How do you create a custom locale resolver in Spring Boot?

Table of Contents

• Introduction
• Steps to Create a Custom Locale Resolver in Spring Boot
  • Step 1: Define a Custom Locale Resolver Class
    • Example: Custom Locale Resolver Implementation
  • Step 2: Register the Custom Locale Resolver
    • Example: Registering Custom Locale Resolver
  • Step 3: Handling Locale Changes in Requests
    • Example: Locale Change Interceptor Configuration
  • Step 4: Using Localized Messages
    • Example: Using MessageSource in a Controller
  • Step 5: Testing the Custom Locale Resolver
  • Step 6: Optional - Store Locale in Session or Cookies
    • Example: Storing Locale in Session
• Conclusion

Introduction

In a Spring Boot application, locale resolution is a crucial part of implementing internationalization (i18n), as it allows your app to adapt to the language and region preferences of the user. By default, Spring Boot uses built-in locale resolvers like SessionLocaleResolver or AcceptHeaderLocaleResolver. However, there are scenarios where you may need a custom locale resolver, such as when the locale is stored in a custom header, cookie, or database. This guide explains how to create and configure a custom locale resolver in a Spring Boot application.

Steps to Create a Custom Locale Resolver in Spring Boot

Step 1: Define a Custom Locale Resolver Class

To create a custom locale resolver, you need to implement the LocaleResolver interface, which defines two main methods:

• resolveLocale(HttpServletRequest request): Resolves the locale from the request.
• setLocale(HttpServletRequest request, HttpServletResponse response, Locale locale): Sets the locale in the request or response.

Example: Custom Locale Resolver Implementation

In this implementation:

• The resolveLocale method checks for a custom header (X-Language) in the HTTP request to determine the user's language.
• If the header is found, it creates a Locale object based on the language specified in the header.
• The setLocale method uses LocaleContextHolder to set the locale for the entire application, so it can be accessed globally.

Step 2: Register the Custom Locale Resolver

Now that you have a custom locale resolver, you need to register it in the Spring application context. You can do this by creating a @Configuration class and defining a LocaleResolver bean.

Example: Registering Custom Locale Resolver

In this configuration:

• The localeResolver bean is created and returned as an instance of CustomLocaleResolver.
• By registering the custom resolver in the Spring context, it will be used throughout the application for resolving and setting the locale.

Step 3: Handling Locale Changes in Requests

You might also want to allow users to change their preferred language through a request parameter, header, or other custom mechanisms. You can do this by implementing a LocaleChangeInterceptor.

Example: Locale Change Interceptor Configuration

In this example:

• The LocaleChangeInterceptor listens for a lang parameter in the request URL (e.g., /greeting?lang=fr).
• When the parameter is found, it changes the locale of the current request.

Step 4: Using Localized Messages

Now that you have a custom locale resolver in place, you can use the MessageSource bean to access localized messages. The locale will be automatically resolved based on the logic in your custom resolver.

Example: Using MessageSource in a Controller

In this controller:

• The greet method uses the MessageSource to fetch a localized message based on the resolved locale.
• The lang request parameter can override the locale, but if not provided, it will fall back to the locale set by the custom locale resolver.

Step 5: Testing the Custom Locale Resolver

To test the custom locale resolver, you can:

1. Send a request with the custom header (X-Language) or query parameter (lang) to change the locale.
2. Verify that the response content is properly localized according to the resolved locale.

For example:

• /greeting should return the default locale message.
• /greeting?lang=fr should return the French message.
• If you pass the X-Language header as es, it should return the Spanish message.

Step 6: Optional - Store Locale in Session or Cookies

If you want to persist the user's language preference across requests, you can store the locale in a session or a cookie. Modify the setLocale method in your custom locale resolver to handle this.

Example: Storing Locale in Session

This ensures that the language preference is maintained across multiple requests for the same user session.

Conclusion

Creating a custom locale resolver in Spring Boot gives you the flexibility to handle user language preferences in a variety of ways. By implementing the LocaleResolver interface, you can define your own logic for resolving and setting the locale, such as using custom headers or query parameters. Combining this with a MessageSource and a LocaleChangeInterceptor allows for dynamic and personalized internationalization in your application.