Spark-java redirect - browser does not redirect - http

Question

Ok, so I try to follow Spark documentation and I want to perform simple redirect in my Single Page Application. My code looks like this:
post("/users/login", (req, res) -> {
ObjectMapper mapper = new ObjectMapper();
User creation = mapper.readValue(req.body(), User.class);
User user = userService.getUser(creation.getLogin());
if (user.getPassword().equals(creation.getPassword())) {
req.session().attribute("userid", creation.getLogin());
System.out.println("OK");
res.status(201);
res.redirect("/index.html");
return "";
}
System.out.println("BAD");
return null;
} , json());
Basically, I have three static html files: registration.html, login.html and index.html. I read stuff concerning staticFileLocation so I added at the beginning of main function following line of code:
staticFileLocation("/public");
When I type correct login and password I find in the network view in Chrome that I have GET request with status 200, OK, to http://localhost:4567/index.html. However, the browser does nothing and does not redirect me to that page. Can you tell me what I am doing wrong?
EDIT:
Here's javascript code that handles log in on the client side:
app.controller('LoginUserCtrl', function($scope, $http) {
$scope.loginUser = {};
$scope.submitForm = function() {
$http({
method : 'POST',
url : 'http://localhost:4567/users/login',
data : $scope.loginUser,
headers : {
'Content-Type' : 'application/x-www-form-urlencoded; charset=UTF-8'
}
}).success(function() {
console.log("User logged successfully");
console.log($scope.loginUser);
}).error(function() {
console.log("Unknown error while logging user");
});
};
});

Answer 1

What's wrong is that you're redirecting to an HTML page in a post endpoint that's supposed to return Json data. You have to return a single json if authentication succeeded or failed, like {"auth": "OK"} or {"auth": "NOK"}, and decide where to redirect to from Javascript based in it's information.

Answer 2

It's because res.redirect will send a redirect http header(HTTP status values 301, 302, 303 and 307) to the browser,
but the browser only can redirect in get, NOT work in post or put or delete (Tested in chrome. Notice that a redirect request was sent by browser, but the page just not change...).
See:
http://www.alanflavell.org.uk/www/post-redirect.html