This error seems to indicate that
Tomcat is finding the ForumSessionListener class, but then failing to find the
Servlet spec API class (javax.servlet.http.HttpSessionListener) that this class impliments.
This might be because you might have two jars that contain implementations of the HttpSessionListener class in it. Tomcat's ClassLoader security mechanism can cause class not found errors if a class lower down the chain tries to replace a higher level class.
The HttpSessionListener class should only be in Tomcat's common/lib/servlet-api.jar file and not an ANY other webapp's lib directory or the shared/lib directory.
Note that this class may be located in another jar name, e.g. Sun's
J2EE implimentation,
JBoss's and the like. It might also be located in another webapp. TC's order of initializing contexts can be somewhat random so the sometimes it work, sometimes it fails may be related to the order in which other web apps get loaded. And it fails if the "bad" context is loaded first...
[originally posted on jforum.net by monroe]
