How to avoid overwriting of non-null values with null values?

Question

I'm using spring MVC for receiving a JSON from client and automatically create an object from it. The problem is that the client doesn't send to server all the fields that are in the entity, but some fields are null and overwrite existing values calling userDao.persist(user). For example, i have this entity:

@Entity
public class User {

    @Id @GeneratedValue
    private int id;

    private String username;
    private String password;
    private String email;

But the user never send me the password, so the object built from JSON has "password" field empty. I don't want the password field to be overwritten by a null value. There's a way to say to hibernate "if you find a null value ignore it and don't overwrite the value that is saved in database?". I can't believe that there isn't a easy solution to this apparently simple problem.

Answer 1

I wrote this answer while being an unexpirienced studen. Today my answer would be similar to the one from @James DW. Also, from the term userDao, I assume that it is some kind of ORM/ODM. In that case it is definitly worth searching "pros and cons of ORM/ODM".

original answer (which was accepted back in 2011):

If your problem is only the database, then I suggest you use a stored procedure, which checks if that value is null, and then dose not change the existing value. That way you can still send a null value, and your validation is on server side which is more robust.