解决Idea创建的maven Web项目无法连接mysql数据库

1、解决报错

java.lang.NullPointerException com.yhq.DBconn.selectSql(DBconn.java:37) com.yhq.UserDaoImpl.login(UserDaoImpl.java:24) com.yhq.DengluServlet.doPost(DengluServlet.java:22) javax.servlet.http.HttpServlet.service(HttpServlet.java:647) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) com.yhq.EncodingFilter.doFilter(EncodingFilter.java:20)java.lang.NullPointerException com.yhq