I remember hearing about the principles corresponding to SOLID while learning design patterns in college, but I didn't hear it systematically, so I thought, "What does this mean?", but I didn't know that I would correct it in the Spring basics lecture. Even in the practical book that I studied personally before this lecture, it was written that Spring is a product of consideration for better object-oriented, but since it was a practical-oriented book, I still had the question, "Why?", and I think this lecture will answer that question.