[스프링 시큐리티] JSP EL 표현식에서 Spring Security Principal 가져 오기

https://code.i-harness.com/ko-kr/q/8a13a3

https://coderanch.com/t/468178/java/Accessing-userPrincipal-EL-doesn-work

This doesn't work: 

?

1	<c:out value="${request.userPrincipal}"/>

But this does: 

?

1	<c:out value="${pageContext.request.userPrincipal}"/>

핵심은 이거