123

    @PostMapping("/login")
    public ResponseEntity<String> login(@RequestBody User requestUser) {
            String username = requestUser.getUsername();
            String password = requestUser.getPassword();
            try {
                if (userRepository.findByUsername(username) == null) {
                    User user = new User(username, passwordEncoder.encode(password));
                    userRepository.save(user);
                }
                UsernamePasswordAuthenticationToken authReq = new UsernamePasswordAuthenticationToken(username, password);
                Authentication auth = authenticationManager.authenticate(authReq);
                SecurityContext sc = SecurityContextHolder.getContext();
                sc.setAuthentication(auth);
                HttpHeaders headers = new HttpHeaders();
                headers.add("Location", "/main");
                return new ResponseEntity<>(headers, HttpStatus.FOUND);
            }

            catch (AuthenticationException e) {
                return new ResponseEntity<>(("Invalid."), HttpStatus.UNAUTHORIZED);
            }
        }