https://rein.kr/posts/2008-04-10-concurrency%EB%A5%BC-%EC%9C%84%ED%95%B4%EC%84%A0-%EB%A9%80%ED%8B%B0%EC%8A%A4%EB%A0%88%EB%94%A9-%EB%BF%90%EC%9D%B8%EA%B0%80/