How to reference constants in EL?
constants, el, jsp
Solution
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.
Problem
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?