title: How to @Validate unique username in spring?
body: <p>Let's assume that I have a form where I might submit <code>username(@NaturalId)</code> and <code>password</code> for a new user.</p>
  
  <p>I would like to add the user with a unique <code>username</code>. How can I use <code>@Valid</code> annotations to validate this constraint? And if username is not unique how can I display this information in jsp via <code>&lt;form:error/&gt;</code>? </p>