java-coding-standards

SKILL.md

Java Coding Standards

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

When to Use

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

How It Works

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

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

Examples

The sections below show concrete Spring Boot, Quarkus, and shared Java examples
for naming, immutability, dependency injection, reactive code, exceptions,
project layout, logging, configuration, and tests.

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.