Difference between having a proxy for a class and an interface

Question

I'm trying to load the proxy by class and by interface for another object.

To ensure that a proxy is created, I have wrapped it with @Cacheable and @CacheEvict annotations on it.

For having a reference C1 which is for a class, works.

But when having a reference for C2 which is for a class that implements another interface I2 it does not work.

If I replace the reference of C2 to I2 then it works again.

Could somebody please point me what is happening?

What else I need to read in order to understand?

Why is it thrown an exception for the c2 case?

Here is the code:

@Component
class HolderC {

    @Autowired
    private C1 c1;

    @Autowired
    private C2 c2;

    @Autowired
    private I2 i3;

    @PostConstruct
    public void postConstruct() {
        System.out.println(c1.getClass());
        System.out.println(c2.getClass());
        System.out.println(i3.getClass());
    }
}

@Component
@Caching
class C1 {
    @CacheEvict
    public void m1() {
    }
}

interface I2 {
    void m2();
}

@Component
@Caching
class C2 implements I2 {
    @CacheEvict
    @Override
    public void m2() {
    }
}

Along with :

@ComponentScan
@EnableCaching
public class ApplicationConfig {

    @Bean
    public Caffeine caffeineConfig() {
        return Caffeine.newBuilder().expireAfterWrite(60, TimeUnit.MINUTES);
    }

    @Bean
    public CacheManager cacheManager(Caffeine caffeine) {
        CaffeineCacheManager caffeineCacheManager = new CaffeineCacheManager();
        caffeineCacheManager.setCaffeine(caffeine);
        return caffeineCacheManager;
    }
}

And it is printing :

for c1:

21:25:24.244 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'c1'
21:25:24.290 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'c2'
21:25:24.297 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'holderC'
class java.config.context.C1$$EnhancerBySpringCGLIB$$a871b65d
21:25:24.302 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'caffeineConfig'
21:25:24.320 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'cacheManager'

for c2

21:26:53.706 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'c1'
21:26:53.755 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'c2'
21:26:53.764 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'holderC'
21:26:53.769 [main] WARN org.springframework.context.annotation.AnnotationConfigApplicationContext - Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'holderC': Unsatisfied dependency expressed through field 'c2'; nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'c2' is expected to be of type 'java.config.context.C2' but was actually of type 'java.config.context.$Proxy22'
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'holderC': Unsatisfied dependency expressed through field 'c2'; nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'c2' is expected to be of type 'java.config.context.C2' but was actually of type 'java.config.context.$Proxy22'
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:596)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:374)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1411)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:849)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:877)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)
    at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:88)
    at java.config.context.Runner.main(Runner.java:9)
Caused by: org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'c2' is expected to be of type 'java.config.context.C2' but was actually of type 'java.config.context.$Proxy22'
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.checkBeanNotOfRequiredType(DefaultListableBeanFactory.java:1672)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1650)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1213)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1167)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:593)
    ... 14 more

for c3

21:28:26.331 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'c1'
21:28:26.386 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'c2'
21:28:26.397 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'holderC'
class java.config.context.$Proxy22
21:28:26.405 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'caffeineConfig'
21:28:26.431 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'cacheManager'

Like I just mentioned [here](https://stackoverflow.com/a/73591431/1082681), a JDK proxy only **delegates** to an instance of an interface-implementing class, but actually **extends** `java.lang.reflect.Proxy` + interface implementation. This is why you cannot cast the JDK proxy to its implementing class, which is what Spring tries to do here due to your type declaration. But _mlecz_'s answer describes how to work around that problem. I would recommend not to use that workaround, but stick with injecting the interface type, in your case `I2`. You have already shown that it works. — kriegaex, Sep 03 '22 at 10:33
The whole point of programming against interfaces is to be able to use several implementations interchangeably. Please try to avoid referring to implementation types as best you can and use the interface types. — kriegaex, Sep 03 '22 at 10:35

score 1 · Answer 1 · answered Sep 03 '22 at 05:35

Spring creates java proxy by default if its possible. Proxy works only on interface, so for class it has to fallback to CGLIB.

You can annotate C2 with @Scope setting its proxyMode to TARGET_CLASS. This will force this class to use CGLIB.

You can also use this setting globally by using this configuration annotation @EnableAspectJAutoProxy(proxyTargetClass=true) you will need to import additional dependency though

Difference between having a proxy for a class and an interface

1 Answers1