How to check for session in JSP EL?

问题:

How do you check if a session exists for the request in EL? I'm trying something like:

<c:if test="${pageContext.request.session != null}"> ... </c:if>

but it seems like it's never null.

回答1:

It's indeed never null. The session is always present in JSP EL, unless you add

<%@page session="false" %>

to top of JSP. You could then check for the session as follows (EL 2.2 only!):

<c:if test="${pageContext.request.getSession(false) != null}">
    <p>The session has been created before.</p>
</c:if>

I'm not sure what's the concrete functional requirement is. If you'd like to check if the session is new or has already been created, use HttpSession#isNew() instead.

<c:if test="${not pageContext.session['new']}">
    <p>You've already visited this site before.</p>
</c:if>
<c:if test="${pageContext.session['new']}">
    <p>You've just started the session with this request!</p>
</c:if>

(the brace notations for new are mandatory because new is a reserved literal in Java language)

Of if you're relying on a specific session attribute, such as the logged-in user which is been set as

session.setAttribute("user", user);

then you should rather be intercepting on that instead:

<c:if test="${not empty user}">
    <p>You're still logged in.</p>
</c:if>
<c:if test="${empty user}">
    <p>You're not logged in!</p>
</c:if>

回答2:

Seems to works with:

<c:if test="${fn:length(sessionScope) > 0}">

I wonder if there's a better way, since this requires that I have session attributes (I always do, but it's not really clean)?

回答3:

In J2EE there will always be a session object when a user visits a site.

What is a Session ? A session is pretty much what it sounds, when a user makes a page request to the server, the server creates a temporary session to identify that user. So when that same user goes to another page on that site, the server identifies that user. So a session is a small and temporary unique connection between a server and the user enabling it to identify that user across multiple page requests or visits to that site.

So basically if your hitting a page you have a session because your using JSP, which eventually gets converted to servlets.

http://www.stardeveloper.com/articles/display.html?article=2001062001&page=1