Get best practices for developing applications with Spring Boot

# 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`).

## Dependency Injection & Components

- **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.

## Configuration

- **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.

## Web Layer (Controllers)