i18n Locale

  1. Overview
  2. Client Properties and the GWT Compilation Process
  3. Adding Locale Choices to a Module
  4. The Default Locale
  5. Specifying the Locale to Load
  6. Runtime Locales
  7. Creating a New Property Provider
  8. Programmatic Access to Locale Information
  9. Server/Generator Manipulation of Locales


GWT represents locale as a client property whose value can be set either using a meta tag embedded in the host page or in the query string of the host page’s URL. Rather than being supplied by GWT, the set of possible values for the locale client property is entirely a function of your module configuration.

Client Properties and the GWT Compilation Process

Client properties are key/value pairs that can be used to configure GWT modules. User agent, for example, is represented by a client property. Each client property can have any number of values, but all of the values must be enumerable when the GWT compiler runs. GWT modules can define and extend the set of available client properties along with the potential values each property might assume when loaded in an end user’s browser using the <extend-property> directive. At compile time, the GWT compiler determines all the possible permutations of a module’s client properties, from which it produces multiple compilations. Each compilation is optimized for a different set of client properties and is recorded into a file ending with the suffix .cache.html.

In deployment, the end-user’s browser only needs one particular compilation, which is determined by mapping the end user’s client properties onto the available compiled permutations. Thus, only the exact code required by the end user is downloaded, no more. By making locale a client property, the standard startup process in <module>.nocache.js chooses the appropriate localized version of an application, providing ease of use, optimized performance, and minimum script size. See the Knowledge Base for more information about the logic of the <modulename>.nocache.js file.

Adding Locale Choices to a Module

In any real-world application, you will define at least one locale in addition to the default locale. “Adding a locale” means extending the set of values of the locale client property using the <extend-property> element in your module XML. For example, the following module adds multiple locale values:

  <inherits name="com.google.gwt.user.User"/>
  <inherits name="com.google.gwt.i18n.I18N"/>
  <!-- French language, independent of country -->
  <extend-property name="locale" values="fr"/>

  <!-- French in France -->
  <extend-property name="locale" values="fr_FR"/>

  <!-- French in Canada -->
  <extend-property name="locale" values="fr_CA"/>
  <!-- English language, independent of country -->
  <extend-property name="locale" values="en"/>

The Default Locale

The com.google.gwt.i18n.I18N module defines only one locale by default, called default. This default locale is used when the locale client property goes unspecified in deployment. The default locale is used internally as a last-resort match between a Localizable interface and a localized resource or class.

In general, you should avoid running the app in the default locale — many things will produce surprising results. For example, only a small set of currencies will be supported, resulting in errors for applications that make use of other currencies, and no plural forms will be supported (since the language isn’t known). If you really want to allow the application to continue running when the user requests an unsupported locale, you are probably better off choosing some real language as a default, such as en. You can set what value is used for the default by including the following in your module XML:

<extend-property name="locale" values="en,es,de,fr,it" /> <!-- 1. full list of supported locales --> 
<set-property name="locale" value="en,es,de,fr,it" />     <!-- 2. remove the 'default' locale permutation -->
<set-property-fallback name="locale" value="en" />        <!-- 3. set the fallback locale, in replacement for the default 'default' locale -->

Specifying the Locale to Load

The locale client property can be specified using either a meta tag or as part of the query string in the host page’s URL. If both are specified, the query string takes precedence. To specify the locale client property using a meta tag in the host page, embed a meta tag for gwt:property as follows:

<meta name="gwt:property" content="locale=x_Y">

For example, the following host HTML page sets the locale to “ja_JP”:

    <meta name="gwt:property" content="locale=ja_JP">
    <!-- Load the GWT compiled module code                           -->
    <script src="com.google.gwt.examples.i18n.ColorNameLookupExample.nocache.js " />

To specify the locale client property using a query string, specify a value for the name locale. For example,


Tip: The preferred time to explicitly set locale is to do so before your GWT module is invoked. You can change the locale from within your GWT module by adding or changing the locale query string in the current URL and reloading the page. Keep in mind that after reloading the page, your module will restart.

Runtime Locales

For cases where the translated values are the same, but you still want country-specific details, you can use runtime locales to reduce the number of compiled permutations, but still get country-specific details like the default currency, number/date formatting rules, etc.

As an example, you might have one set of translations for all of Spanish as spoken in Latin America (es_419), yet allow users to choose a country-specific locale such as Argentinian Spanish (es_AR).

The easy way to use runtime locales is simply to add:

<inherits name="com.google.gwt.i18n.CldrLocales"/>

to your module XML file, and all locales that GWT knows about that inherit from your compile-time locale will be automatically included. You can see the result in the Showcase sample application.


  • All the tables for all included runtime locales are included in the each appropriate compiled permutation, so this can increase download size.
  • The tables for non-obvious locale inheritance and aliases are too large to be included in the selection script, so inheritance won’t work properly in all cases. This means you either need to specifically control the set of possible locales, such as in the locale selector in the Showcase sample application, or have the server choose the locale using the proper inheritance tables (GwtLocaleFactoryImpl will be helpful here, and you will need a way to get the set of locales your application was built with).
  • Only currency data, number format, and date/time formats are affected by runtime locales currently — everything else will only use the compile-time locale from the locale deferred binding property.

Creating a New Property Provider

If you are embedding your module into an existing application, there may be another way of determining locale that does not lend itself to using the <meta> tag or specifying locale= as a query string. In this case, you could write your own property provider.

A property provider is specified in the module XML file as a JavaScript fragment that will return the value for the named property at runtime. In this case, you would want to define the locale property using a property provider. To see examples of <property-provider> definitions in action, see the files I18N.gwt.xml and UserAgent.gwt.xml in the GWT source code.

Programmatic Access to Locale Information

To get information about the current locale or the available set of locales, see the LocaleInfo class. For example:

  • To check if the current locale is a Right-to-Left locale:
if (LocaleInfo.getCurrentLocale().isRTL()) {
  • To get a list of supported locales, such as for a locale selector:
for (String localeName : LocaleInfo.getAvailableLocaleNames()) {
  String displayName = LocaleInfo.getLocaleNativeDisplayName(localeName);

Server/Generator Manipulation of Locales

GWT provides two classes to manipulate locales, which fully support aliases and locale inheritance.

  • GwtLocale represents a GWT locale, and supports converting to canonical form, producing search lists for locale inheritance and aliases, and provides accessors to the components of a locale.

  • GwtLocaleFactory provides a way of creating new GwtLocale objects from locale names or their components (useful for converting from a java.util.Locale object).

  • LocaleUtils provides easy access to GWT’s locale infrastructure for a generator.

    • Get a GwtLocaleFactory instance:
GwtLocaleFactory factory = LocaleUtils.getLocaleFactory();
  • Get all locales for this compile, including runtime locales:
Set<GwtLocale> locales = localeUtils.getAllLocales();