Spring Boot Engineer

# Spring Boot Engineer

## Core Workflow

1. **Analyze requirements** — Identify service boundaries, APIs, data models, security needs
2. **Design architecture** — Plan microservices, data access, cloud integration, security; confirm design before coding
3. **Implement** — Create services with constructor injection and layered architecture (see Quick Start below)
4. **Secure** — Add Spring Security, OAuth2, method security, CORS configuration; verify security rules compile and pass tests. If compilation or tests fail: review error output, fix the failing rule or configuration, and re-run before proceeding
5. **Test** — Write unit, integration, and slice tests; run `./mvnw test` (or `./gradlew test`) and confirm all pass before proceeding. If tests fail: review the stack trace, isolate the failing assertion or component, fix the issue, and re-run the full suite
6. **Deploy** — Configure health checks and observability via Actuator; validate `/actuator/health` returns `UP`. If health is `DOWN`: check the `components` detail in the response, resolve the failing component (e.g., datasource, broker), and re-validate

## Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Web Layer | `references/web.md` | Controllers, REST APIs, validation, exception handling |
| Data Access | `references/data.md` | Spring Data JPA, repositories, transactions, projections |
| Security | `references/security.md` | Spring Security 6, OAuth2, JWT, method security |
| Cloud Native | `references/cloud.md` | Spring Cloud, Config, Discovery, Gateway, resilience |
| Testing | `references/testing.md` | @SpringBootTest, MockMvc, Testcontainers, test slices |

## Quick Start — Minimal Working Structure

A standard Spring Boot feature consists of these layers. Use these as copy-paste starting points.

### Entity

```java
@Entity
@Table(name = "products")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank
    private String name;

    @DecimalMin("0.0")
    private BigDecimal price;

    // getters / setters or use @Data (Lombok)
}
```

### Repository

```java
public interface ProductRepository extends JpaRepository<Product, Long> {
    List<Product> findByNameContainingIgnoreCase(String name);
}
```

### Service (constructor injection)

```java
@Service
public class ProductService {
    private final ProductRepository repo;

    public ProductService(ProductRepository repo) { // constructor injection — no @Autowired
        this.repo = repo;
    }

    @Transactional(readOnly = true)
    public List<Product> search(String name) {
        return repo.findByNameContainingIgnoreCase(name);
    }

    @Transactional
    public Product create(ProductRequest request) {
        var product = new Product();
        product.setName(request.name());
        product.setPrice(request.price());
        return repo.save(product);
    }
}
```

### REST Controller

```java
@RestController
@RequestMapping("/api/v1/products")
@Validated
public class ProductController {
    private final ProductService service;

    public ProductController(ProductService service) {
        this.service = service;
    }

    @GetMapping
    public List<Product> search(@RequestParam(defaultValue = "") String name) {
        return service.search(name);
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public Product create(@Valid @RequestBody ProductRequest request) {
        return service.create(request);
    }
}
```

### DTO (record)

```java
public record ProductRequest(
    @NotBlank String name,
    @DecimalMin("0.0") BigDecimal price
) {}
```

### Global Exception Handler

```java
@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String, String> handleValidation(MethodArgumentNotValidException ex) {
        return ex.getBindingResult().getFieldErrors().stream()
            .collect(Collectors.toMap(FieldError::getField, FieldError::getDefaultMessage));
    }

    @ExceptionHandler(EntityNotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    public Map<String, String> handleNotFound(EntityNotFoundException ex) {
        return Map.of("error", ex.getMessage());
    }
}
```

### Test Slice

```java
@WebMvcTest(ProductController.class)
class ProductControllerTest {
    @Autowired MockMvc mockMvc;
    @MockBean ProductService service;

    @Test
    void createProduct_validRequest_returns201() throws Exception {
        var product = new Product(); product.setName("Widget"); product.setPrice(BigDecimal.TEN);
        when(service.create(any())).thenReturn(product);

        mockMvc.perform(post("/api/v1/products")
                .contentType(MediaType.APPLICATION_JSON)
                .content("""{"name":"Widget","price":10.0}"""))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.name").value("Widget"));
    }
}
```

## Constraints

### MUST DO

| Rule | Correct Pattern |
|------|----------------|
| Constructor injection | `public MyService(Dep dep) { this.dep = dep; }` |
| Validate API input | `@Valid @RequestBody MyRequest req` on every mutating endpoint |
| Type-safe config | `@ConfigurationProperties(prefix = "app")` bound to a record/class |
| Appropriate stereotype | `@Service` for business logic, `@Repository` for data, `@RestController` for HTTP |
| Transaction scope | `@Transactional` on multi-step writes; `@Transactional(readOnly = true)` on reads |
| Hide internals | Catch domain exceptions in `@RestControllerAdvice`; return problem details, not stack traces |
| Externalize secrets | Use environment variables or Spring Cloud Config — never `application.properties` |

### MUST NOT DO
- Use field injection (`@Autowired` on fields)
- Skip input validation on API endpoints
- Use `@Component` when `@Service`/`@Repository`/`@Controller` applies
- Mix blocking and reactive code (e.g., calling `.block()` inside a WebFlux chain)
- Store secrets or credentials in `application.properties`/`application.yml`
- Hardcode URLs, credentials, or environment-specific values
- Use deprecated Spring Boot 2.x patterns (e.g., `WebSecurityConfigurerAdapter`)

[Documentation](https://jeffallan.github.io/claude-skills/skills/backend/spring-boot-engineer/)