Post data to Spring with retrofit

I have  a Android project for client and a Spring project for server.

In Android, I want to post the String param "type" and a POJO "User" with  to my Spring project, what is the best way?

The param "type" is used to decide my server function, eg: for query or compare.

Thus, It's used for only once,  it's impossible to put it into the POJO "User"
