Spring security Google signin, controller doesn't redirect to my index.html - spring-security

Question

My application has a login page which uses spring security. Then I would like to add "Sign in with Google" button likes following:
add google signin button
Then if user click signin with google, it should send a post request, following is my javascript file
<script type="text/javascript">
function onSignIn(googleUser) {
// The ID token you need to pass to your backend:
var id_token = googleUser.getAuthResponse().id_token;
var xhr = new XMLHttpRequest();
xhr.open('POST', 'http://localhost:8080/myapp/tokensigningoogle');
xhr.setRequestHeader('Content-Type',
'application/x-www-form-urlencoded');
xhr.onload = function() {
console.log('Signed in as: ' + xhr.responseText);
};
xhr.send('idtoken=' + id_token);
}
</script>
Then I added a MVC controller to get the GrantedAuthorities since I need to get the roles of the current user. Then I want to redirect to my index.html. It doesn't have errors when I run it in eclipse, however, it can not redirect to my index.html and it still stayes in my login.jsp.
#Controller
#RequestMapping(value = "/tokensigningoogle")
public class SigninWithGoogle {
static final Logger LOGGER = Logger.getLogger(SigninWithGoogle.class);
#RequestMapping(method = RequestMethod.POST, headers = "content-type=application/x-www-form-urlencoded")
public String processRegistration(#RequestParam("idtoken") String idTokenString, ModelMap model)
throws GeneralSecurityException, IOException {
GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(new NetHttpTransport(), new GsonFactory())
.setAudience(Arrays.asList(Config.GOOGLE_CLIENT_ID)).setIssuer("accounts.google.com").build();
GoogleIdToken idToken = verifier.verify(idTokenString);
if (idToken != null) {
Payload payload = idToken.getPayload();
// Print user identifier
String userId = payload.getSubject();
// Get profile information from payload
String email = payload.getEmail();
boolean emailVerified = Boolean.valueOf(payload.getEmailVerified());
List<User> users = DbFunction.listHqlNew("FROM User WHERE email = :email", "email", email);
if (!emailVerified || users.isEmpty()) {
return "/error.html";
} else {
List<String> roles = DbFunction.listSQLNew(
"SELECT role.name FROM user_role_association JOIN role ON role.id = role_id JOIN user on user.id = user_id WHERE user.email = :email",
"email", email);
List<GrantedAuthority> authorities = new ArrayList<GrantedAuthority>();
for (String role : roles) {
authorities.add(new SimpleGrantedAuthority(role));
}
UserDetails userDetails = new org.springframework.security.core.userdetails.User(users.get(0).getName(),
"xx", users.get(0).isEnabled(), true, true, true, authorities);
Authentication authentication = new UsernamePasswordAuthenticationToken(users.get(0).getName(), null,
userDetails.getAuthorities());
SecurityContextHolder.clearContext();
SecurityContextHolder.getContext().setAuthentication(authentication);
return "/index.html";
}
} else {
System.out.println("Invalid ID token.");
}
return "/error.html";
}
}