Programmatically log-in a user using spring security

In my controller i have this, which logs user in as normal :

Authentication auth = 
  new UsernamePasswordAuthenticationToken(user, null, user.getAuthorities());

SecurityContextHolder.getContext().setAuthentication(auth);

Where user is my custom user object(implementing UserDetails) that is newly created. The getAuthorities() method does this (just because all my users have the same role):

public Collection<GrantedAuthority> getAuthorities() {
        //make everyone ROLE_USER
        Collection<GrantedAuthority> grantedAuthorities = new ArrayList<GrantedAuthority>();
        GrantedAuthority grantedAuthority = new GrantedAuthority() {
            //anonymous inner type
            public String getAuthority() {
                return "ROLE_USER";
            }
        }; 
        grantedAuthorities.add(grantedAuthority);
        return grantedAuthorities;
    }