Spring Boot 애플리케이션이 JAR로 실행될 때 클래스 경로 리소스를 찾지 못하는 이유는 무엇입니까?

<code class="java">@Configuration
@ComponentScan
@EnableAutoConfiguration
public class Application implements CommandLineRunner {

    private static final Logger logger = Logger.getLogger(Application.class);

    @Value("${message.file}")
    private Resource messageResource;

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @Override
    public void run(String... arg0) throws Exception {
        // both of these work when running as Spring boot app from STS, but
        // fail after mvn package, and then running as java -jar
        testResource(new ClassPathResource("message.txt"));
        testResource(this.messageResource);
    }

    private void testResource(Resource resource) {
        try {
            InputStream inputStream = resource.getInputStream();
            logger.debug("Found the resource " + resource.getFilename());
        } catch (IOException ex) {
            logger.error(ex.toString());
        }
    }
}</code>