eclipse中，项目有红叉之- The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path

1.The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path

2.原因：确实servlet-api.jar

3.导入servlet-api.jar，其中一种方法：项目右击->build path->configure build path->add library->server runtime->apache tomcat