This rule flags classes that extend HttpServlet which do not set the content type for the response with the setContentType() method. The default value in WebSphere Application Server V5.1 was "text/html", but the default value in Version 6.0 changed it to "none". This rule will not search classes outside of the class that extends HttpServlet for the setContentType() method call, so it is possible that the HttpServletResponse could be passed to another class which calls the setContentType() method. The code will need to be manually scanned for this.
The web container custom property com.ibm.ws.webcontainer.contenttypecompatibility can be set to force the old behavior. If this property is set to V4, V5, V6, or V7, then the default contentType will be set based on the version specified.
For additional information on setting the custom property, see
Migrating web application components from WebSphere Application Server Version 5.x