using连接数据库，执行SQL命令，使用适配器返回数据集。我们不需要去关心它的conn的资源是否关闭。当我们不用到它时。系统自动帮我们安全关闭-using to connect the database, the implementation of SQL commands, use the adapter to return data sets. We do not need to care about it whether the resources conn closed. When w