小开

You can't. It follows the Java Bean convention. So you must have a getter for it.

小开

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}

小开

最佳答案

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.

小开

Yes, you can. You need a custom tag (if you can't find it somewhere else). I've done this:

package something;


import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.Map;
import java.util.TreeMap;


import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.TagSupport;


import org.apache.taglibs.standard.tag.el.core.ExpressionUtil;


/**
* Get all class constants (statics) and place into Map so they can be accessed
* from EL.
* @author Tim.sabin
*/
public class ConstMapTag extends TagSupport {
public static final long serialVersionUID = 0x2ed23c0f306L;


private String path = "";
private String var = "";


public void setPath (String path) throws JspException {
this.path = (String)ExpressionUtil.evalNotNull ("constMap", "path",
path, String.class, this, pageContext);
}


public void setVar (String var) throws JspException {
this.var = (String)ExpressionUtil.evalNotNull ("constMap", "var",
var, String.class, this, pageContext);
}


public int doStartTag () throws JspException {
// Use Reflection to look up the desired field.
try {
Class<?> clazz = null;
try {
clazz = Class.forName (path);
} catch (ClassNotFoundException ex) {
throw new JspException ("Class " + path + " not found.");
}
Field [] flds = clazz.getDeclaredFields ();
// Go through all the fields, and put static ones in a Map.
Map<String, Object> constMap = new TreeMap<String, Object> ();
for (int i = 0; i < flds.length; i++) {
// Check to see if this is public static final. If not, it's not a constant.
int mods = flds [i].getModifiers ();
if (!Modifier.isFinal (mods) || !Modifier.isStatic (mods) ||
!Modifier.isPublic (mods)) {
continue;
}
Object val = null;
try {
val = flds [i].get (null);    // null for static fields.
} catch (Exception ex) {
System.out.println ("Problem getting value of " + flds [i].getName ());
continue;
}
// flds [i].get () automatically wraps primitives.
// Place the constant into the Map.
constMap.put (flds [i].getName (), val);
}
// Export the Map as a Page variable.
pageContext.setAttribute (var, constMap);
} catch (Exception ex) {
if (!(ex instanceof JspException)) {
throw new JspException ("Could not process constants from class " + path);
} else {
throw (JspException)ex;
}
}
return SKIP_BODY;
}
}

and the tag is called:

<yourLib:constMap path="path.to.your.constantClass" var="consts" />

All public static final variables will be put into a Map indexed by their Java name, so if

public static final int MY_FIFTEEN = 15;

then the tag will wrap this in an Integer and you can reference it in a JSP:

<c:if test="${consts['MY_FIFTEEN'] eq 15}">

and you don't have to write getters!

小开

Static properties aren't accessible in EL. The workaround I use is to create a non-static variable which assigns itself to the static value.

public final static String MANAGER_ROLE = 'manager';
public String manager_role = MANAGER_ROLE;

I use lombok to generate the getter and setter so that's pretty well it. Your EL looks like this:

${bean.manager_role}

Full code at https://rogerkeays.com/access-java-static-methods-and-constants-from-el

小开

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"/>

小开

I implemented like:

public interface Constants{
Integer PAGE_SIZE = 20;
}

-

public class JspConstants extends HashMap<String, String> {


public JspConstants() {
Class c = Constants.class;
Field[] fields = c.getDeclaredFields();
for(Field field : fields) {
int modifier = field.getModifiers();
if(Modifier.isPublic(modifier) && Modifier.isStatic(modifier) && Modifier.isFinal(modifier)) {
try {
Object o = field.get(null);
put(field.getName(), o != null ? o.toString() : null);
} catch(IllegalAccessException ignored) {
}
}
}
}


@Override
public String get(Object key) {
String result = super.get(key);
if(StringUtils.isEmpty(result)) {
throw new IllegalArgumentException("Check key! The key is wrong, no such constant!");
}
return result;
}
}

Next step put instance of this class into servlerContext

public class ApplicationInitializer implements ServletContextListener {




@Override
public void contextInitialized(ServletContextEvent sce) {
sce.getServletContext().setAttribute("Constants", new JspConstants());
}


@Override
public void contextDestroyed(ServletContextEvent sce) {
}
}

add listener to web.xml

<listener>
<listener-class>com.example.ApplicationInitializer</listener-class>
</listener>

access in jsp

${Constants.PAGE_SIZE}

小开

You can. Try in follow way

 #{T(com.example.Addresses).URL}

Tested on TomCat 7 and java6

小开

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.

小开

Even knowing its a little late, and even knowing this is a little hack - i used the following solution to achieve the desired result. If you are a lover of Java-Naming-Conventions, my advice is to stop reading here...

Having a class like this, defining Constants, grouped by empty classes to create kind of a hierarchy:

public class PERMISSION{
public static class PAGE{
public static final Long SEE = 1L;
public static final Long EDIT = 2L;
public static final Long DELETE = 4L;
...
}
}

can be used from within java as PERMISSION.PAGE.SEE to retrieve the value 1L

To achieve a simliar access-possibility from within EL-Expressions, I did this: (If there is a coding-god - he hopefully might forgive me :D )

@Named(value="PERMISSION")
public class PERMISSION{
public static class PAGE{
public static final Long SEE = 1L;
public static final Long EDIT = 2L;
public static final Long DELETE = 4L;
...


//EL Wrapper
public Long getSEE(){
return PAGE.SEE;
}


public Long getEDIT(){
return PAGE.EDIT;
}


public Long getDELETE(){
return PAGE.DELETE;
}
}


//EL-Wrapper
public PAGE getPAGE() {
return new PAGE();
}
}

finally, the EL-Expression to access the very same Long becomes: #{PERMISSION.PAGE.SEE} - equality for Java and EL-Access. I know this is out of any convention, but it works perfectly fine.

小开

@Bozho already provided a great answer

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}

However, I feel an example is needed so it brings a bit more clarity and spare someone's time

@Component
public Configuration implements ServletContextAware {
private String addressURL = Addresses.URL;


// Declare other properties if you need as also add corresponding
// getters and setters


public String getAddressURL() {
return addressURL;
}


public void setServletContext(ServletContext servletContext) {
servletContext.setAttribute("config", this);
}
}

小开

There is a workaround that is not exactly what you want, but lets you active almost the same with touching scriptlets in a quite minimal way. You can use scriptlet to put value into a JSTL variable and use clean JSTL code later in the page.

<%@ taglib prefix="c"       uri="http://java.sun.com/jsp/jstl/core" %>
<%@ page import="com.whichever.namespace.Addresses" %>
<c:set var="ourUrl" value="<%=Addresses.URL%>"/>
<c:if test='${"http://www.google.com" eq ourUrl}'>
Google is our URL!
</c:if>

如何在 EL 中引用常数？

EL 3.0 or newer

EL 2.2 or older