Reflection on CGLIB proxies

Woyzeck · **Joined:** Tue Aug 17, 2004 5:06 am **Posts:** 46

Once again these CGLIB proxies make me go crazy.

I have a one-to-one association to a polymorph object. The proxy is derived from the top level object, i.e.

Person
Worker extends Person
Student extends Person

The proxy is derived from the top level object, so it's a Person.

If the one-to-one accessor returns a proxy (a Person) that proxies a Worker then I can't access the Worker's getter and setter methods using reflection. They are just not found.

Any hints ?

christian · **Posted:** Fri Oct 29, 2004 1:06 pm

Why did you not fill out the form in the text field? We don't know what you are talking about.

Woyzeck · **Joined:** Tue Aug 17, 2004 5:06 am **Posts:** 46

Sorry, but I can't fill out the form due to 'security constraints'. Abstracting the mapping files would be too difficult...

I really like Hibernate, but my experience is that the CGLIB proxies break polymorphism in case of associations. Once I disable them everything works fine.

christian · **Posted:** Fri Oct 29, 2004 2:10 pm

Sure, a proxy is a subclass generated at runtime. Use a tool that uses buildtime bytecode enhancement if you can't work with this.

christian · **Posted:** Fri Oct 29, 2004 2:11 pm

P.S. And they don't break polymorphism as long as you are not using "instanceof" all over the place.

Woyzeck · **Joined:** Tue Aug 17, 2004 5:06 am **Posts:** 46

christian wrote:

P.S. And they don't break polymorphism as long as you are not using "instanceof" all over the place.

I think that breaking the "instanceof" functionality does indeed mean that they break polymorphism. It's getting even worse if you use business interfaces.

I understand the problem. There's no way that a proxy can know it's correct subclass in advance to being initialized. However frameworks that depend on reflection will get into deep trouble here. In my case I can't call getters and setters using reflection *before* the object is initialized and the object isn't initialized when accessing it using reflection.

I see two approaches for solving my problem:
* initialize the proxy before it's being touched (better solution)
* disable proxies for polymorph objects at all (worse solution)

I'll try this today.

christian · **Posted:** Sat Oct 30, 2004 8:17 am

Code:

      Item aProxy = new ItemDAO().getItemById(auctionOne.getId(), false); // session.load()

      System.out.println("### PROXY IS INITIALIZED: " + Hibernate.isInitialized(aProxy)); // false
      System.out.println("### PROXY INSTANCE OF IS: " + (aProxy instanceof Item) ); // true
      System.out.println("### PROXY CLASS IS: " + aProxy.getClass().getName() ); // CGLIB class

      // Initializes the proxy and prints out the name of the Item
      System.out.println("### INVOKING METHOD ON PROXY: " +
              aProxy.getClass().getDeclaredMethod("getName", null).invoke(aProxy, null));

christian · **Posted:** Sat Oct 30, 2004 8:19 am

Oh, you use inheritance, so this code doesn't apply to you. You only have the two solutions you already mentioned.

baliukas · **Posted:** Sat Oct 30, 2004 12:42 pm

Woyzeck wrote:

Person
Worker extends Person
Student extends Person

The proxy is derived from the top level object, so it's a Person.

If the one-to-one accessor returns a proxy (a Person) that proxies a Worker then I can't access the Worker's getter and setter methods using reflection. They are just not found.
Any hints ?

If you need proxy for "Worker", then can tell it in mapping, but this mapping is static and you will get class cast exception if it will be a "Student" at runtime.
You can unwrapp proxied instance to get the real type too.

BTW Hibernate assigns proxy itself and it knows runtime type ( if poster is right then proxied instance type is "Worker" ), is it some problem to generate proxy using runtime type ? It looks like a bug, doe's not it ?

Woyzeck · **Joined:** Tue Aug 17, 2004 5:06 am **Posts:** 46

baliukas wrote:

Woyzeck wrote:

You can unwrapp proxied instance to get the real type too.

BTW Hibernate assigns proxy itself and it knows runtime type ( if poster is right then proxied instance type is "Worker" ), is it some problem to generate proxy using runtime type ? It looks like a bug, doe's not it ?

How can I unwrap a proxy ?

Woyzeck · **Joined:** Tue Aug 17, 2004 5:06 am **Posts:** 46

Those CGLIB proxies are driving me crazy...

We have associations that return proxies. These proxies do not implement anything but the base class. No business interface, no concrete type. We can't cast them to anything useful.

I tried to call Hibernate.initialize(myProxy), but it did not change a thing.

Is there any way how I can force a proxy object to expose the correct (polymorph) getters and setters in case of an association ?

baliukas · **Posted:** Sun Oct 31, 2004 2:22 am

Proxy is generated by mapping information, if you will use "Worker" in one-to-one association then proxy will extend it too, but it can not be generated by runtime type, probably it because framework needs to hit database to know "discriminator" and lazy proxy is meaningless for this use case (probably all persistence frameworks have this problem).
There is "ProxyHelper" API or something like that to unwrapp proxy, find it in javadoc, I do not remember it, there are a few discussions about it in this forum too.

Woyzeck · **Joined:** Tue Aug 17, 2004 5:06 am **Posts:** 46

Ok, I've read the chapter 6.4 "Mapping polymorphic associations" and it clearly warns of my problems. I should have done this earlier...

The proposed solution (using session.load()) to get a typesafe instance is unusable for all but the most trivial data structures, so I'll drop proxies for my polymorphic objects at all.

I understand all the problems and why polymorphic proxies have to be implemented that way, but my personal opinion is that proxies that don't hide themselves from the application code are bad design.

Breaking language fundamentals like instanceof and typecasts is very intrusive and leads to problems that can be very difficult to find during testing. In my opinion it would be better to disable proxying for polymorphic associations by default and warn the user of the problems that he can expect when enabling it.