为什么只在Spring Application Context中为Bean加载接口

Question

I was trying to load a class in application context which implements an interface, I encountered two situations

1. When interface GenericDao don't have any declared method: Here in this case my class CriteriaQuerySampleDao loads as per below code in application context there is no issue

CriteriaQuerySampleDao dao= context.getBean(CriteriaQuerySampleDao.class);

2. When my interface GenericDao do have declared method and which is implemented by my class CriteriaQuerySampleDao :

   Here in this case my class don't get loaded in application context of spring as per below code

CriteriaQuerySampleDao dao= context.getBean(CriteriaQuerySampleDao.class);

and throws

Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.journaldev.spring.config.CriteriaQuerySampleDao' available

but when I load it by below code which is using interface GenericDao of my class CriteriaQuerySampleDao it loads successfully asper below code

GenericDao dao= context.getBean(GenericDao.class);

can some one explain why I am forced to load CriteriaQuerySampleDao class by GenericDao interface if my class is implementing aeven a single method of interface.

I am not sure about details, is it custom in spring?

GenericDao Interface

interface GenericDao<T>
{

    Session getSession();

    Class<T> getDefaultMappedObject();
}

CriteriaQuerySampleDao Class implementing GenericDao

@Transactional
@Repository
public class CriteriaQuerySampleDao implements GenericDao<MyDBTable>
{
    protected final static Logger logger= LogManager.getLogger(CriteriaQuerySampleDao.class);

    @Inject
    SessionFactory                sessionFactory;

    @Override
    public Session getSession()
    {
        Session session= null;
        try
        {
            logger.debug(" ++++ getSession try ++++ ");
            session= sessionFactory.getCurrentSession();

        }catch(Exception ex)
        {
            logger.debug(" ++++ getSession catch ++++ ", ex);
            session= sessionFactory.openSession();
        }
        return session;
    }

    @Override
    public Class<MyDBTable> getDefaultMappedObject()
    { return MyDBTable.class; }

}

LocalHibernateConfig Class Configuration And Class Having Main Method:

@Configuration
@PropertySource(value= {"classpath:hibernate.properties"}, ignoreResourceNotFound= true)
@EnableTransactionManagement
@ComponentScan
@Component(value= "LocalHibernateConfig")
public class LocalHibernateConfig
{
    public static void main(String[] args) throws Exception
    {
        AnnotationConfigApplicationContext context= new AnnotationConfigApplicationContext();
        // context.getEnvironment().setActiveProfiles("local");
        context.register(LocalHibernateConfig.class);
        context.refresh();

        LocalHibernateConfig lch= context.getBean(LocalHibernateConfig.class);
        logger.debug(" ++++ Properties is null "+(lch.properties==null)+" properties: "+lch.getProperties());

        CriteriaQuerySampleDao dao= context.getBean(CriteriaQuerySampleDao.class);
        // GenericDao dao= context.getBean(GenericDao.class);
        logger.debug(" ++++ dao: "+(dao==null));

    }

}

Answer 1

This will happen if you have a code coverage tool, like jacoco, attached to the JVM when building. It creates a Proxy class to handle the coverage, and that Proxy class implements your interface, so while its an instance of your interface, its not actually an instance of your Impl.

Answer 2

It is related to the way proxy interfaces are generated for enabling transaction support. You can change the behavior by setting the proxyTargetClass attribute of the @EnableTransactionManagement to true :

@EnableTransactionManagement(proxyTargetClass= true)
@ComponentScan
@Component(value= "LocalHibernateConfig")
public class LocalHibernateConfig {

}

Also see https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/transaction/annotation/EnableTransactionManagement.html#proxyTargetClass--

The reason behind this is that @EnableTransactionManagement is implemented using the decorator pattern with dynamically generated decorator classes.

Spring can create the dynamic decorator in two ways, default java proxies or CGLib. Default java proxies can only be generated for classes that implement an interface. In the case of CriteriaQuerySampleDao it implements an interface so Spring will resort to a default java proxy.