问题
When we can access all the implicit variables in JSP, why do we have pageContext ?
My assumption is the following: if we use EL expressions or JSTL, to access or set the attributes we need pageContext. Let me know whether I am right.
回答1:
You need it to access non-implicit variables. Does it now make sense?
Update: Sometimes would just like to access the getter methods of HttpServletRequest and HttpSession directly. In standard JSP, both are only available by ${pageContext}. Here are some real world use examples:
Refreshing page when session times out:
<meta http-equiv="refresh" content="${pageContext.session.maxInactiveInterval}">
Passing session ID to an Applet (so that it can communicate with servlet in the same session):
<param name="jsessionid" value="${pageContext.session.id}">
Displaying some message only on first request of a session:
<c:if test="${pageContext.session['new']}">Welcome!</c:if>
note that new has special treatment because it's a reserved keyword in EL, at least, since EL 2.2
Displaying user IP:
Your IP is: ${pageContext.request.remoteAddr}
Making links domain-relative without hardcoding current context path:
<a href="${pageContext.request.contextPath}/login">login</a>
Dynamically defining the <base> tag (with a bit help of JSTL functions taglib):
<base href="${fn:replace(pageContext.request.requestURL, pageContext.request.requestURI, pageContext.request.contextPath)}/">
Etcetera. Peek around in the aforelinked HttpServletRequest and HttpSession javadoc to learn about all those getter methods. Some of them may be useful in JSP/EL as well.
回答2:
To add to @BalusC's excellent answer, the PageContext that you are getting might not be limited to what you see in the specification.
For example, Lucee is a JSP Servlet that adds many features to the interface and abstract classes. By getting a reference to the PageContext you can gain access to a lot of information that is otherwise unavailable.
回答3:
All 11 implicit EL variables are defined as Map, except the pageContext variable. pageContext variable provides convenient methods for accessing request/response/session attributes or forwarding the request.
来源：https://stackoverflow.com/questions/3735405/why-there-is-a-need-of-pagecontext-in-jsp