everything-claude-code/skills/java-coding-standards/SKILL.md

name, description, origin

name	description	origin
java-coding-standards	Java coding standards for Spring Boot and Quarkus services: naming, immutability, Optional usage, streams, exceptions, generics, CDI, reactive patterns, and project layout. Automatically applies framework-specific conventions.	ECC

Java Coding Standards

Standards for readable, maintainable Java (17+) code in Spring Boot and Quarkus services.

Framework Detection

Before applying standards, determine the framework from the build file:

• Build file contains quarkus → apply [QUARKUS] conventions
• Build file contains spring-boot → apply [SPRING] conventions
• Neither detected → apply shared conventions only

When to Activate

• Writing or reviewing Java code in Spring Boot or Quarkus projects
• Enforcing naming, immutability, or exception handling conventions
• Working with records, sealed classes, or pattern matching (Java 17+)
• Reviewing use of Optional, streams, or generics
• Structuring packages and project layout
• [QUARKUS]: Working with CDI scopes, Panache entities, or reactive pipelines

Core Principles

• Prefer clarity over cleverness
• Immutable by default; minimize shared mutable state
• Fail fast with meaningful exceptions
• Consistent naming and package structure
• [QUARKUS]: Favor build-time over runtime processing; avoid runtime reflection where possible

Naming

// PASS: Classes/Records: PascalCase
public class MarketService {}
public record Money(BigDecimal amount, Currency currency) {}

// PASS: Methods/fields: camelCase
private final MarketRepository marketRepository;
public Market findBySlug(String slug) {}

// PASS: Constants: UPPER_SNAKE_CASE
private static final int MAX_PAGE_SIZE = 100;

// PASS: [QUARKUS] JAX-RS resources named as *Resource, not *Controller
public class MarketResource {}

// PASS: [SPRING] REST controllers named as *Controller
public class MarketController {}

Immutability

// PASS: Favor records and final fields
public record MarketDto(Long id, String name, MarketStatus status) {}

public class Market {
  private final Long id;
  private final String name;
  // getters only, no setters
}

// PASS: [QUARKUS] Panache active-record entities use public fields (Quarkus convention)
@Entity
public class Market extends PanacheEntity {
  public String name;
  public MarketStatus status;
  // Panache generates accessors at build time; public fields are idiomatic here
}

// PASS: [QUARKUS] Panache MongoDB entities
@MongoEntity(collection = "markets")
public class Market extends PanacheMongoEntity {
  public String name;
  public MarketStatus status;
}

Optional Usage

// PASS: Return Optional from find* methods
// [SPRING]
Optional<Market> market = marketRepository.findBySlug(slug);

// [QUARKUS] Panache
Optional<Market> market = Market.find("slug", slug).firstResultOptional();

// PASS: Map/flatMap instead of get()
return market
    .map(MarketResponse::from)
    .orElseThrow(() -> new EntityNotFoundException("Market not found"));

Streams Best Practices

// PASS: Use streams for transformations, keep pipelines short
List<String> names = markets.stream()
    .map(Market::name)
    .filter(Objects::nonNull)
    .toList();

// FAIL: Avoid complex nested streams; prefer loops for clarity

Dependency Injection

// PASS: [SPRING] Constructor injection (preferred over @Autowired on fields)
@Service
public class MarketService {
  private final MarketRepository marketRepository;

  public MarketService(MarketRepository marketRepository) {
    this.marketRepository = marketRepository;
  }
}

// PASS: [QUARKUS] Constructor injection
@ApplicationScoped
public class MarketService {
  private final MarketRepository marketRepository;

  @Inject
  public MarketService(MarketRepository marketRepository) {
    this.marketRepository = marketRepository;
  }
}

// PASS: [QUARKUS] Package-private field injection (acceptable in Quarkus — avoids proxy issues)
@ApplicationScoped
public class MarketService {
  @Inject
  MarketRepository marketRepository;
}

// FAIL: [SPRING] Field injection with @Autowired
@Autowired
private MarketRepository marketRepository; // use constructor injection

// FAIL: [QUARKUS] @Singleton when interception or lazy init is needed
@Singleton // non-proxyable — use @ApplicationScoped instead
public class MarketService {}

Reactive Patterns [QUARKUS]

// PASS: Return Uni/Multi from reactive endpoints
@GET
@Path("/{slug}")
public Uni<Market> findBySlug(@PathParam("slug") String slug) {
  return Market.find("slug", slug)
      .<Market>firstResult()
      .onItem().ifNull().failWith(() -> new MarketNotFoundException(slug));
}

// PASS: Non-blocking pipeline composition
public Uni<OrderConfirmation> placeOrder(OrderRequest req) {
  return validateOrder(req)
      .chain(valid -> persistOrder(valid))
      .chain(order -> notifyFulfillment(order));
}

// FAIL: Blocking call inside a Uni/Multi pipeline
public Uni<Market> find(String slug) {
  Market m = Market.find("slug", slug).firstResult(); // BLOCKING — breaks event loop
  return Uni.createFrom().item(m);
}

// FAIL: Subscribing more than once to a shared Uni
Uni<Market> shared = fetchMarket(slug);
shared.subscribe().with(m -> log(m));
shared.subscribe().with(m -> cache(m)); // double subscribe — use Uni.memoize()

Exceptions

• Use unchecked exceptions for domain errors; wrap technical exceptions with context
• Create domain-specific exceptions (e.g., MarketNotFoundException)
• Avoid broad catch (Exception ex) unless rethrowing/logging centrally

throw new MarketNotFoundException(slug);

Centralised Exception Handling

// [SPRING]
@RestControllerAdvice
public class GlobalExceptionHandler {
  @ExceptionHandler(MarketNotFoundException.class)
  public ResponseEntity<ErrorResponse> handle(MarketNotFoundException ex) {
    return ResponseEntity.status(404).body(ErrorResponse.from(ex));
  }
}

// [QUARKUS] Option A: ExceptionMapper
@Provider
public class MarketNotFoundMapper implements ExceptionMapper<MarketNotFoundException> {
  @Override
  public Response toResponse(MarketNotFoundException ex) {
    return Response.status(404).entity(ErrorResponse.from(ex)).build();
  }
}

// [QUARKUS] Option B: @ServerExceptionMapper (RESTEasy Reactive)
@ServerExceptionMapper
public RestResponse<ErrorResponse> handle(MarketNotFoundException ex) {
  return RestResponse.status(Status.NOT_FOUND, ErrorResponse.from(ex));
}

Generics and Type Safety

• Avoid raw types; declare generic parameters
• Prefer bounded generics for reusable utilities

public <T extends Identifiable> Map<Long, T> indexById(Collection<T> items) { ... }

Project Structure

[SPRING] Maven/Gradle

src/main/java/com/example/app/
  config/
  controller/
  service/
  repository/
  domain/
  dto/
  util/
src/main/resources/
  application.yml
src/test/java/... (mirrors main)

[QUARKUS] Maven/Gradle

src/main/java/com/example/app/
  config/              # @ConfigMapping, @ConfigProperty beans, Producers
  resource/            # JAX-RS resources (not "controller")
  service/
  repository/          # PanacheRepository implementations (if not using active record)
  domain/              # JPA/Panache entities, MongoDB entities
  dto/
  util/
  mapper/              # MapStruct mappers (if used)
src/main/resources/
  application.properties   # Quarkus convention (YAML supported with quarkus-config-yaml)
  import.sql               # Hibernate auto-import for dev/test
src/test/java/... (mirrors main)

Formatting and Style

• Use 2 or 4 spaces consistently (project standard)
• One public top-level type per file
• Keep methods short and focused; extract helpers
• Order members: constants, fields, constructors, public methods, protected, private

Code Smells to Avoid

• Long parameter lists → use DTO/builders
• Deep nesting → early returns
• Magic numbers → named constants
• Static mutable state → prefer dependency injection
• Silent catch blocks → log and act or rethrow
• [QUARKUS]: @Singleton where @ApplicationScoped is intended — breaks proxying and interception
• [QUARKUS]: Mixing quarkus-resteasy-reactive and quarkus-resteasy (classic) — pick one stack
• [QUARKUS]: Panache active-record + repository pattern in the same bounded context — pick one

Logging

// [SPRING] SLF4J
private static final Logger log = LoggerFactory.getLogger(MarketService.class);
log.info("fetch_market slug={}", slug);
log.error("failed_fetch_market slug={}", slug, ex);

// [QUARKUS] JBoss Logging (default, zero-cost at build time)
private static final Logger log = Logger.getLogger(MarketService.class);
log.infof("fetch_market slug=%s", slug);
log.errorf(ex, "failed_fetch_market slug=%s", slug);

// [QUARKUS] Alternative: simplified logging with @Inject
@Inject
Logger log; // CDI-injected, scoped to declaring class

Null Handling

• Accept @Nullable only when unavoidable; otherwise use @NonNull
• Use Bean Validation (@NotNull, @NotBlank) on inputs
• [QUARKUS]: Apply @Valid on @BeanParam, @RestForm, and request body parameters

Configuration

// [SPRING] @ConfigurationProperties
@ConfigurationProperties(prefix = "market")
public record MarketProperties(int maxPageSize, Duration cacheTtl) {}

// [QUARKUS] @ConfigMapping (type-safe, build-time validated)
@ConfigMapping(prefix = "market")
public interface MarketConfig {
  int maxPageSize();
  Duration cacheTtl();
}

// [QUARKUS] Simple values with @ConfigProperty
@ConfigProperty(name = "market.max-page-size", defaultValue = "100")
int maxPageSize;

Testing Expectations

Shared

• JUnit 5 + AssertJ for fluent assertions
• Mockito for mocking; avoid partial mocks where possible
• Favor deterministic tests; no hidden sleeps

[SPRING]

• @WebMvcTest for controller slices, @DataJpaTest for repository slices
• @SpringBootTest reserved for full integration tests
• @MockBean for replacing beans in Spring context

[QUARKUS]

• Plain JUnit 5 + Mockito for unit tests (no @QuarkusTest)
• @QuarkusTest reserved for CDI integration tests
• @InjectMock for replacing CDI beans in integration tests
• Dev Services for database/Kafka/Redis — avoid manual Testcontainers setup when Dev Services suffice
• @QuarkusTestResource for custom external service lifecycle

// [SPRING] Controller test
@WebMvcTest(MarketController.class)
class MarketControllerTest {
  @Autowired MockMvc mockMvc;
  @MockBean MarketService marketService;
}

// [QUARKUS] Integration test
@QuarkusTest
class MarketResourceTest {
  @InjectMock
  MarketService marketService;

  @Test
  void should_return_404_when_market_not_found() {
    given().when().get("/markets/unknown").then().statusCode(404);
  }
}

// [QUARKUS] Unit test (no CDI, no @QuarkusTest)
@ExtendWith(MockitoExtension.class)
class MarketServiceTest {
  @Mock MarketRepository marketRepository;
  @InjectMocks MarketService marketService;
}

Remember: Keep code intentional, typed, and observable. Optimize for maintainability over micro-optimizations unless proven necessary.