How to reference constants in EL?
How do you reference an constants with EL on a JSP page?
I have an interface Addresses with a constant named URL. I know I can reference it with a scriplet by going: <%=Addresses.URL%>, but how do I do this using EL?
Answer
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.