No message found under code '...' for locale

This error can occur even if the code is in the resources file for example when rendering error pages resulting from Tomcat internal errors (500) like JSP errors. We better configure Spring to use the Reloadable Resource Message Source:

<bean id="messageSource"
         class="org.springframework.context.support.ReloadableResourceBundleMessageSource">
         <!--  <property name="basename" value="classpath:messages" />  -->
         <property name="basenames">
             <value>/WEB-INF/i18n/messages</value>
         </property> 
         <property name="cacheSeconds">
             <value>60</value>
         </property>
         <property name="fallbackToSystemLocale" value="false" />
     </bean>

JSP Spring Taglib for your error pages must contain a default value (not very i18n friendly) otherwise when the error pops up the JSP will fail with "No message found under code 'password' for locale 'en_US'"

<spring:message code="password" text="Password"/>

A special case for this issue is when Spring tries to parse an empty code. To avoid the issue in that case just define an empty code line in message.properties. Something like "=" or "=Empty" or "=Null" should do the trick.