Liferay 7 – javax.servlet.ServletException: java.lang.NoClassDefFoundError

If you run into this issue, how to solve it? 2018-07-09 21:44:54.986 ERROR [http-nio-8443-exec-2][status_jsp:911] com.liferay.portal.kernel.portlet.PortletContainerException: com.liferay.portal.kernel.portlet.PortletContainerException: javax.servlet.ServletException: java.lang.NoClassDefFoundError: com/liferay/docs/guestbook/model/Guestbook 2018-07-09 21:44:55.030 ERROR [liferay/monitoring-1][ParallelDestination:55] Unable to…