How does Spring MVC use LocaleResolver?

Question

Accepted Answer

LocaleResolver is the Spring MVC abstraction responsible for determining which Locale to use for a given request, driving internationalization across the application. It decides the current locale based on a configurable strategy and makes that locale available for message resolution and formatting.

Key Points:
• AcceptHeaderLocaleResolver derives the locale from the browser's Accept-Language header, requiring no extra state.
• SessionLocaleResolver stores the resolved locale in the HTTP session, so it persists across requests for that user's session.
• CookieLocaleResolver stores the locale in a cookie, persisting it across sessions and even browser restarts.
• Once resolved, the locale drives Spring's MessageSource for translated text and also affects date, number, and currency formatting throughout the request.
• A LocaleChangeInterceptor is commonly paired with a LocaleResolver to let users explicitly switch languages via a request parameter.

Example:
An app using CookieLocaleResolver remembers a user selected Spanish even after they close and reopen the browser, rendering all message-bundle text and formatted dates in Spanish on their next visit without asking again.

Code Example:
```
@Bean
public LocaleResolver localeResolver() {
    CookieLocaleResolver resolver = new CookieLocaleResolver();
    resolver.setDefaultLocale(Locale.US);
    resolver.setCookieMaxAge(Duration.ofDays(30));
    return resolver;
}
```

Interview Tip:
A concise interview answer is:

"LocaleResolver determines which locale applies to a request, and Spring offers several strategies — AcceptHeaderLocaleResolver reading the browser header, or SessionLocaleResolver and CookieLocaleResolver persisting an explicit choice across requests or sessions. Once resolved, that locale drives message translation and date/number formatting throughout the request."