Java Coding Standards

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

When to Activate

• Writing or reviewing Java code in Spring Boot 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

Core Principles

• Prefer clarity over cleverness
• Immutable by default; minimize shared mutable state
• Fail fast with meaningful exceptions
• Consistent naming and package structure

Naming

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

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

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

Immutability

// ✅ 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
}

Optional Usage

// ✅ Return Optional from find* methods
Optional<Market> market = marketRepository.findBySlug(slug);

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

Streams Best Practices

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

// ❌ Avoid complex nested streams; prefer loops for clarity

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);

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 (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)

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

Logging

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

Null Handling

• Accept @Nullable only when unavoidable; otherwise use @NonNull
• Use Bean Validation (@NotNull, @NotBlank) on inputs

Testing Expectations

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

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

Reference Guide

Load detailed guidance based on context:

Topic	Reference	Load When
General Patterns	references/patterns.md	Patterns for design classes, records, DTO, services, builders, constructors
Security	references/security.md	Input validations, secret manager, exception details exposing, auth
Testing	references/testing.md	Frameworks to use, how to structure packages and classes, unit and integration testing, naming