This error can happen if you don't have all the classes needed for your servlet available to the servlet container, even though they
were available to the compiler when you compiled your code.
Servlet containers almost always ignore any external "classpath", and only recognize and load classes deployed within their own configuration and application structure. Typically
you should ensure that your own application classes are placed in WEB-INF/classes, and that any third-party jar files needed by your code are placed in WEB-INF/lib.