There are numerous articles describing charset problems when using Tomcat to run Grails applications that are supposed to accept UTF-8 characters (e.g. German umlauts äöüÄÖÜß). Most of them don’t mention a subtle cause that may jeopardize all your efforts to properly interpret UTF-8 form input. And fiddling around with Tomcat’s defaults was not an option for me, as the same Tomcat instance is to run other non-Grails legacy applications that rely on ISO-8859-1 being the default character set.

It took me some time to find out that Grails applications (1.2.0) behave just as expected under Tomcat (6.x) without any modification whatsoever, provided you don’t use Tomcat’s RequestDumperValve within your server.xml or context.xml.

<!-- Reiner: If you include this in your context.xml or server.xml,
     form input will always be interpreted as ISO-8859-1 :-(
<Valve className="org.apache.catalina.valves.RequestDumperValve"/>
-->

The reason for RequestDumperValve to garble form input is that it causes POST-parameters to be evaluated (using default ISO-8859-1) before Grails request filters have had a chance to set the request encoding to UTF-8. This behaviour is documented and RequestDumperValve is now deprecated due to its side effects.

The ultimate cause to this problem is that most browsers do not bother to state the character set that has been used to encode POST-parameters, thus leaving it up to the server to guess. Grails does this out-of-the-box within its applicationContext.xml:

<bean id="characterEncodingFilter"
      class="org.springframework.web.filter.CharacterEncodingFilter">
      <property name="encoding">
        <value>utf-8</value>
      </property>
</bean>

Credits: Ales at Grails User forum.