Add those lines to the web.xml:

  <servlet>
    <servlet-name>GeoExplorerService</servlet-name>
    <servlet-class>org.gcube.portlets.user.geoexplorer.server.DefaultGeoExplorerServiceImpl</servlet-class>
  </servlet>
  
  <servlet-mapping>
    <servlet-name>GeoExplorerService</servlet-name>
    <url-pattern>/<YOUR APPLICATION NAME>/GeoExplorerService</url-pattern>
  </servlet-mapping>
