如果您在进行首次连接之前未加载驱动程序，则可能会发生这种情况.

That can happen if you didn't load the driver before making the first connection ever.

Class.forName("com.mysql.jdbc.Driver");

可以肯定的是，驱动程序必须进入/WEB-INF/lib，而不是进入/WEB-INF.顺便说一下，您那里有一些SQL注入漏洞.看PreparedStatement. finally也可以进行改进，就像现在一样，con不会在rs.close()引发异常时关闭.

To be sure, the driver has to go in /WEB-INF/lib, not in /WEB-INF. You've there by the way some SQL injection holes. Look at PreparedStatement. The finally can also be improved, as you have it now, the con will never be closed when rs.close() throws an exception.