Java Coding Standards

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