外部Jar化したコードの一部。

BaseApplicationConfig.java

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.i18n.LocaleChangeInterceptor;

import lombok.RequiredArgsConstructor;

@RequiredArgsConstructor
@Configuration
public class BaseApplicationConfig implements WebMvcConfigurer {

	private final LocaleChangeInterceptor localeChangeInterceptor;
	
	@Override
	public void addInterceptors(InterceptorRegistry registry) {
		registry.addInterceptor(localeChangeInterceptor)
				.addPathPatterns("/**")
				.excludePathPatterns("/static/**");
	}

}

BaseBeanConfig.java

import java.util.Locale;

import javax.servlet.ServletContext;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.context.ServletContextAware;
import org.springframework.web.servlet.LocaleResolver;
import org.springframework.web.servlet.i18n.CookieLocaleResolver;
import org.springframework.web.servlet.i18n.LocaleChangeInterceptor;

import lombok.Setter;

@Configuration
public class BaseBeanConfig implements ServletContextAware {

	/* ロケールのリクエストパラメータをlangにしたい */
	@Bean
	public LocaleChangeInterceptor localeChangeInterceptor() {
		LocaleChangeInterceptor interceptor = new LocaleChangeInterceptor();
		interceptor.setParamName("lang");
		return interceptor;
	}

	/* for ServletContextAware */
	@Setter(onMethod = @__({ @Override }))
	private ServletContext servletContext;

	/* Cookieにロケール情報を持つ */
	@Bean
	public LocaleResolver localeResolver() {
		CookieLocaleResolver resolver = new CookieLocaleResolver();
		resolver.setDefaultLocale(Locale.JAPAN);
		resolver.setCookieName("app.setting.locale");

		/* CookieLocaleResolverはデフォでcookieのpathを / にしてしまうのでアプリケーションのコンテキストパスにする */
		resolver.setCookiePath(servletContext.getContextPath());
		/* --> が、ContextPathがnullでされておらず、NullPointerException */
		
		return resolver;
	}

}