Spring - get all resolvable message keys

I have a web application which uses Spring MVC. I would like to have my interface consist of just a single page which retrieves all data dynamically as JSON via AJAX. My problem is with internationalization. When I render content in jsps, I can use JSTL tags to resolve my keys (super-easy with Spring MVC):

<fmt:message key="name"/>: ${name}
<fmt:message key="title"/>: ${title}
<fmt:message key="group"/>: ${group}

When properly configured, it renders in finnish locale as

Nimi: yay
Otsikko: hoopla
Ryhmä: doo

Now, when I use json, I have only this coming in from the server:

{
 name: "yay", 
 title: "hoopla",
 group: "doo"
}

There's no keynames! But I have to provide them somehow. I considered changing the keynames to their开发者_Python百科 localized forms or adding the localized keynames to json output (eg. name_localized="Nimi") but both of these options feel like bad practice. I'm using jackson json to automatically parse my domain objects into json and I like the encapsulation it provides.

The only feasible solution I came up with is this: dynamically create a javascript file with the localized keynames as variables.

<script type="text/javascript">
var name="Nimi";
var title="Otsikko";
var group="Ryhmä";
</script>

Once I have this loaded, I now have all the information in my javascript to handle the json! But there's a gotcha: my list of field names is dynamic. So actually static rendering in jsp would look like this:

<c:forEach var="field" values="${fields}">
 <fmt:message key="${field.key}"/>: ${field.value}
</c:forEach>

I need to find all of the messages specified in my messages.properties. Spring MessageSource interface only supports retrieving messages by key. How can I get a list of keynames in my JSP which renders the localized javascript variables?

Well, I "solved" my problem by extending ResourceBundleMessageSource.

package org.springframework.context.support;

import java.util.Locale;
import java.util.ResourceBundle;
import java.util.Set;

public class ExposedResourceBundleMessageSource extends
        ResourceBundleMessageSource {
    public Set<String> getKeys(String basename, Locale locale) {
        ResourceBundle bundle = getResourceBundle(basename, locale);
        return bundle.keySet();
    }
}

Now I have access to the keys, but I have to do an ugly cast in my controller, in addition to having to specify the message source basename. Ugh, that's a lot of coupling.

Set<String> keys = 
  ((ExposedResourceBundleMessageSource)
  messageSource).getKeys("messages", locale);

I have solve this issue.

public class ExposedResourceBundleMessageSource extends
        ResourceBundleMessageSource {
    public static final String WHOLE = "whole";
    private Set baseNames;
    private Map> cachedData = new HashMap>();

    public Set getKeys(String baseName, Locale locale) {
        ResourceBundle bundle = getResourceBundle(baseName, locale);
        return bundle.keySet();
    }

    public Map getKeyValues(String basename, Locale locale) {
        String cacheKey = basename + locale.getCountry();
        if (cachedData.containsKey(cacheKey)) {
            return cachedData.get(cacheKey);
        }
        ResourceBundle bundle = getResourceBundle(basename, locale);
        TreeMap treeMap = new TreeMap();
        for (String key : bundle.keySet()) {
            treeMap.put(key, getMessage(key, null, locale));
        }
        cachedData.put(cacheKey, treeMap);
        return treeMap;
    }

    public Map getKeyValues(Locale locale) {
        String cacheKey = WHOLE + locale.getCountry();
        if (cachedData.containsKey(cacheKey)) {
            return cachedData.get(cacheKey);
        }
        TreeMap treeMap = new TreeMap();
        for (String baseName : baseNames) {
            treeMap.putAll(getKeyValues(baseName, locale));
        }
        cachedData.put(cacheKey, treeMap);
        return treeMap;
    }

    public void setBasenames(String[] basenames) {
        baseNames = CollectionUtils.arrayAsSet(basenames);
        super.setBasenames(basenames);
    }


}

My solution:

1. use spring <util:properties />

   <util:properties id="message" location="classpath:messages.properties" />
   

2. Add an interceptor, with 'message' set to request attributes

   request.setAttribute("msgKeys", RequestContextUtils.getWebApplicationContext(request).getBean("message"));
   

3. In JSP, use the msgKeys to retrieve message through <fmt:message /> tag

   var msg = {<c:forEach items="${msgKeys}" var="m" varStatus="s">
       <c:set var="key" value="${fn:substringBefore(m,'=')}"/>
       "${fn:substringAfter(key)}":"<fmt:message key="${key}"/>",
   </c:forEach>};
   

(Of course you need to further escape the output to match js string.)

So, for properties

app.name=Application Name
app.title=Some title

would output

var msg = { "name":"Application Name", "title":"Some title" };

The good thing about that is that you can do some more logic in c:forEach loop.

If you do not want to extend the ResourceBundleMessageSource class, you can do it like this:

@Component
class FrontendMessageLoader {
  import scala.collection.JavaConverters._

  @Resource(name = "messageSource")
  var properties: ResourceBundleMessageSource = _

  @PostConstruct def init(): Unit = {
    val locale = new Locale("en", "US")
    val baseNames: mutable.Set[String] = properties.getBasenameSet.asScala
    val messageMap: Map[String, String] =
      baseNames.flatMap { baseName =>
        readMessages(baseName, locale)
      }.toMap
    }
  }

  private def readMessages(baseName: String, locale: Locale) = {
    val bundle = ResourceBundle.getBundle(baseName, locale)
    bundle.getKeys.asScala.map(key => key -> bundle.getString(key))
  }
}

This is Scala but you get the idea and you can easily convert it to Java if needed. The point is that you get the base-names (which are the property files) and then load those files and then you can get the keys. I am passing also locale, but there are multiple overloaded versions for ResourceBundle.getBundle and you can choose the one that matches your needs.

I don't think it's bad practice to return the localized values. You could return something like this from your controller (so you just have to resolve the keys there):

{
   name: {label: "Nimi", value: "yay"},
   title: {label: "Ostikko", value: "hoopla"},
   group: {label: "Rhymä", value: "doo"},
}

You can create an intermediate object (the easiest would be Map<String, Map<String, String>>) that holds these values and jackson will serialize it into JSON. I think it's better than the dynamic Javascript-file option. This solution also ends up doing what you were trying to do with the dynamic Javascript-file anyway, that is, associating the key with its resolved value.