SEVERE: Servlet.service() for servlet jsp threw exception java.lang.NoClassDefFoundError: esavo/slap/ConfBean

I'm using apache and tomcat to run a simple servlet. 
I have the following exception when I'm trying to run the servlet:

exception
javax.servlet.ServletException: esavo/slap/ConfBean
...
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

root cause
java.lang.NoClassDefFoundError: esavo/slap/ConfBean
....
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


The solution I have found is to copy the directory lib into WEB-INF, then stop and start the service again.
Servlets usually look int WEB-INF to find the classes they need.