When you define multiple message resource bundles, you must assign a key to all but one. The one you don't assign a key to will be the default bundle. example:
<message-resources parameter="application" key="application"/>
<message-resources parameter="MessageResources" />
If I want to use a message key in MessageResources.properties, since it's the default, I don't have to specify a bundle in <bean:message> and the following will work:
<bean:message key="my.key" />
But if I want to use a message key in application.properties, I must specify a bundle like this:
<bean:message bundle="application" key="other.key" />
This link is a very good reference on message resources in
Struts.