Spring Boot Best Practices

Your goal is to help me write high-quality Spring Boot applications by following established best practices.

Project Setup & Structure

Build Tool: Use Maven (pom.xml) or Gradle (build.gradle) for dependency management.
Starters: Use Spring Boot starters (e.g., spring-boot-starter-web, spring-boot-starter-data-jpa) to simplify dependency management.
Package Structure: Organize code by feature/domain (e.g., com.example.app.order, com.example.app.user) rather than by layer (e.g., com.example.app.controller, com.example.app.service).

Constructor Injection: Always use constructor-based injection for required dependencies. This makes components easier to test and dependencies explicit.
Immutability: Declare dependency fields as private final.
Component Stereotypes: Use @Component, @Service, @Repository, and @Controller/@RestController annotations appropriately to define beans.

Externalized Configuration: Use application.yml (or application.properties) for configuration. YAML is often preferred for its readability and hierarchical structure.
Type-Safe Properties: Use @ConfigurationProperties to bind configuration to strongly-typed Java objects.
Profiles: Use Spring Profiles (application-dev.yml, application-prod.yml) to manage environment-specific configurations.
Secrets Management: Do not hardcode secrets. Use environment variables, or a dedicated secret management tool like HashiCorp Vault or AWS Secrets Manager.

RESTful APIs: Design clear and consistent RESTful endpoints.
DTOs (Data Transfer Objects): Use DTOs to expose and consume data in the API layer. Do not expose JPA entities directly to the client.
Validation: Use Java Bean Validation (JSR 380) with annotations (@Valid, @NotNull, @Size) on DTOs to validate request payloads.
Error Handling: Implement a global exception handler using @ControllerAdvice and @ExceptionHandler to provide consistent error responses.

Business Logic: Encapsulate all business logic within @Service classes.
Statelessness: Services should be stateless.
Transaction Management: Use @Transactional on service methods to manage database transactions declaratively. Apply it at the most granular level necessary.

Spring Data JPA: Use Spring Data JPA repositories by extending JpaRepository or CrudRepository for standard database operations.
Custom Queries: For complex queries, use @Query or the JPA Criteria API.
Projections: Use DTO projections to fetch only the necessary data from the database.

SLF4J: Use the SLF4J API for logging.
Logger Declaration: private static final Logger logger = LoggerFactory.getLogger(MyClass.class);
Parameterized Logging: Use parameterized messages (logger.info("Processing user {}...", userId);) instead of string concatenation to improve performance.

Unit Tests: Write unit tests for services and components using JUnit 5 and a mocking framework like Mockito.
Integration Tests: Use @SpringBootTest for integration tests that load the Spring application context.
Test Slices: Use test slice annotations like @WebMvcTest (for controllers) or @DataJpaTest (for repositories) to test specific parts of the application in isolation.
Testcontainers: Consider using Testcontainers for reliable integration tests with real databases, message brokers, etc.

Spring Security: Use Spring Security for authentication and authorization.
Password Encoding: Always encode passwords using a strong hashing algorithm like BCrypt.
Input Sanitization: Prevent SQL injection by using Spring Data JPA or parameterized queries. Prevent Cross-Site Scripting (XSS) by properly encoding output.