未启动存储库

Question

当我试图将我的版本库启动从我的Application类转移到config类时，我得到了一个错误。这看起来很奇怪，因为urlRepository.save调用没有抛出，而urlRepository.findAll()却得到一个null。我到底做错了什么？

之前 (工作正常--打印出预加载的URL到控制台)

@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));
}
}

在(repository.findAll()的空指针异常)之后。

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));
    }
}

最后一行抛出了一个空指针异常，我不知道为什么!

堆栈跟踪

第22行指的是 "运行"，第35行指的是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'

存储库接口和类（两种情况都适用

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应该是一个接口，这就是Spring可以自动管理它的crud实现的方式。由于你把它声明为一个类，你基本上重载了内置的finder方法，只是返回null，这就是你的NPE的来源。

请看spring.io.的例子。https:/spring.ioguidesgsaccessing-data-mongodb。

为什么在第1种情况下可以工作。

@Autowired
private UrlRepository urlRepository = new MongoUrlRepository();

你实例化了一个MongoUrlRepository，然而在依赖注入过程中，该对象将被替换，由于你没有指定限定词，spring将构建一个托管的MongoRepository Bean（基于你的UrlRepository接口）并忽略你的MongoUrlRepository类。