In case you don't know ResourceBundleMessageSource, here's an excerpt of its javadoc:
MessageSource implementation that accesses resource bundles using specified basenames. This class relies on the underlying JDK's ResourceBundle implementation, in combination with the JDK's standard message parsing provided by MessageFormat.
We are leveraging the basename functionality of ResourceBundleMessageSource in our application, the way it's suggested in the javadoc. It's pretty neat and you should definitely check it out, if you've never used it.
The problem we encountered indicated that the bundle could not be found. This is what we saw in our log statements:
WARNING: ResourceBundle [broken_messages] not found for MessageSource: Can't find bundle for base name broken_messages, locale en_US
Of course, we tried to fiddle with the classpath, changed the names of the properties files and moved them to different directories in our WAR file - it would only print the above log statement. After hours of trying and failing, Marc discovered an invalid Unicode character representation (something like d\u00Fvid instead of d\u00FCvid) in the ResourceBundle, which wouldn't let ResourceBundle load the damn thing.
So where's the problem with ResourceBundleMessageSource? The problem is that it swallowed the cause of its own MissingResourceException. Here's what Java's good old ResourceBundle class tells you in case of a broken \uxxxx representation:
java.util.MissingResourceException: Can't find bundle for base name broken_messages, locale en_US
Caused by: java.lang.IllegalArgumentException: Malformed \uxxxx encoding.
You can clearly figure out what the problem is and how to solve it. Whereas, with the message of MissingResourceException the least you would expect, is to look for the problem in the properties file itself.
I created a small test case to reproduce the problem and filed a request for improvement. I hope there'll be a fix, so that other people don't have to suffer the same way we did.