Repository Not Being Initiated

Question

I am getting an error when trying to move my repository initiation from my Application class to a config class. It seems strange because the urlRepository.save call does not throw, while the urlRepository.findAll() gets a null. What am I doing wrong here?

Before (Works Fine - prints to console with the preloaded URL)

@SpringBootApplication
public class UrlShortenerApplication implements CommandLineRunner {

@Autowired
private UrlRepository urlRepository = new MongoUrlRepository();

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

@Override
public void run(String... args) throws Exception {

    String shortlinkStub = "short.li/";
    String cascadeDishPods = "https://www.amazon.com/gp/product/B07CTQ8THP/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1";
    String flipFlops = "https://www.amazon.com/gp/product/B0013MWDO0/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1";
    urlRepository.save(new ShortUrl(shortlinkStub + "flipflops", flipFlops));
    urlRepository.findAll().forEach(shortUrl -> System.out.println("Preloaded " + shortUrl));
}
}

After (null pointer exception for repository.findAll())

ShortenerConfig.java

@Configuration
public class ShortenerConfig {

public static UrlRepository urlRepository = new MongoUrlRepository();

@Bean("urlRepo")
public UrlRepository urlRepo() {
    return urlRepository;
}
}

UrlShortenerApplication.java

@SpringBootApplication
public class UrlShortenerApplication implements CommandLineRunner {

    @Autowired
    @Qualifier("urlRepo")
    private UrlRepository urlRepository;

    public static void main(String[] args) {

        SpringApplication.run(UrlShortenerApplication.class, args);
    }

    @Override
    public void run(String... args) throws Exception {

        String shortlinkStub = "short.li/";
        String cascadeDishPods = "https://www.amazon.com/gp/product/B07CTQ8THP/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1";
        String flipFlops = "https://www.amazon.com/gp/product/B0013MWDO0/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1";
        urlRepository.save(new ShortUrl(shortlinkStub + "flipflops", flipFlops));
        urlRepository.findAll().forEach(shortUrl -> System.out.println("Preloaded " + shortUrl));
    }
}

That last line throws a null pointer exception and I don't know why!

Stacktrace

line 22 refers to the "run" and line 35 refers to the reponsitory.findAll()

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2020-06-12 10:48:25.813 ERROR 75530 --- [           main] o.s.boot.SpringApplication               : Application run failed

java.lang.IllegalStateException: Failed to execute CommandLineRunner
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:798) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:779) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:322) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    at com.example.urlshortener.UrlShortenerApplication.main(UrlShortenerApplication.java:22) ~[main/:na]
Caused by: java.lang.NullPointerException: null
    at com.example.urlshortener.UrlShortenerApplication.run(UrlShortenerApplication.java:35) ~[main/:na]
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:795) ~[spring-boot-2.3.0.RELEASE.jar:2.3.0.RELEASE]
    ... 5 common frames omitted

2020-06-12 10:48:25.843  INFO 75530 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Shutting down ExecutorService 'applicationTaskExecutor'

Repository Interface and Class (For Both Cases)

public interface UrlRepository extends MongoRepository<ShortUrl, Long> {
    public ShortUrl findByShortUrl(String shortUrl); 
    public List<ShortUrl> findByLongUrl(String longUrl);

}

public class MongoUrlRepository implements UrlRepository {
    @Override
    public ShortUrl findByShortUrl(String shortUrl) {
        return null;
    }

    @Override
    public List<ShortUrl> findByLongUrl(String longUrl) {
        return null;
    }

....

Answer 1

MongoUrlRepository supposed to be an interface, that's the way Spring could manage it's crud implementations automatically. Since you are declaring it as a class you basically overriding the builtin finder methods to just return null, that's where your NPE is coming from.

see the examples from spring.io: https://spring.io/guides/gs/accessing-data-mongodb/

Why it worked in the 1st case:

@Autowired
private UrlRepository urlRepository = new MongoUrlRepository();

You instantiate a MongoUrlRepository, however that object will be replaced during dependency injection and since you're not specifying a qualifier spring will build a managed MongoRepository bean (based on your UrlRepository interface) and ignore your MongoUrlRepository class.