PART 10 :CONTROLLER ALL CONCEPT IN SPRINGBOOT PROJECT

Controller se Service layer ko call karne ke 4 main tareeke hote hain.
Industry me Constructor Injection = BEST PRACTICE maana jaata hai ✅

✅ 1️⃣ Constructor Injection (⭐ RECOMMENDED)

Service:

@Service
public class UserService {
    public UserDTO getUser(Long id) {
        return new UserDTO(1L, "Bhupendra", "bhu@gmail.com");
    }
}

Controller:

@RestController
@RequestMapping("/users")
public class UserController {

    private final UserService userService;

    // Constructor injection
    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/{id}")
    public ResponseEntity<UserDTO> getUser(@PathVariable Long id) {
        UserDTO dto = userService.getUser(id);   // 👈 service call
        return ResponseEntity.ok(dto);
    }
}

✔ Most safe
✔ Easy to test
✔ Immutable (final)
✔ Industry standard

⚠️ 2️⃣ Field Injection (`@Autowired`) – NOT recommended

@RestController
@RequestMapping("/users")
public class UserController {

    @Autowired
    private UserService userService;  // 👈 directly injected

    @GetMapping("/{id}")
    public ResponseEntity<UserDTO> getUser(@PathVariable Long id) {
        return ResponseEntity.ok(userService.getUser(id));
    }
}

❌ Hard to test
❌ Field hidden dependency
❌ Not good practice in big projects

⚠️ 3️⃣ Setter Injection (rare use)

@RestController
@RequestMapping("/users")
public class UserController {

    private UserService userService;

    @Autowired
    public void setUserService(UserService userService) {
        this.userService = userService;
    }
}

✔ Optional dependency me use hota hai
❌ Mostly avoid kiya jata hai

✅ 4️⃣ Lombok ke saath (cleanest way 😎)

@RestController
@RequestMapping("/users")
@RequiredArgsConstructor
public class UserController {

    private final UserService userService; // Lombok constructor bana dega

    @GetMapping("/{id}")
    public ResponseEntity<UserDTO> getUser(@PathVariable Long id) {
        return ResponseEntity.ok(userService.getUser(id));
    }
}

✔ Clean code
✔ Constructor injection hi hai
✔ Industry favorite

🧠 Flow yaad rakho:

Controller  --->  Service  --->  Repository  --->  DB
   ↑              ↑
 HTTP Request   Business Logic

Controller:

sirf request handle kare

Service:

business logic likhe

🎯 Interview One-Liner:

Service layer ko Controller me call karne ka best tareeka hai Constructor Injection using @Autowired or Lombok @RequiredArgsConstructor.

❓ Extra: Multiple services inject karna

public class OrderController {

    private final OrderService orderService;
    private final UserService userService;

    public OrderController(OrderService orderService,
                           UserService userService) {
        this.orderService = orderService;
        this.userService = userService;
    }
}

✅ Summary Table

Way	Use?	Status
Constructor Injection	✔ YES	⭐ Best
Lombok Constructor	✔ YES	⭐ Best
Field Injection	❌ Avoid	Bad
Setter Injection	⚠ Rare	Special case

Agar chaho main tumhe next step dikha sakta hoon:
✅ Controller → Service → Repository full example
✅ POST request (DTO → Entity)
✅ Exception handling with service layer

Bas bolo:
👉 "Full flow example dikhao"
ya
👉 "POST request flow dikhao"