Concurrency, design patterns, reflection, etc.. I could feel how elaborately Spring was made, from basic concepts to Spring AOP. It was really interesting and fun to learn that tasks that felt like magic were all made up of Java code when I took them apart. I had the highest expectations for the advanced version, and it was really good. ㅎㅎ Next, I was originally going to listen to the Spring Boot version, but I thought that the concepts I learned this time were covered in detail in the newly released Java Advanced 1 and 2 versions, so I thought it would be better to listen to the Advanced 1 and 2 versions first and study the Spring Boot version last. Boot's Prometheus Grafana?? There are so many terms that I've never seen before, so it's scary...ㅎㅎ;