I was working on a project and had a lot of trouble with security and authentication/authorization. I learned the general flow by cloning the free lecture (Spring Security 5.x) from another place and then modified the source codes in the blog to fit my project, but I felt that it was not enough. Most of the existing books and lectures were based on Spring Security 5.x version, so there were many differences from 6.x. In particular, in the security setting part, 5.x uses a simple method, but from 6.x, lambda expressions are required, so I had to googling every single thing while wondering if it was right. I applied JWT and OAuth2 to the project with Googling and Spring Security in Action 2nd Edition and GPT4, and the server worked, but I was very uneasy about applying authentication/authorization without properly understanding it, so I decided to take the lecture. I was worried that there would be a lot of overlap with Spring Security in Action 2nd Edition, but the detailed parts that were not covered in the book were covered in detail, so I was able to clearly understand the concepts that were ambiguous. For those who are learning for the first time, I recommend 5.x version + ChatGPT4 or 4o (since most of the learning data is 5.x version, it presents a lot of deprecated code) + don't waste time Googling and just listen to this lecture😭 + If you are completely new to Spring Security, I recommend Spring Security in Action 2nd (6.x version or higher) sold by Manning + official documentation. I do not recommend studying 5.x version.