Re: [Jython-users] Casting PyObjects to Java Interfaces fails when constructed from a URLClassLoader

[John H. <jhu...@no...>]

On 05/19/2016 05:22 PM, Jimmy Mauri wrote:
> For context, see sample project in: 
> https://github.com/ElCubanoPoderoso/jython-classloader-issue
>
> I have a use-case where I must support dynamically loaded .jar files 
> as hot-deployable subapplications.  I was having trouble in instances 
> where if the .jar file was redeployed (say I made a code change and 
> need to update the environment), Jython starts to break down with 
> class casting.  Upon the first deployment, the application is able to 
> properly load the .jar file, instantiate the main class which creates 
> the interpreter with the packaged URLClassLoader into the 
> PySystemState object, interprets the script and creates the class 
> object with the appropriate java interface cast to it.
>
> However, the second invocation of said project causes the casting to 
> fail on subsequent executions, converting the object to a PySingleton 
> and rejecting the cast.  I'm having trouble understanding what I need 
> to do to resolve my issue.  Not being able to properly coerce the 
> object to the expected java interface is causing forced workarounds to 
> add wrapper objects to utilize them in Java code.
>
> Note:  This is only observed when trying to cast to interfaces that 
> reside inside the .jar file and not in the system classloader.
>
> The sample GitHub project linked was simplified to generate a 
> proof-of-concept that can be invoked via JUnit to reproduce the error.
>
> My question is:  what is wrong with my current implementation that is 
> causing this issue?  As part of the requirement, I must be able to 
> redeploy without shutting down the JVM.  Any help in this matter would 
> be appreciated.

I'm not sure if this is your issue but remember that in Java two 
instantiations of the same class from two different class loaders are 
for all intents and purposes not the same class.  Given an object of 
class C loaded by class loader A and another object of class C loaded by 
class loader B you cannot cast from the first to the second.  Jython's 
duck typing protects you from some of this (e.g. within the Jythin 
interpreter you can call all methods on either instantiation of C), 
however if you try to pass a C instantiated by CL A to a any class 
loaded in CL B that will fail.

If this sounds like your issue let me know and I can tell you more about 
how we deal with this in our application (which uses multiple instances 
of a custom classloader to isolate different components). Its been a 
while since I've seen it but I seem to recall that the hint that I'd run 
into this from within our application was getting a ClassCastException 
with a message something like: 'cannot cast a foo.bar to foo.bar'.



-- 
-john

To be or not to be, that is the question
                 2b || !2b
(0b10)*(0b1100010) || !(0b10)*(0b1100010)
         0b11000100 || !0b11000100
         0b11000100 ||  0b00111011
                0b11111111
255, that is the answer.