Example of using message resources in Spring Boot service layer code, in as simple way as possible (hopefully!). NOTE: this approach supports only a single locale, not dynamically changing it.

Messages.java

package com.company.project.components;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.MessageSource;
import org.springframework.context.support.MessageSourceAccessor;
import org.springframework.stereotype.Component;

import javax.annotation.PostConstruct;
import java.util.Locale;

/**
 * Helper to simplify accessing i18n messages in code.
 * 
 * This finds messages automatically found from src/main/resources (files named messages_*.properties)
 * 
 * This example uses hard-coded English locale.
 *
 * @author Joni Karppinen
 * @since 2015-11-02
 */
@Component
public class Messages {

    @Autowired
    private MessageSource messageSource;

    private MessageSourceAccessor accessor;

    @PostConstruct
    private void init() {
        accessor = new MessageSourceAccessor(messageSource, Locale.ENGLISH);
    }

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

}

messages_en.properties

default.title = Title

SomeServiceImpl.java

package com.company.project.services;

import com.company.project.components.Messages;
import org.springframework.beans.factory.annotation.Autowired;

@Service
public class SomeServiceImpl implements SomeService {

    @Autowired
    Messages messages;
    
    // ... 
   
    private String getDefaultTitle() {
        return messages.get("default.title"));
    }

}

Im getting NullPointerException that MessageSource is null

Can you provide a simple unit test for this configuration? I cannot generate a dynamic locale change via this configuration.