How to reference constants in EL?
EL 3.0 or newer
If you're already on Java EE 7 / EL 3.0, then the @page import will also import class constants in EL scope.
<%@ page import="com.example.YourConstants" %>
This will under the covers be imported via ImportHandler#importClass() and be available as ${YourConstants.FOO}.
Note that all java.lang.* classes are already implicitly imported and available like so ${Boolean.TRUE} and ${Integer.MAX_VALUE}. This only requires a more recent Java EE 7 container server as early versions had bugs in this. E.g. GlassFish 4.0 and Tomcat 8.0.0-1x fails, but GlassFish 4.1+ and Tomcat 8.0.2x+ works. And you need to make absolutely sure that your web.xml is declared conform the latest servlet version supported by the server. Thus with a web.xml which is declared conform Servlet 2.5 or older, none of the Servlet 3.0+ features will work.
Also note that this facility is only available in JSP and not in Facelets. In case of JSF+Facelets, your best bet is using OmniFaces <o:importConstants> as below:
<o:importConstants type="com.example.YourConstants" />
Or adding an EL context listener which calls ImportHandler#importClass() as below:
@ManagedBean(eager=true)
@ApplicationScoped
public class Config {
@PostConstruct
public void init() {
FacesContext.getCurrentInstance().getApplication().addELContextListener(new ELContextListener() {
@Override
public void contextCreated(ELContextEvent event) {
event.getELContext().getImportHandler().importClass("com.example.YourConstants");
}
});
}
}
EL 2.2 or older
This is not possible in EL 2.2 and older. There are several alternatives:
Put them in a
Map<String, Object>which you put in the application scope. In EL, map values are accessible the usual Javabean way by${map.key}or${map['key.with.dots']}.-
Use
<un:useConstants>of the Unstandard taglib (maven2 repo here):<%@ taglib uri="http://jakarta.apache.org/taglibs/unstandard-1.0" prefix="un" %> <un:useConstants className="com.example.YourConstants" var="constants" />This way they are accessible the usual Javabean way by
${constants.FOO}. -
Use Javaranch's CCC
<ccc:constantsMap>as desribed somewhere at the bottom of this article.<%@ taglib uri="http://bibeault.org/tld/ccc" prefix="ccc" %> <ccc:constantsMap className="com.example.YourConstants" var="constants" />This way they are accessible the usual Javabean way by
${constants.FOO}as well. -
If you're using JSF2, then you could use
<o:importConstants>of OmniFaces.<html ... xmlns:o="http://omnifaces.org/ui"> <o:importConstants type="com.example.YourConstants" />This way they are accessible the usual Javabean way by
#{YourConstants.FOO}as well. Create a wrapper class which returns them through Javabean-style getter methods.
Create a custom EL resolver which first scans the presence of a constant and if absent, then delegate to the default resolver, otherwise returns the constant value instead.
The following does not apply to EL in general, but instead to SpEL (Spring EL) only (tested with 3.2.2.RELEASE on Tomcat 7). I think it is worth mentioning it here in case someone searches for JSP and EL (but uses JSP with Spring).
<%@ taglib prefix="spring" uri="http://www.springframework.org/tags"%>
<spring:eval var="constant" expression="T(com.example.Constants).CONSTANT"/>
You usually place these kinds of constants in a Configuration object (which has getters and setters) in the servlet context, and access them with ${applicationScope.config.url}
You can't. It follows the Java Bean convention. So you must have a getter for it.
I'm defining a constant in my jsp right at the beginning:
<%final String URI = "http://www.example.com/";%>
I include the core taglib in my JSP:
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
Then, I make the constant available to EL by following statement:
<c:set var="URI" value="<%=URI%>"></c:set>
Now, I can use it later. Here an example, where the value is just written as HTML comment for debugging purposes:
<!-- ${URI} -->
With your constant class, you can just import your class and assign the constants to local variables. I know that my answer is a sort of quick hack, but the question also bumps up when one wants to define constants directly in the JSP.