Of note,
we often think of Japan's influence on Korea, but early on in history, it was quite the other way around. The "Japan" that we are familiar with thru movies and tv was quite different than the Early Japan of the Yamato and pre yamato periods. It was during those early stages in Japanese history where Korea, and also China and other peoples greatly influenced the beginnings of Japan. In fact, it's quite documented that the early Yamato royal line came from the Royal line of the Kingdom of Paekche, and Paekche always maintained close ties with Japan. Also, Strong evidence now supports that the early ruling warrior class in Japan were descendents of the Puyo peoples of northern korea/manchuria. Back in the Early days, Korea was also a glom of all kinds of peoples and cultures and even languages, with many ancient tribes and peoples - most lost in time - making up the "Koreans" that we know today.
As far as the Hwa Rang warriors, history shows that they did pre date the "samurai" class a few hundred years, but there were many exchanges off and on between korea and japan during those times and certainly the japanese warriors of the day would have had many exchanges with the warrior classes of Korea and China and Manchuria.