When I wrote the login page and searched for users, I encapsulated the login information into a simplified version of the user class, but issues involving empty strings and nulls require some judgment, so should I make a judgment at the service layer and then pass in the parameters? dao object and then return the result like this?

What you do is correct. Business operations are placed in service, and there are just some simple ones in the dao layer CURD操作.

Second point, I think your directory result is very good, there is no problem. Anyway, I am using the directory structure generated by Maven by default.