您好, cad文件本身就分dgn格式和dwg格式的
http://support.supermap.com.cn/DataWarehouse/WebDocHelp/iObjectsJava/ProgrammingReference/com/supermap/data/conversion/ImportSettingDGN.html
http://support.supermap.com.cn/DataWarehouse/WebDocHelp/iObjectsJava/ProgrammingReference/com/supermap/data/conversion/ImportSettingDWG.html
希望可以帮助到您。