数据库查询中，我们是这样进行模糊匹配的：

select * from user where 1=1  and username like '%林%' limit 0,20

理所当然的，在java代码中你会这么写：

select * from user where 1=1  and username like '%?%' limit 0,20

ps.setString(1, userCondition.getUsername());

如果真这么写，你就等着扑街吧：Parameter index out of range (1 > number of parameters, which is 0)

这时候怎么破？

select * from user where 1=1  and username like ? limit 0,20

ps.setString(1, "%"+userCondition.getUsername()+"%");