Auto login after successful registeration in Spring Security 3

When user successfully register, auto login user instead of redirecting them to login page and asking them to login again. Here is the solution I found for Spring Security 3.

@RequestMapping("/user")
@Controller
public class UserController {

 @Autowired
 protected UserService userService;
 

 @Autowired
 @Qualifier("org.springframework.security.authenticationManager")
 protected AuthenticationManager authenticationManager;

 @RequestMapping(method = RequestMethod.POST)
 public String register(@Valid @ModelAttribute("user") User user,
   BindingResult result, HttpServletRequest request, ModelMap modelMap) {

  //valiate user information
  if (result.hasErrors()) {
   return "user/register";
  }

  //save user information into database
  userService.register(user);
  // After successfully Creating user
  UsernamePasswordAuthenticationToken token = new UsernamePasswordAuthenticationToken(
    user.getUsername(), user.getPassword());

  // generate session if one doesn't exist
  request.getSession();

  token.setDetails(new WebAuthenticationDetails(request));
  Authentication authenticatedUser = authenticationManager
    .authenticate(token);

  SecurityContextHolder.getContext().setAuthentication(authenticatedUser);

  return "redirect:/";
 }

Note:
@Qualifier("org.springframework.security.authenticationManager")

I met NoSuchBeanDefinitionException at first:

org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [org.springframework.security.authentication.AuthenticationManager] is defined: expected single matching bean but found 2: [org.springframework.security.authentication.ProviderManager#0, org.springframework.security.authenticationManager]

This is because:

There are two AuthenticationManagers in the context:

org.springframework.security.authenticationManager is populated with authentication providers explicitly declared in
org.springframework.security.authentication.ProviderManager#0 is populated with implicitly declared providers (remember me, anonymous and so on) and delegates the authentication request to org.springframework.security.authenticationManager as a fallback.

so what we need is @Qualifier("org.springframework.security.authenticationManager").