The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path
鼠标右键项目->Build Path->Configure Build Path…->Libraries->Add Library…
Next
选择Apache Tomcat v7.0 ->Finish
鼠标右键项目->Build Path->Configure Build Path…->Libraries->Add Library…
Next
选择Apache Tomcat v7.0 ->Finish