How do you configure message sources for internationalization in Spring?

Table of Contents

• Introduction
• Configuring Message Sources for Internationalization in Spring
  • 1. Defining a MessageSource Bean
    • Example:
  • 2. Creating Message Property Files
    • Example of messages.properties (default):
    • Example of messages_en.properties (English):
    • Example of messages_fr.properties (French):
  • 3. Resolving Messages in Your Application
    • Example in Controller:
  • 4. Using Messages in Views (Thymeleaf)
    • Example in Thymeleaf Template:
  • 5. Configuring Locale Resolver for Dynamic Locale Switching
    • Example of SessionLocaleResolver:
  • 6. Using Locale in Spring Boot Configuration
    • Example (application.properties):
• Practical Examples
  • Example 1: Switching Languages via URL Parameter
    • Example (Thymeleaf):
  • Example 2: Handling Form Validation Messages
    • Example:
• Conclusion

Introduction

Internationalization (i18n) is a key feature of modern web applications, allowing users to interact with your application in their preferred language. In Spring applications, message sources play a vital role in this process by storing and resolving locale-specific messages. A MessageSource bean is used to retrieve messages from properties files based on the user’s locale. In this guide, we will explore how to configure message sources in Spring, how to set up message properties for multiple languages, and how to use these messages in your Spring application.

Configuring Message Sources for Internationalization in Spring

1. Defining a MessageSource Bean

The MessageSource interface in Spring is responsible for resolving messages from property files based on the current locale. You can configure a MessageSource bean in your Spring configuration to provide these messages. Typically, you define the base name of the properties files, set the default encoding, and configure the file locations where messages will be loaded from.

Example:

In this example, messages.properties is the default message file, and it can be extended with language-specific files like messages_en.properties for English, messages_fr.properties for French, and so on.

2. Creating Message Property Files

Message properties are typically stored in .properties files, where each key-value pair represents a message. The base file should be named messages.properties (or any other name you choose) and should be placed in the src/main/resources folder.

Example of messages.properties (default):

Example of messages_en.properties (English):

Example of messages_fr.properties (French):

By creating different files for each language (e.g., messages_fr.properties, messages_es.properties), you can store locale-specific translations of your messages.

3. Resolving Messages in Your Application

Once the MessageSource is configured, you can inject it into your Spring components (like controllers, services, or views) to retrieve localized messages.

Example in Controller:

In this example:

• messageSource.getMessage() is used to retrieve the message based on the key (greeting, welcome.message).
• The locale parameter is passed to ensure the message is resolved based on the current locale.

4. Using Messages in Views (Thymeleaf)

If you're using Thymeleaf as the view technology, you can directly access messages in your templates by using the #{} syntax.

Example in Thymeleaf Template:

In the above example:

• #{greeting} and #{welcome.message} are Thymeleaf expressions that are resolved based on the current locale.
• When the page is rendered, the corresponding message from the appropriate messages_{locale}.properties file is displayed.

5. Configuring Locale Resolver for Dynamic Locale Switching

To enable dynamic switching of locales (e.g., based on user preferences), you can use a LocaleResolver. This will allow your Spring application to detect the user's locale, which can be changed via a query parameter or session data.

Example of SessionLocaleResolver:

In this configuration:

• The SessionLocaleResolver stores the locale in the user's session.
• The LocaleChangeInterceptor allows the locale to be changed dynamically via the lang query parameter in the URL (e.g., ?lang=fr for French).

6. Using Locale in Spring Boot Configuration

In a Spring Boot application, you can configure the message source and locale resolver by adding properties to application.properties or application.yml for convenience.

Example (application.properties):

This simplifies the configuration, and Spring Boot will automatically configure the MessageSource bean using these properties.

Practical Examples

Example 1: Switching Languages via URL Parameter

You can create language-switching links in your templates to allow users to change the locale.

Example (Thymeleaf):

When the user clicks one of these links, the lang parameter is passed in the URL, and the locale is updated accordingly, triggering the message source to fetch the appropriate messages.

Example 2: Handling Form Validation Messages

In Spring MVC, you can use the MessageSource to resolve form validation messages based on the current locale.

Example:

In this example, if the user input is invalid, the error.invalid.input message is resolved dynamically based on the current locale.

Conclusion

Configuring message sources in Spring is essential for building internationalized applications that can serve content in multiple languages. By defining a MessageSource bean, creating locale-specific properties files, and leveraging LocaleResolver and MessageSource together, you can ensure that your application provides a seamless, localized experience for users around the world. Spring’s integration with views like Thymeleaf also makes it easy to dynamically load messages in templates, creating a fully internationalized solution.