Full Flow

// ===== 1차 =====
@Controller
@PostMapping("/user/login")
public String login(@ModelAttribute UserDto userDto, HttpSession session) {
    User user = userService.login(userDto.getUsername(), userDto.getPassword());
    session.setAttribute("loginUser", user); // 서버 메모리에 저장
    return "redirect:/board/list";            // HTML redirect
}

// ===== 2차 =====
@RestController
@PostMapping("/api/user/login")
public ResponseEntity<?> login(@RequestBody UserDto userDto) {
    String token = userService.login(...);     // JWT 토큰(String) 반환
    return ResponseEntity.ok(Map.of("token", token)); // JSON 반환
}

// ===== 3차 =====
@RestController
@PostMapping("/api/user/login")
public ResponseEntity<?> login(@RequestBody UserDto userDto) {
    Map<String,String> result = userService.login(...); // token + role 반환
    return ResponseEntity.ok(result);
}

// ===== 1차 — 세션에서 꺼내기 =====
@PostMapping("/board/write")
public String write(@ModelAttribute BoardDto boardDto, HttpSession session) {
    User loginUser = (User) session.getAttribute("loginUser"); // 세션에서
    if (loginUser == null) return "redirect:/user/login";      // 직접 체크
    boardService.write(boardDto, loginUser);
    return "redirect:/board/list";
}

// ===== 2차 — request 속성에서 꺼내기 =====
@PostMapping("/api/board/write")
public ResponseEntity<?> write(@RequestBody BoardDto boardDto,
                               HttpServletRequest request) {
    User loginUser = (User) request.getAttribute("loginUser"); // request에서
    if (loginUser == null)                                       // 직접 체크
        return ResponseEntity.status(401).build();
    boardService.write(boardDto, loginUser);
    return ResponseEntity.ok(...);
}

// ===== 3차 — @AuthenticationPrincipal 자동 주입 =====
@PostMapping("/write")
public ResponseEntity<?> write(@RequestBody BoardDto boardDto,
        @AuthenticationPrincipal CustomUserDetails userDetails) {
    // null 체크 없음 — SecurityConfig가 이미 처리
    boardService.write(boardDto, userDetails.getUser());
    return ResponseEntity.ok(Map.of("message", "글쓰기 성공"));
}