EL Expression Language
四个范围:
pageScope < requestScope < sessionScope < applicationScope
${ name } 如果不指定范围,默认从小到大的范围查找
写法一: ${ sessionScope.user.userName }
写法二: ${ sessionScope ["user"] ["userName"] }
pageContext 获得jsp的内置对象
${ pageContext.request.contextPath } = pageContext.getRequest( ).getContextPath( )