What is IOC ResourceLoader?

Interface that provide function for read resource

To get the Resource, need Resource Loader.

Fist of all, resource is any file for example txt, xml and so on.

Can use method ‘getResource’ for get some resource like below code.

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@Component
public class LoaderRunner implements ApplicationRunner {

    @Autowired
    ResourceLoader resourceLoader;

    @Override
    public void run(ApplicationArguments args) throws Exception {
        Resource res = resourceLoader.getResource("classpath:test.txt");
        System.out.println(res.exists());
    }
}

1. @Primary

As BookService like below code. There are repository which extends BookRepository.

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public interface BookRepository {
}

@Repository
public class EwanBookRepository implements BookRepository {
}

@Repository
public class MyBookRepository implements BookRepository {
}

Then which BookRepository being inject?.

We can see BookRepository class name from setUp method with @PostConstruct.

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@Service
public class BookService {

    @Autowired
    private BookRepository bookRepository;

    @PostConstruct
    public void setUp(){
        System.out.println(bookRepository.getClass());
    }    
}

Result is fail because spring don’t know which class inject.

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Exception in thread "task-2" org.springframework.beans.factory.BeanCreationNotAllowedException: Error creating bean with name 'springApplicationAdminRegistrar': Singleton bean creation not allowed while singletons of this factory are in destruction (Do not request a bean from a BeanFactory in a destroy method implementation!)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:208)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:207)
	at org.springframework.context.event.AbstractApplicationEventMulticaster.retrieveApplicationListeners(AbstractApplicationEventMulticaster.java:245)
	at org.springframework.context.event.AbstractApplicationEventMulticaster.getApplicationListeners(AbstractApplicationEventMulticaster.java:197)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:134)
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:403)
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:360)
	at org.springframework.boot.autoconfigure.orm.jpa.DataSourceInitializedPublisher.publishEventIfRequired(DataSourceInitializedPublisher.java:99)
	at org.springframework.boot.autoconfigure.orm.jpa.DataSourceInitializedPublisher.access$100(DataSourceInitializedPublisher.java:50)
	at org.springframework.boot.autoconfigure.orm.jpa.DataSourceInitializedPublisher$DataSourceSchemaCreatedPublisher.lambda$postProcessEntityManagerFactory$0(DataSourceInitializedPublisher.java:200)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

So if want enroll which class priority first, write @Primary like below code.

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@Repository @Primary
public class EwanBookRepository implements BookRepository {
}

The result is success and can see class name that injected in BookService.

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class com.example.demo.book.EwanBookRepository$$EnhancerBySpringCGLIB$$ab287e3a

2. List

If want get all class that type BookRepository, can inject as List.

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@Autowired
List<BookRepository> bookRepositoryList;

@PostConstruct
public void setUp(){
    bookRepositoryList.forEach(x ->
            System.out.println(x.getClass())
    );
}

The result like below.

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class com.example.demo.book.EwanBookRepository$$EnhancerBySpringCGLIB$$289e3a54
class com.example.demo.book.MyBookRepository$$EnhancerBySpringCGLIB$$a27f925b

3. Qualifier

Also can use class name in @Qualifier, but be careful first spelling should small letter even though class name is upper case.

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@Autowired @Qualifier("ewanBookRepository")
    private BookRepository bookRepository;

    @PostConstruct
    public void setUp(){
        System.out.println(bookRepository.getClass());
    }

The result like below.

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class com.example.demo.book.EwanBookRepository$$EnhancerBySpringCGLIB$$289e3a54

4. Write class name

Can write class name directly, spring can inject that class.

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@Autowired
    private BookRepository ewanBookRepository;

    @PostConstruct
    public void setUp(){
        System.out.println(ewanBookRepository.getClass());
    }

The result like below.

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class com.example.demo.book.EwanBookRepository$$EnhancerBySpringCGLIB$$fc17c0d9