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5. Internationalization with Messages

§Internationalization with Messages

§Specifying languages supported by your application

You specify languages for your application using language tags, specially formatted strings that identify a specific language. Language tags can specify simple languages, such as “en” for English, a specific regional dialect of a language (such as “en-AU” for English as used in Australia), a language and a script (such as “az-Latn” for Azerbaijani written in Latin script), or a combination of several of these (such as “zh-cmn-Hans-CN” for Chinese, Mandarin, Simplified script, as used in China).

To start you need to specify the languages supported by your application in the conf/application.conf file:

play.i18n.langs = [ "en", "en-US", "fr" ]

These language tags will be used to create play.i18n.Lang instances. To access the languages supported by your application, you can inject a play.i18n.Langs component into your class:

import play.i18n.Langs;
import play.i18n.Messages;
import play.i18n.MessagesApi;

import javax.inject.Inject;
import java.util.Collection;
import java.util.Collections;
import java.util.Locale;

public class MyService {
    private final Langs langs;

    @Inject
    public MyService(Langs langs) {
        this.langs = langs;
    }
}

An individual play.i18n.Lang can be converted to a java.util.Locale object by using lang.toLocale() method:

java.util.Locale locale = lang.toLocale();

§Externalizing messages

You can externalize messages in the conf/messages.xxx files.

The default conf/messages file matches all languages. You can specify additional language messages files, such as conf/messages.fr or conf/messages.en-US.

Messages are available through the MessagesApi instance, which can be added via injection. You can then retrieve messages using the play.i18n.Messages object:

class SomeService {
    private final play.i18n.MessagesApi messagesApi;

    @Inject
    SomeService(MessagesApi messagesApi) {
        this.messagesApi = messagesApi;
    }

    public void message() {
        Collection<Lang> candidates = Collections.singletonList(new Lang(Locale.US));
        Messages messages = messagesApi.preferred(candidates);
        String message = messages.at("home.title");

    }
}

The current language is available via the lang field in the current Context. If there’s no current Context then the default language is used. The Context’s lang value is determined by:

1. Seeing if the Context’s lang field has been set explicitly.
2. Looking for a PLAY_LANG cookie in the request.
3. Looking at the Accept-Language headers of the request.
4. Using the application’s default language.

You can change the Context’s lang field by calling changeLang or setTransientLang. The changeLang method will change the field and also set a PLAY_LANG cookie for future requests. The setTransientLang will set the field for the current request, but doesn’t set a cookie. See below for example usage.

If you don’t want to use the current language you can specify a message’s language explicitly:

String title = messagesApi.get(Lang.forCode("fr"), "hello");

Note that you should inject the play.i18n.MessagesApi class, using dependency injection. For example, using Guice you would do the following:

public class MyClass {

    private final play.i18n.MessagesApi messagesApi;

    @Inject
    public MyClass(MessagesApi messagesApi) {
        this.messagesApi = messagesApi;
    }
}

§Use in Controllers

If you are in a Controller, you get the Messages instance through Http.Context, using Http.Context.current().messages():

public Result index() {
    Messages messages = Http.Context.current().messages();
    String hello = messages.at("hello");
    return ok(hellotemplate.render());
}

Please note that because the Http.Context depends on a thread local variable, if you are referencing Http.Context.current().messages() from inside a CompletionStage block that may be in a different thread, you may need to use HttpExecutionContext.current() to make the HTTP context available to the thread. Please see Handling asynchronous results for more details.

To use Messages as part of form processing, please see Handling form submission.

§Use in templates

Once you have the Messages object, you can pass it into the template:

@(messages: play.i18n.Messages)
@messages.at("hello")

You can also use the Scala Messages object from within templates. The Scala Messages object has a shorter form that’s equivalent to messages.at which many people find useful.

If you use the Scala Messages object remember not to import the Java play.i18n.Messages class or they will conflict!

@Messages("hello")

Localized templates that use messages.at or the Scala Messages object are invoked like normal:

public Result index() {
    return ok(hellotemplate.render()); // "hello"
}

If you want to change the language for the template you can call changeLang on the current Context. This will change the language for the current request, and set the language into a cookie so that the language is changed for future requests:

public Result index() {
    ctx().changeLang("fr");
    return ok(hellotemplate.render()); // "bonjour"
}

If you just want to change the language, but only for the current request and not for future requests, call setTransientLang:

public Result index() {
    ctx().setTransientLang("en-US");
    return ok(hellotemplate.render()); // "howdy"
}

§Formatting messages

Messages are formatted using the java.text.MessageFormat library. For example, if you have defined a message like this:

files.summary=The disk {1} contains {0} file(s).

You can then specify parameters as:

Messages.get("files.summary", d.files.length, d.name)

§Notes on apostrophes

Since Messages uses java.text.MessageFormat, please be aware that single quotes are used as a meta-character for escaping parameter substitutions.

For example, if you have the following messages defined:

info.error=You aren''t logged in!

example.formatting=When using MessageFormat, '''{0}''' is replaced with the first parameter.

you should expect the following results:

String errorMessage = messages.at("info.error");
Boolean areEqual = errorMessage.equals("You aren't logged in!");

String errorMessage = messages.at("example.formatting");
Boolean areEqual = errorMessage.equals("When using MessageFormat, '{0}' is replaced with the first parameter.");

§Retrieving supported languages from an HTTP request

You can retrieve a specific HTTP request’s supported languages:

public Result index() {
    List<Lang> langs = request().acceptLanguages();
    String codes = langs.stream().map(Lang::code).collect(joining(","));
    return ok(codes);
}

§Using explicit MessagesApi

The default implementation of MessagesApi is backed by a DefaultMessagesApi instance which is a Scala API. But you can instantiate a DefaultMessagesApi and manually inject it into the MessagesApi like:

private MessagesApi explicitMessagesApi() {
    return new play.i18n.MessagesApi(
            new play.api.i18n.DefaultMessagesApi(
                    Collections.singletonMap(Lang.defaultLang().code(), Collections.singletonMap("foo", "bar")),
                    new play.api.i18n.DefaultLangs().asJava())
    );
}

If you need a MessagesApi instance for unit testing, you can also use play.test.Helpers.stubMessagesApi(). See Testing your application for more details.

Next: Dependency Injection