruby.java:810:in `runScript'
	from Ruby.java:803:in `runScript'
	from Ruby.java:674:in `runNormally'
	from Ruby.java:523:in `runFromMain'
	from Main.java:390:in `doRunFromMain'
	from Main.java:279:in `internalRun'
	from Main.java:221:in `run'
	from Main.java:201:in `main'

Thank you for sharing this.