Spring Jpa Patterns

Spring Boot + JPA Patterns

Backend knowledge base for Java 21+ services built on Spring Boot 3 with PostgreSQL and Redis. Covers layered architecture, persistence with Spring Data JPA + Hibernate, Flyway-managed schema, HikariCP tuning, Spring Security 6, REST design, the JUnit 5 + Mockito + Testcontainers test stack, and Micrometer-based observability. Designed to be auto-loaded by java-engineer and referenced by reviewers and architects working on Java services.

Project Architecture (Spring Boot 3)

Use layered or hexagonal architecture consistently across the project:

• Controller (@RestController): HTTP mapping, request validation, response DTOs
• Service (@Service, @Transactional): business logic, orchestration, transaction boundaries
• Repository (@Repository, extends JpaRepository): data access, custom queries
• Entity (@Entity): JPA-managed domain objects, lifecycle callbacks
• DTO / Record: data transfer between layers — never expose entities in API responses
• Mapper: convert between entities and DTOs (MapStruct or manual)
• Config (@Configuration): bean definitions, property binding, infrastructure setup
• Exception (@ControllerAdvice): global error handling with @ExceptionHandler

Dependency injection rules:

• Constructor injection only. final fields with @RequiredArgsConstructor or explicit constructor.
• No field injection (@Autowired on fields).
• Java 21+ features: records, sealed classes, pattern matching, text blocks, virtual threads. No raw types or unchecked casts.

Configuration:

• Use application.yml over .properties — structured, profile-aware.
• Profile-based config: application-dev.yml, application-prod.yml, application-test.yml.
• Externalize environment values: ${DB_URL}, ${REDIS_HOST}, ${JWT_SECRET}.
• @ConfigurationProperties + @Validated for type-safe binding.
• Virtual threads: spring.threads.virtual.enabled=true for I/O-bound workloads (Java 21+).

Spring Data JPA + Hibernate

Entity design:

• @Entity with explicit @Table(name = "...") — never rely on naming strategy alone.
• Primary keys: @GeneratedValue(strategy = IDENTITY) for serial, @UuidGenerator for UUID.
• Auditing: @CreatedDate, @LastModifiedDate via @EntityListeners(AuditingEntityListener.class).
• @Column(nullable = false) matches DB constraints.
• equals() / hashCode() based on business key or ID — never on all fields.
• @Enumerated(EnumType.STRING) — never EnumType.ORDINAL.

Query and fetch patterns:

• N+1 prevention: @EntityGraph or JOIN FETCH in JPQL for association loading.
• Lazy by default: all @ManyToOne and @OneToMany use FetchType.LAZY.
• Projections: interface/DTO projections for read-only queries — avoid loading full entities.
• Batch ops: hibernate.jdbc.batch_size=25 for bulk inserts/updates.
• Read-only transactions: @Transactional(readOnly = true) enables Hibernate optimizations.
• Indexes: create for columns in WHERE, JOIN, ORDER BY via Flyway migration.
• Avoid native SQL except for performance-critical paths, with a comment explaining why.

Flyway Migrations

• Location: src/main/resources/db/migration/.
• Versioned: V{version}__{description}.sql (e.g., V1__create_users_table.sql).
• Repeatable: R__{description}.sql for views, functions, seed data.
• Each migration is idempotent where possible (CREATE INDEX IF NOT EXISTS, ALTER TABLE ... ADD COLUMN IF NOT EXISTS).
• Never modify or delete existing migrations.
• Never use ddl-auto=update in production — Flyway owns schema.
• CI test plan: run migrations against an empty DB and from the previous version.

HikariCP Connection Pool

• Configure maximum-pool-size, connection-timeout, max-lifetime.
• Initial pool sizing heuristic: (2 * CPU cores) + number_of_disks — tune via load testing.
• Set max-lifetime below PostgreSQL's idle_in_transaction_session_timeout.
• Monitor via Micrometer: hikaricp.connections.active, hikaricp.connections.pending.
• Detect connection leaks via leakDetectionThreshold in non-prod profiles.

Redis Caching

Declarative caching:

• @EnableCaching + @Cacheable, @CachePut, @CacheEvict.
• Spring Data Redis with Lettuce client (default in Spring Boot 3).
• Cache key design: {entity}:{id} or {entity}:list:{hash_of_params} — predictable, eviction-friendly.
• TTL: configured per cache in RedisCacheConfiguration.
• Eviction: @CacheEvict on mutations; allEntries = true for list caches.
• Serialization: GenericJackson2JsonRedisSerializer — human-readable, debuggable.

Patterns:

• Cache-aside: default @Cacheable flow — check cache, miss → load from DB → store.
• Distributed locks: RedisTemplate + Lua scripts, or Redisson.
• Rate limiting: Redis sliding-window counters.
• Session storage: Spring Session with Redis for stateless horizontal scaling.
• Never cache mutable state without an explicit invalidation strategy.
• Monitor hit/miss ratio, eviction rate, memory usage (Redis INFO + Micrometer).

Spring Security

• Bean-based config via SecurityFilterChain — never extend WebSecurityConfigurerAdapter (removed in Spring Security 6).
• JWT auth: stateless sessions, token validation in OncePerRequestFilter.
• OAuth2 resource server: spring-boot-starter-oauth2-resource-server for JWT/opaque tokens.
• Method-level: @PreAuthorize("hasRole('ADMIN')"), @Secured for role-based access.
• CORS: configure explicitly in SecurityFilterChain — never @CrossOrigin("*").
• CSRF: disable only for stateless REST (csrf(csrf -> csrf.disable())); keep enabled for session apps.
• Passwords: BCryptPasswordEncoder. Never store plaintext.
• Validate all inputs; rely on JPA's parameterized queries (no string concat into JPQL).

REST API Design

• Resource-oriented URLs: /api/v1/users, /api/v1/users/{id}/orders.
• HTTP verbs: GET (read), POST (create), PUT (full update), PATCH (partial), DELETE (remove).
• Status codes: 200, 201, 204, 400, 401, 403, 404, 409, 422, 500 — precise, never generic 200 for errors.
• Pagination: Pageable parameter → Page<T> with page, size, sort.
• Versioning: URL-based (/api/v1/...).
• Validation: @Valid + Jakarta Bean Validation (@NotNull, @Size, @Email, @Pattern).
• Error responses: RFC 7807 problem-details shape — { timestamp, status, error, message, path, details[] }.
• Custom exception hierarchy: BusinessException (4xx) → specific subclasses; SystemException (5xx).
• Global @ControllerAdvice with @ExceptionHandler per exception family.
• Never expose stack traces or internal details in API responses.
• Log exceptions with correlation IDs.
• OpenAPI: springdoc-openapi for generated docs; keep schemas in sync with DTOs.

Testing

JUnit 5

• Structure: Given → When → Then.
• AssertJ assertions; @DisplayName for readability.
• Skip tests for framework behavior, getters/setters, generated code.

Mockito

• @Mock + @InjectMocks, @ExtendWith(MockitoExtension.class) for unit tests.
• Mock at architectural seams (services depend on repositories; controllers depend on services).
• Verify behavior with verify(...) only when interaction matters; otherwise assert state.

Spring test slices

• @WebMvcTest + MockMvc for controllers.
• @DataJpaTest for repositories (paired with Testcontainers PostgreSQL).
• @DataRedisTest for Redis-only slices.
• @SpringBootTest + MockMvc for end-to-end request-response cycles.

Testcontainers

• @Testcontainers + @Container for PostgreSQL and Redis to match production runtimes.
• Reuse containers across tests with withReuse(true) and ~/.testcontainers.properties for local speed.
• Lifecycle: containers start before Spring context; use @DynamicPropertySource to inject JDBC URL and Redis host.

Observability

• Logging: SLF4J + Logback. Structured JSON in production. MDC for correlation IDs.
• Metrics: Micrometer + Prometheus via /actuator/prometheus. Track latency, error rate, DB pool, cache hit ratio.
• Health: /actuator/health with custom indicators for PostgreSQL, Redis, external services.
• Tracing: Micrometer Tracing (Brave or OpenTelemetry bridge). Propagate trace/span IDs across services.
• Actuator: expose only necessary endpoints. Secure with a separate security config.

Performance

• Virtual threads (Java 21+): enable for I/O-bound ops. Prefer ReentrantLock over synchronized to avoid pinning carriers.
• Connection pool: monitor HikariCP metrics and adjust pool size per throughput/latency.
• Query optimization: EXPLAIN ANALYZE on slow queries. Add indexes. Paginate large results.
• Caching: cache frequently read, rarely changed data. Measure before caching.
• Async: @Async + custom TaskExecutor. CompletableFuture for parallel calls.
• GraalVM native: consider for microservices needing fast startup.

When this applies

Integration

• Consumed by: java-engineer (primary), software-engineer (when reviewing Java code), db-engineer (JPA model and migration review), solution-architect (REST + service-layer design).
• Companion knowledge: owasp-coverage skill for Spring Security threat coverage; test-strategy skill for layered test pyramid; observability-methods skill for Micrometer/OTel cross-stack patterns.
• External references: Spring Boot 3 reference docs, Spring Data JPA docs, Hibernate ORM docs, JUnit 5 user guide, Testcontainers docs, HikariCP wiki, Flyway docs.