# Spring Boot Java Coding Rules and Best Practices

A comprehensive checklist and guide for writing clean, maintainable, and scalable Spring Boot applications.

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## 🚀 1. Project Structure and Organization

- Use the standard package structure: `com.yourcompany.projectname`
- Organize by layer: `controller`, `service`, `repository`, `model`, `config`, `dto`, `exception`, `util`
- For large apps, consider **package-by-feature** (e.g., `user`, `order`, `payment`)
- Class Naming: `PascalCase`  
- Variable/Method Naming: `camelCase`  
- Avoid abbreviations and acronyms

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## 🔧 2. Spring Boot Configuration

- Use `@ConfigurationProperties` over `@Value` for structured config
- Externalize all configuration in `application.yml` or `application.properties`
- Use Spring Profiles (`dev`, `test`, `prod`)
- Validate config with `@Validated`
- Fail fast on startup misconfigurations

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## 🧠 3. Dependency Injection and Bean Management

- Prefer **constructor injection**
- Avoid field injection (`@Autowired` on fields)
- Be explicit with component scanning (define `basePackages`)
- Use `@Qualifier` when multiple beans of the same type exist

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## 🧾 4. DTOs and Entities

- Never expose entity objects in API responses
- Use DTOs for both request and response payloads
- Use `ModelMapper` or `MapStruct` to handle mapping
- Validate incoming DTOs using `javax.validation` annotations and `@Valid`

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## 🧰 5. Service and Repository Layer

- Keep services stateless
- Push reusable logic to utility/helper classes
- Use `interface + implementation` pattern for services and repositories
- Define transaction boundaries with `@Transactional` at the service layer

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## 📡 6. Controller Layer

- Use `@RestController` for REST APIs
- Handle exceptions globally with `@ControllerAdvice`
- Always return DTOs or wrapper responses
- Use `ResponseEntity` for custom status codes and headers

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## 🧪 7. Testing

- Unit test services and utilities with **JUnit + Mockito**
- Use `@SpringBootTest` for integration tests
- Test repositories using `@DataJpaTest`
- Use `MockMvc` or `WebTestClient` to test REST endpoints
- Isolate DB with Testcontainers if possible

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## 🛡️ 8. Security and Validation

- Enable and configure **Spring Security**
- Use input validation to prevent injection and data corruption
- Hash passwords using `BCryptPasswordEncoder`
- Protect endpoints with proper authentication and authorization
- Enable CSRF and configure CORS as required

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## 🧹 9. Code Quality and Maintainability

- Follow Java code conventions: indentation, braces, spacing, naming
- Use a linter and formatter (e.g., Checkstyle, Spotless)
- Use **Lombok** with care:
  - Avoid `@Data` on JPA entities
  - Prefer `@Getter`, `@Setter`, `@Builder` as needed
- Log using SLF4J: `LoggerFactory.getLogger(...)`
- Avoid using `System.out.println`
- Create and use custom exceptions

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## ⚡ 10. Performance and Optimization

- Avoid N+1 query problems using `join fetch`, `@EntityGraph`
- Use caching with `@Cacheable`, Redis, or Caffeine
- Perform long-running tasks with `@Async` or Spring Batch
- Configure connection pooling (HikariCP is default)
- Profile and monitor the application with **Actuator**

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## 🧭 Checklist Summary

- [ ] Use constructor injection
- [ ] Separate DTOs from entities
- [ ] Validate all incoming request data
- [ ] Write unit & integration tests
- [ ] Configure Spring Security properly
- [ ] Externalize all environment-specific config
- [ ] Avoid field injection and circular dependencies
- [ ] Use proper exception handling
- [ ] Monitor and profile using Actuator
- [ ] Document REST APIs with Swagger/OpenAPI

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