错误原因：
如API中所述
API：
A table of data representing a database result set, which is usually generated by executing a statement that queries the database. A
ResultSet object maintains a cursor pointing to its current row of data. Initially the cursor is positioned before the first row. The
next method moves the cursor to the next row。
就是说游标初始化的位置在首行之前。
解决方法：
在使用rs前要调用rs.next()方法
