As Younghan said, if you know JPA well, Spring Data JPA and Querydsl are completely free, and after listening to the JPA roadmap lecture, I think I was able to easily understand and move on from the Spring Data JPA and Querydsl parts, except for QuerydslRepositorySupport. On the other hand, when you introduced that you can combine various BooleanExpressions to create one large BooleanExpression, the part that you can create a BooleanExpression with a business meaning like isValid (valid) + isAvailableTime (if the time zone is right) = isServiceable (serviceable) and put it in the where clause was particularly impressive. Thank you again for the great lecture!! Younghan's JPA roadmap seems to be the best roadmap!!