[15.11.19]Error : The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path

The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path 의 해결방법 2가지

1. Project -> Properties -> java Build Path -> Add Libraries -> Server Runtime 

2. java 환경과 tomcat server도 잡아줬는데 이 에러가 날 때,

2-1. 해당 프로젝트 Properties -> Project Facets -> Runtimes 탭 -> 설정한 웹서버 체크