当用户在系统中浏览时，不会迷路，随时可以到自己想去的地方将对数据库的访问代码封装在一个类里，在系统其他地方只需调用此类提供的接口即可。-When the user browse in the system, it will not get lost, you can always want to go to their local database access code will be encapsulated in a class, the only other places in the system call interface can be provided for such.