Spring Boot Skill

Enterprise Spring Boot 4.x development with focus on clean architecture and production-ready code.

Core Workflow

1. Analyze - Understand requirements, identify service boundaries, APIs, data models
2. Design - Plan architecture, confirm design before coding
3. Implement - Build with constructor injection and layered architecture; for a relational database, write Flyway migrations alongside entities (ddl-auto: validate)
4. Secure - Add Spring Security, OAuth2, method security; verify tests pass
5. Test - Write unit, integration tests; run ./mvnw clean verify and confirm all pass, with integration tests running (not skipped)
6. Deploy - Configure health checks via Actuator; validate /actuator/health returns UP and /actuator/info shows build.version

Quick Start Templates

Entity

@Entity
@Table(name = "products")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank
    private String name;

    @DecimalMin("0.0")
    private BigDecimal price;

    // Getters/Setters (no Lombok)
}

Repository

public interface ProductRepository extends JpaRepository<Product, Long> {
    List<Product> findByNameContainingIgnoreCase(String name);
}

Service

@Service
@Transactional(readOnly = true)
public class ProductService {
    private final ProductRepository repo;

    public ProductService(ProductRepository repo) {
        this.repo = repo;
    }

    public List<ProductResponse> search(String name) {
        return repo.findByNameContainingIgnoreCase(name).stream()
            .map(ProductResponse::from)
            .toList();
    }

    @Transactional
    public ProductResponse create(ProductRequest request) {
        var product = new Product();
        product.setName(request.name());
        product.setPrice(request.price());
        return ProductResponse.from(repo.save(product));
    }
}

REST Controller

@RestController
@RequestMapping("/api/v1/products")
@Validated
public class ProductController {
    private final ProductService service;

    public ProductController(ProductService service) {
        this.service = service;
    }

    @GetMapping
    public List<ProductResponse> search(@RequestParam(defaultValue = "") String name) {
        return service.search(name);
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public ProductResponse create(@Valid @RequestBody ProductRequest request) {
        return service.create(request);
    }
}

DTOs (Records)

public record ProductRequest(
    @NotBlank String name,
    @DecimalMin("0.0") BigDecimal price
) {}

public record ProductResponse(Long id, String name, BigDecimal price) {
    public static ProductResponse from(Product product) {
        return new ProductResponse(product.getId(), product.getName(), product.getPrice());
    }
}

Global Exception Handler (Problem Details)

@RestControllerAdvice
public class ApiExceptionHandler extends ResponseEntityExceptionHandler {

    public record FieldMessage(String field, String code, String message) {}

    @Override
    protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex, HttpHeaders headers,
                                                                  HttpStatusCode status, WebRequest request) {
        ex.getBody().setProperty("errors", ex.getFieldErrors().stream()
                .map(e -> new FieldMessage(e.getField(), code(e.getCode()), e.getDefaultMessage()))
                .toList());
        return super.handleMethodArgumentNotValid(ex, headers, status, request);
    }

    private static String code(String constraint) {
        return switch (constraint) {
            case "NotBlank", "NotNull" -> "required";
            case null, default -> "invalid";
        };
    }

    @ExceptionHandler
    ProblemDetail handleOptimisticLock(ObjectOptimisticLockingFailureException ex) {
        return ProblemDetail.forStatusAndDetail(HttpStatus.CONFLICT, "Record was changed by someone else; reload and retry");
    }

    @ExceptionHandler
    ProblemDetail handleUnexpected(Exception ex) {
        logger.error("Unexpected error", ex);
        return ProblemDetail.forStatusAndDetail(HttpStatus.INTERNAL_SERVER_ERROR, "Unexpected server error");
    }
}

Every error is RFC 9457 Problem Details with field errors in errors: [{field, code, message}]. Full handler (unique constraints, bad sort, documented codes): references/web.md.

Test Slice

@WebMvcTest(ProductController.class)
class ProductControllerTest {
    @Autowired MockMvc mockMvc;
    @MockitoBean ProductService service;

    @Test
    void createProduct_validRequest_returns201() throws Exception {
        var response = new ProductResponse(1L, "Widget", new BigDecimal("10.00"));
        when(service.create(any())).thenReturn(response);

        mockMvc.perform(post("/api/v1/products")
                .contentType(MediaType.APPLICATION_JSON)
                .content("""
                    {"name":"Widget","price":10.0}
                    """))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.name").value("Widget"));
    }
}

Reference Guide

Load detailed patterns based on context:

Constraints

MUST DO

• Constructor injection (no field injection)
• @Valid on all request bodies
• @Transactional for multi-step writes
• @Transactional(readOnly = true) for reads
• Type-safe config with @ConfigurationProperties
• Global exception handling with @RestControllerAdvice
• Externalize secrets (use env vars, not properties files)
• Flyway migrations + ddl-auto: validate when persisting to a relational database (skip Flyway otherwise)
• Errors as Problem Details; list endpoints return PagedModel
• Permit /actuator/health/** so the liveness and readiness probes work behind security
• Only when the API is built for a UI (see conventions, "REST API Built for a UI"): follow references/ui-contract.md

MUST NOT DO

• Field injection (@Autowired on fields)
• Skip input validation on endpoints
• Mix blocking and reactive code
• Store secrets in application.properties
• Use deprecated Spring Boot 2.x/3.x patterns
• Hardcode URLs, credentials, environment values
• Use ddl-auto: update/create outside tests, or edit an already-applied migration
• Return Page/PageImpl from a controller, or put @EnableSpringDataWebSupport on the application class (@WebMvcTest registers it twice)
• Put @ApiResponse on controller methods (springdoc 3 then drops the success response; use an OpenApiCustomizer)
• Return an exception's message from the catch-all handler

Spring Boot 4 / Spring Data 4 Notes

• PropertyReferenceException and TypeInformation moved to org.springframework.data.core
• @MockBean/@SpyBean are removed: use @MockitoBean/@MockitoSpyBean
• @WebMvcTest creates no repositories: every repository a controller injects needs a @MockitoBean in its test
• Build with ./mvnw clean verify: classes compiled by the IDE in target can hide javac errors

Architecture Patterns

Project Structure:

src/main/java/pl/piomin/services/
├── controller/     # REST endpoints
├── service/        # Business logic
├── repository/     # Data access
├── model/          # Entities
├── dto/            # Request/Response DTOs
├── config/         # Configuration
└── exception/      # Custom exceptions + handler

Layering:

• Controller → Service → Repository
• Controller handles HTTP, validation
• Service handles business logic, transactions
• Repository handles data persistence

Clean Architecture Principles:

• Domain models independent of frameworks
• Use case driven design
• Dependency inversion (interfaces)
• Clear boundaries between layers

Common Annotations

Reactive WebFlux Endpoint

@RestController
@RequestMapping("/api/v1/orders")
public class OrderController {
    private final OrderService orderService;

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

    @GetMapping("/{id}")
    public Mono<ResponseEntity<OrderDto>> getOrder(@PathVariable UUID id) {
        return orderService.findById(id)
                .map(ResponseEntity::ok)
                .defaultIfEmpty(ResponseEntity.notFound().build());
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public Mono<OrderDto> createOrder(@Valid @RequestBody CreateOrderRequest request) {
        return orderService.create(request);
    }
}

Spring Security JWT

@Configuration
@EnableMethodSecurity
public class SecurityConfig {
    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        return http
                .csrf(AbstractHttpConfigurer::disable)
                .sessionManagement(s -> s.sessionCreationPolicy(STATELESS))
                .authorizeHttpRequests(auth -> auth
                        .requestMatchers("/actuator/health/**").permitAll()  // + "/actuator/info" when built for a UI
                        .anyRequest().authenticated())
                .oauth2ResourceServer(oauth2 -> oauth2.jwt(Customizer.withDefaults()))
                .build();
    }
}

Knowledge Base

Spring Boot 4.x, Java 21+, Spring WebFlux, Project Reactor, Spring Data JPA, Spring Security, OAuth2/JWT, Hibernate, R2DBC, Spring Cloud, Resilience4j, Micrometer, JUnit 5, Testcontainers, Mockito, Maven, Paketo Buildpacks, Skaffold