急急急！！！！！！！！怎么解决这个问题？java.lang.NoClassDefFoundError: com/imooc/db/DBAccess

Question

java.lang.NoClassDefFoundError: com/imooc/db/DBAccess
com.imooc.dao.MessageDao.queryMessage(MessageDao.java:23)
com.imooc.service.ListService.queryMessage(ListService.java:19)
com.imooc.servlet.ListServlet.doGet(ListServlet.java:58)
javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)