If you want to become proficient with Spring, my recommendation is to put Spring Boot aside for a moment and focus on learning the Spring Framework fundamentals. After that, not only you'll appreciate all that Spring Boot does for you, but you'll know how to customize it. (1/6) A great starting point is the Spring Framework official documentation: docs.spring.io/spring-framewo… Focus on the application context, dependency injection, bean lifecycle, and configuration (annotation-based and java-based), aspect-oriented programming, and bean validation. (2/6)