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         


        

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.