https://www.openmaru.io/was-java-servlet-%eb%8f%99%ec%9e%91-%eb%b0%a9%ec%8b%9d-%ed%95%9c%eb%88%88%ec%97%90-%ec%95%8c%ec%95%84%eb%b3%b4%ea%b8%b0_chapter-1/
WAS, Java Servlet 동작 방식 한눈에 알아보기_chapter 1