Example of using message resources in Spring Boot service layer code, in as simple way.

Messages.java

import org.springframework.context.MessageSource;
import org.springframework.context.support.MessageSourceAccessor;
import org.springframework.stereotype.Component;

import java.util.Locale;

/**
 * @author saniaky
 * @since 12/5/17
 */
@Component
public class Messages {

    private final MessageSourceAccessor accessor;

    public Messages(MessageSource messageSource) {
        this.accessor = new MessageSourceAccessor(messageSource, LocaleContextHolder.getLocale());
    }

    public String get(String code) {
        return accessor.getMessage(code);
    }

}

This will always use the default Locale and wouldn't dynamically change with say an "Accept-Language" header for the same reason I mentioned on this similar thread:
https://gist.github.com/jonikarppinen/0d600b0c82edce890310#gistcomment-2972839