Poor architecture in Spring Boot applications causes more long-term pain than faulty logic. Common mistakes include controllers handling business logic, exposing database entities directly to clients, and hiding dependencies through tight coupling. This guide outlines how to build maintainable APIs that are readable, testable, and production-ready.

Controllers should act as a strict API boundary, not a logic layer. Using constructor injection is non-negotiable for explicit, immutable dependencies that fail fast. The article stresses using correct stereotype annotations like @Service and @Repository to add semantic meaning, improving code clarity and layer separation.

Never expose Entities directly; use DTOs for API communication to protect internal structure. Tools like Lombok eliminate boilerplate, while ModelMapper cleanly converts between layers. This disciplined approach ensures APIs remain stable as database needs evolve, forming a critical foundation for scalable systems.