Why there is a need of pageContext in JSP?

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?

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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:

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Refreshing page when session times out:

<meta http-equiv="refresh" content="${pageContext.session.maxInactiveInterval}">

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Passing session ID to an Applet (so that it can communicate with servlet in the same session):

<param name="jsessionid" value="${pageContext.session.id}">

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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

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Displaying user IP:

Your IP is: ${pageContext.request.remoteAddr}

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Making links domain-relative without hardcoding current context path:

<a href="${pageContext.request.contextPath}/login">login</a>

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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)}/">

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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. Sure you can do much more than that