Spring Boot Idioms

Spring Boot Idioms and Patterns

Spring Boot (3.x) rewards auto-configuration, constructor injection, and actuator-driven observability. Idiomatic Spring = annotation-driven, testable, production-ready.

Scope: Spring Boot-specific patterns. For Java: @.gemini/skills/java-idioms/SKILL.md.

Dependency Injection

1. Constructor injection only — never field injection:

   @Service
   public class TaskService {
       private final TaskRepository repository;
       private final TaskMapper mapper;
   
       public TaskService(TaskRepository repository, TaskMapper mapper) {
           this.repository = repository;
           this.mapper = mapper;
       }
   }
   

2. @ConfigurationProperties over @Value for typed config.

Spring Data JPA

1. Query methods for simple queries:

   interface TaskRepository extends JpaRepository<Task, UUID> {
       List<Task> findByStatusOrderByCreatedAtDesc(TaskStatus status);
       @Query("SELECT t FROM Task t WHERE t.priority = :priority AND t.status = 'ACTIVE'")
       List<Task> findActivByPriority(@Param("priority") Priority priority);
   }
   

2. Projections for read-only views — avoid loading full entities.

3. @Transactional on service methods, never on repositories.

REST Controllers

1. @RestController + DTOs — never expose entities directly:

   @RestController
   @RequestMapping("/api/v1/tasks")
   public class TaskController {
       @PostMapping
       @ResponseStatus(HttpStatus.CREATED)
       public TaskResponse create(@Valid @RequestBody CreateTaskRequest request) {
           return taskService.create(request);
       }
   }
   

2. @ControllerAdvice for global exception handling.

Actuator and Observability

1. Actuator endpoints enabled for health, metrics, info.
2. Micrometer for custom metrics.
3. Structured logging with MDC for correlation IDs.

Testing

1. @SpringBootTest for integration, @WebMvcTest for controller slices:

   @WebMvcTest(TaskController.class)
   class TaskControllerTest {
       @Autowired MockMvc mockMvc;
       @MockBean TaskService taskService;
   
       @Test
       void createTask_returns201() throws Exception {
           mockMvc.perform(post("/api/v1/tasks")
               .contentType(MediaType.APPLICATION_JSON)
               .content("{\"title\":\"Test\",\"priority\":\"HIGH\"}"))
               .andExpect(status().isCreated());
       }
   }
   

2. TestContainers for database integration tests.

Related

• Java Idioms @.gemini/skills/java-idioms/SKILL.md
• Database Design Principles @.gemini/skills/database-design-principles/SKILL.md
• API Design Principles @.gemini/skills/api-design-principles/SKILL.md