Critical Patterns

Constructor Injection (REQUIRED)

// ✅ ALWAYS: Constructor injection for testability @Service public class OrderService { private final OrderRepository repository; private final PaymentService paymentService;

public OrderService(OrderRepository repository, PaymentService paymentService) {
    this.repository = repository;
    this.paymentService = paymentService;
}

}

// ❌ NEVER: Field injection @Service public class OrderService { @Autowired private OrderRepository repository; }

Records for DTOs (REQUIRED)

// ✅ ALWAYS: Use records for immutable DTOs public record UserDTO( String id, String name, String email ) {}

// ❌ NEVER: Mutable POJOs for data transfer public class UserDTO { private String id; // ... getters, setters, equals, hashCode }

Exception Handling (REQUIRED)

// ✅ ALWAYS: Use @ControllerAdvice @ControllerAdvice public class GlobalExceptionHandler {

@ExceptionHandler(ResourceNotFoundException.class)
public ResponseEntity<ErrorResponse> handleNotFound(ResourceNotFoundException ex) {
    return ResponseEntity.status(HttpStatus.NOT_FOUND)
        .body(new ErrorResponse(ex.getMessage()));
}

}

Decision Tree

Need REST API? → Use @RestController Need data validation? → Use @Valid + Bean Validation Need database access? → Use Spring Data JPA Need async processing? → Use @Async Need caching? → Use @Cacheable Need config properties? → Use @ConfigurationProperties

Code Examples

REST Controller

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

private final UserService userService;

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

@GetMapping("/{id}")
public ResponseEntity<UserDTO> getUser(@PathVariable String id) {
    return userService.findById(id)
        .map(ResponseEntity::ok)
        .orElse(ResponseEntity.notFound().build());
}

@PostMapping
public ResponseEntity<UserDTO> createUser(@Valid @RequestBody CreateUserRequest request) {
    UserDTO created = userService.create(request);
    return ResponseEntity.status(HttpStatus.CREATED).body(created);
}

}

Bean Validation

public record CreateUserRequest( @NotBlank(message = "Name is required") String name,

@Email(message = "Invalid email format")
@NotBlank
String email,

@Size(min = 8, message = "Password must be at least 8 characters")
String password

) {}

Commands

mvn clean install # Build project mvn spring-boot:run # Run application mvn test # Run tests mvn dependency:tree # View dependencies ./mvnw spring-boot:run # Run with wrapper

Resources

• Spring Boot Docs: https://spring.io/projects/spring-boot