Spring AOP CGLIB proxy's field is null

Question

Description

Using the vlcj component, the custom component appears as a result of the AOP proxy object null.

MediaList Class

public class MediaList {
    private libvlc_media_list_t mediaListInstance;
    public MediaList(LibVlc libvlc, libvlc_instance_t instance, libvlc_media_list_t mediaListInstance) {
        this.libvlc = libvlc;
        this.instance = instance;
        createInstance(mediaListInstance);
    }
    private void createInstance(libvlc_media_list_t mediaListInstance) {
        logger.debug("createInstance()");
        if(mediaListInstance == null) {
            mediaListInstance = libvlc.libvlc_media_list_new(instance);
        }
        else {
            libvlc.libvlc_media_list_retain(mediaListInstance);
        }

        this.mediaListInstance = mediaListInstance; // <- assignment
        logger.debug("mediaListInstance={}", mediaListInstance);

        mediaListEventManager = libvlc.libvlc_media_list_event_manager(mediaListInstance);
        logger.debug("mediaListEventManager={}", mediaListEventManager);

        registerEventListener();
    }
    public final libvlc_media_list_t mediaListInstance() {
        return mediaListInstance; // <- proxy object return null, if use aop
    }
}

Custom MediaList Class

public class TestMediaList extends MediaList {

    public TestMediaList(LibVlc libvlc, libvlc_instance_t instance) {
        super(libvlc, instance);
    }

    public void xTest(String test){
        System.out.println(test);
    }
}

Spring Configuration Class

@Configuration
public class PlayerBeanConfig {

    @Bean
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    @Resource
    public TestMediaList testMediaList(LibVlc libvlc, libvlc_instance_t instance) {
        return new TestMediaList(libvlc, instance);
    }
}

AOP Configuration Class

@Aspect
public class MediaListAspect {
    @Pointcut("execution(* TestMediaList.xTest(..))")
    private void anyMethod() {
    }

    @Around("anyMethod()")
    public Object lockAndUnlock(ProceedingJoinPoint joinPoint) throws Throwable {
        Object object = joinPoint.proceed();
        return object;
    }
}

Test Code

public static void main(String[] args) {
    boolean b = new NativeDiscovery().discover();

    if (b) {
        springContext = new AnnotationConfigApplicationContext(PlayerBeanConfig.class);

        String[] kkk = new String[]{};
        TestMediaList list = springContext.
                getBean(TestMediaList.class, LibVlc.INSTANCE, LibVlc.INSTANCE.libvlc_new(kkk.length, kkk));

        System.out.println(list.mediaListInstance()); // <- proxy object return null
    } else {
        logger.error("Cannot find vlc lib, exit application");
    }
}

I try to single step tracking, when TestMediaList the build is complete. MediaListInstance () of the method to return to normal values, but when the spring returns to the proxy object, null is returned. At the same time, I also try to return the value correctly if you don't use AOP. Therefore, I determine the basic problem in AOP dynamic proxy, but I don't know why, did not previously encountered such a situation.

---

Minimal example

all class in package : vod.demo

TargetClass

public class TargetClass {
    private String returnValue;

    public TargetClass() {
        this.returnValue = "Hello World";
    }

    public final String test() {
        System.out.println("TargetClass.test();");
        return returnValue;
    }
}

Aspect Class

@Aspect
public class AspectClass {
    @Pointcut("execution(* vod.demo.TargetClass.*(..))")
    private void targetMethod() {
    }

    @Around("targetMethod()")
    public Object aroundTarget(ProceedingJoinPoint joinPoint) throws Throwable {
        System.out.println("AspectClass.aroundTarget();");
        return joinPoint.proceed();
    }
}

Spring Config Class

@Configuration
@EnableAspectJAutoProxy
@Import(AspectClass.class)
public class SpringConfig {
    @Bean
    public TargetClass target() {
        return new TargetClass();
    }
}

Client Class

public class Client {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(SpringConfig.class);
        TargetClass target = context.getBean(TargetClass.class);
        System.out.println("Client invoke:" + target.test()); // <- output null
    }
}

Answer 1

This is a combination of potentially unexpected behaviors. First, Spring uses CGLIB to proxy your beans for AOP. CGLIB proxies are instances of a dynamic subtype of your class that delegate all method calls to a real instance of your class. However, even though the proxy is of a subtype, its fields are not initialized (ie. your TargetClass super constructor is not invoked). A lengthier explanation can be found here.

Additionally, your method

public final libvlc_media_list_t mediaListInstance() {
    return mediaListInstance; // <- proxy object return null, if use aop
}

or

public final String test() {
    System.out.println("TargetClass.test();");
    return returnValue;
}

are final. CGLIB therefore cannot override them to delegate to the real instance. This would be hinted at in Spring logs. For example, you would see

22:35:31.773 [main] INFO  o.s.aop.framework.CglibAopProxy - Unable to proxy method [public final java.lang.String com.example.root.TargetClass.test()] because it is final: All calls to this method via a proxy will NOT be routed to the target instance.

Put all of the above together and you get a proxy instance where the field is null and where the proxy cannot delegate to the real instance's method. So your code will actually invoke

public final String test() {
    System.out.println("TargetClass.test();");
    return returnValue;
}

for an instance where the returnValue field is null.

---

If you can, change your method, remove the final modifier. If you can't, you'll have to rethink your design.