javax.faces.application.Application class's getMessageBundle() method returns null in JBoss 5.1. This causes a NullPointerException. Message bundle is retrieved correctly by the same means in JBoss 4.2.3. Sample code:

Application app = context.getApplication();

// The following line returns null in JBoss 5.1. Returns message bundle name correctly in JBoss 4.2.3.
// (Should retrieve message bundle name configured in faces.config.xml file). NullPointerException follows this

String messageBundleName = app.getMessageBundle();

ResourceBundle messageBundle = ResourceBundle.getBundle(messageBundleName);