Using a Java constant in EL?

[D. Ogranos]

Is there a way to access java constants in an EL expression? I want to access a data bean in the session context. In my servlet I store the bean as session attribute "myitems" and use the constant KEY (defined in class Constants) to avoid hardcoding the name in the servlet. In the jsp, using the old way I could then do something like

<%@ page import="Constants" %>

<% Databean myitems = (Databean) session.getAttribute(Constants.KEY); %>

where Constants.KEY has the value "myitems". But this doesn't seem to work with EL:

<%@ page import="Constants" %>

<jsp:useBean id="${Constants.KEY}" scope="session" type="Databean"/>

This produces a jsp compilation error. Is there a way to use the value from the Constants file instead of hardcoding the id of the bean?

[Ramkumar Subburaj]

what is the error message you are getting?

[D. Ogranos]

17.04.2009 10:49:54 org.apache.catalina.core.ApplicationContext log

INFO: login: Exception: Unable to compile class for JSP:

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error on token "$", = expected after this token
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error on token(s), misplaced construct(s)
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error, insert "AssignmentOperator Expression" to complete Assignment
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error, insert ";" to complete BlockStatements
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error on token "}", delete this token
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error, insert ") Statement" to complete IfStatement
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

An error occurred at line: 9 in the jsp file: /jsps/include.jsp
Syntax error on tokens, delete these tokens
9: <jsp:useBean id="${SessionData.MESSAGES_KEY}" scope="session" type="java.util.Map"/>

So apparently this construct isn't allowed here. Of course I can just type id="myitems" (or whatever the actual value of MESSAGES_KEY is) but I'd rather use the same constant in my servlet and jsps.

[Tim Holloway]

In this particular case (id attribute), no, I don't think you can use EL. It's partly because "id=" has a mystical significance in XML to begin with, but also because JSP tag constructs are defined as either static (constant) or dynamic (EL). And I'd expect id to be static, since, like I said, it has certain restrictions.

[chris webster]

I'm new to JSP etc, but it looks like part of the problem is that you can't use EL to access static (class) variables directly. For example, in my JSP:

<p> Scriptlet approach: <%= Constants.KEY %> </p>  *THIS WORKS*
<p> Direct EL reference = ${Constants.KEY} </p> *THIS DOES NOT WORK*

In the HeadFirst book (my sole source of JSP wisdom right now!), it says the thing on the left of the EL dot operator, i.e. Constants in this case, must be a bean or a Map. If you're using Constants just to provide a bunch of static (class) variables without instantiating it, then I guess you don't have a Bean/Map instance so EL doesn't like it.

Seeing as how these are constants, you could define them as parameters in your XML instead, then let EL access them via the implicit "initParam" Map instead. For example, in web.xml:

<context-param>
  <param-name>myParam</param-name>
  <param-value>some value</param-value>
</context-param>

In the JSP you can then access the value of the parameter with EL:

<p>My parameter value is ${initParam.myParam}</p>

But as Tim says, it looks like you still can't put this into your <jsp:useBean> "id" attribute.

As a JSP newbie, can I ask if it is common to mix beans into JSP views like this? My limited understanding of MVC would cause me to instantiate the bean somewhere else e.g. in a servlet, then pass it into the JSP as an attribute, rather than creating it in the JSP view itself. But what do I know, right?

[Tim Holloway]

chris webster wrote:

As a JSP newbie, can I ask if it is common to mix beans into JSP views like this? My limited understanding of MVC would cause me to instantiate the bean somewhere else e.g. in a servlet, then pass it into the JSP as an attribute, rather than creating it in the JSP view itself. But what do I know, right?

it's common, alas, but it doesn't scale well. Which is why major JEE projects are usually done with a MVC framework like Struts or JSF. The useBean construct doesn't actually result in a MVC-like architecture - the model, view, and controller are all basically the same object (the JSP with an embedded bean). The results can get really ugly on complex projects. I had to rewrite one that did that, and it's virtually unmaintainable in its original form. One JSP "page" printed out as its source file ran 42 pages, half being HTML and half being embedded Java code.

EL is based on the standard JavaBean concept of a class instance with accessor (get/set) methods. So yes, there's a problem with accessing a static property in EL. You can get around that one by making member accessors that refer to the class property, however.

The restriction against EL for the id attribute, however, is another matter, and I don't think there's a way around that.

[chris webster]

@Tim - thanks for the helpful explanation!

[D. Ogranos]

Ok, thanks for the answers!

chris webster wrote:As a JSP newbie, can I ask if it is common to mix beans into JSP views like this? My limited understanding of MVC would cause me to instantiate the bean somewhere else e.g. in a servlet, then pass it into the JSP as an attribute, rather than creating it in the JSP view itself. But what do I know, right?

Actually, jsp:useBean doesn't instantiate the bean if you use the type attribute. It only tells the JSP to look in the given context for an (existing) bean of that type with the given id. My ide (IntelliJ) tells me when there are undeclared variables or references to beans in the JSP (for example in EL expressions), and with jsp:useBean they become known. Maybe there is another way to "declare" objects used in EL expressions?

[Bear Bibeault]

I wrote this article on this exact subject.

[D. Ogranos]

Thanks, that looks awesome!