spring-boot

Spring Boot

Purpose

Build Spring Boot services with correct transaction boundaries and a JPA layer that does not issue a hundred queries to render a list. Spring's defaults are safe; its abstractions hide the cost of getting them wrong.

When to Use

• Building or reviewing a Spring Boot application.
• Diagnosing lazy-loading exceptions or N+1 query behavior.
• Getting transaction boundaries and propagation right.
• Structuring a test suite that is faster than a full context load per class.

Capabilities

• Constructor injection and bean lifecycle.
• Declarative transactions: propagation, isolation, rollback rules.
• JPA/Hibernate: fetch strategies, entity graphs, projections.
• Configuration properties with validation and profiles.
• Test slices: @WebMvcTest, @DataJpaTest, Testcontainers.

Inputs

• The service, its entities, and the slow endpoint if there is one.
• Database engine and connection pool configuration.

Outputs

• Services with explicit transaction boundaries.
• Repositories that fetch exactly what the caller needs.
• Tests that load the narrowest context that proves the behavior.

Workflow

1. Use constructor injection — Field injection with @Autowired hides dependencies and makes the class untestable without a container.
2. Place @Transactional at the service layer — Not on the repository (too narrow, one transaction per call) and not on the controller (too wide, the transaction spans view rendering).
3. Fetch deliberately — Every association is LAZY. Then use an entity graph or a fetch join for the specific query that needs it, and a DTO projection for read-only views.
4. Validate configuration at startup — @ConfigurationProperties with @Validated. A missing property should prevent boot, not surface as a null at runtime.
5. Test in slices — @DataJpaTest for repositories against Testcontainers, @WebMvcTest for controllers with mocked services, @SpringBootTest only for the handful of genuine end-to-end paths.