Spring is a widely used framework that provides many features to help developers create stand-alone, production-grade applications. However, it also introduces new complexities, such as transactional operations, persistent entities, and bean definitions, which can lead to bugs, misconfigurations, and security issues if not handled properly. Sonar, a tool that analyzes code quality, can detect these issues and provide warnings to developers, helping them avoid common pitfalls and improve the overall quality of their applications.