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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?
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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"); } }); } } 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.
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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"/> If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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