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From Gregg Wonderly <ge...@cox.net>
Subject Re: River-436 - need some explanation of preferred class provider
Date Wed, 05 Mar 2014 01:39:55 GMT

> On Mar 4, 2014, at 12:02 AM, Michał Kłeczek <michal.kleczek@xpro.biz> wrote:
> The real problem is that Util interface is in two codebases. It should be
> in a single codebase shared between UtilProxy and WrapperProxy.
> But to make it possible we would need to have peer class loading like in
> ClassWorlds or OSGI.
> It is not solvable in a standard hierarchical class loading scheme.

This is one of the good examples of where hierarchical loading can present challenges.

But the question really is, can an arbitrary client really expect for arbitrary services to
interact correctly?  If you want them to do this, it has been shown over and over that global
types are the best, least troublesome choice.  

If you want ubiquitous interactions why not use string based values such as XML or better
yet, JSON?

Then code and data is immune to class loading snafus and not bound to a container or hosting


> Anyway... It is not really River-436 problem so my patch proposal is going
> to have the same issue since it is just a replacement for String
> annotations and not change in class loading scheme.
> Thanks,
> Michal
> 4 mar 2014 06:38 "Michał Kłeczek" <michal.kleczek@xpro.biz> napisał(a):
>> 1. The problem is there is no such thing as "the service interface". It is
>> context dependent. What is "the service interface" for service browser?
>> 2. In this particular case Util interface is an implementation detail of
>> WrapperProxy. It is Wrapper interface the client is interested in. So I
>> would say it should be preferred in WrapperProxy codebase.
>> 3. Even if Util is not preferred in WrapperProxy codebase we still have
>> ClassCastException if the client does not have Util in its classpath. Why
>> should it? it is interested in Wrapper not in Util. So either
>> a. We always get ClassCastException if Util is preferred in WrapperProxy
>> codebase, or
>> b. We get ClassCastException anyway if a client does not have Util in its
>> classpath.
>> Let's say I want to register RemoteEventListener that wraps a Javaspace
>> proxy to write events in a space. Does that mean the service "event source"
>> has to be aware of Javaspace interface??? That would be absurd...
>> It all does not have anything to do with codebase services.
>> Thanks,
>> Michal
>> 4 mar 2014 00:09 "Peter" <jini@zeus.net.au> napisał(a):
>>> The Util interface should not be preferred.  Implementations of Util can
>>> be preferred but not Util itself.
>>> Services need a common api that all implementations and clients can use
>>> to interract, even if this is a kind of codebase service.
>>> Modifying an interface is generally considered bad practise but now Java
>>> 8 makes it possible to add default methods for added functionality, that
>>> line blurs somewhat.  What can you do if an earlier interface is loaded by
>>> a parent ClassLoader and you need a later version, make it preferred?
>>> My thoughts are that interfaces should never be preferred and all classes
>>> defined in their methods shouldn't be preferred either.
>>> It would be relatively easy to write a new implementation that ensures
>>> that interfaces are loaded into their own ProtectionDomain in a parent
>>> ClassLoader.  But that would be confusing as dynamic policy grants are made
>>> to ClassLoader's not ProtectionDomains.
>>> But using ProtectionDomains in this manner, preserves security, ensures
>>> maximum visibility and avoids codebase annotation loss, if we ask the
>>> ProtectionDomain for the annotation, instead of the ClassLoader.  But this
>>> is not how we do things presently.
>>> Cheers,
>>> Peter.
>>> ----- Original message -----
>>>> But it will also be loaded by WrapperProxy ClassLoader, since it is
>>>> preferred there. So it will end up with ClassCastException, right?
>>>> Regards,
>>>> Michal
>>>> If Util is installed locally, it will only be loaded by the application
>>>> ClassLoader, since it isn't preferred.
>>>> Peter.
>>>> ----- Original message -----
>>>>> Folks,
>>>>> while woking on the River-436 patch proposal I've came across the
>>>>> scenario that I am not sure how to handle:
>>>>> Utility service:
>>>>> //inteface is NOT preferred
>>>>> interface Util {...}
>>>>> //class IS preferred
>>>>> class UtilProxy implements Util {}
>>>>> Wrapper service:
>>>>> //NOT preferred
>>>>> interface Wrapper {}
>>>>> //preferred
>>>>> class WrapperProxy implements Serializable{
>>>>> //initialized with Util impl from a lookup service
>>>>> private Util util;
>>>>> }
>>>>> Wrapper service codebase includes Util interface but it is
>>> _preferred_.
>>>>> Would deserialization of WrapperProxy end with ClassCastException?
>>>>> From what I understand UtilProxy is annotated with its codebase. When
>>>>> deserializing UtilProxy a ClassLoader is going to be created with
>>>>> parent set to TCCL. It means Util interface is going to be loaded
>>>>> twice by two ClassLoaders - one for WrapperProxy codebase and another
>>>>> for UtilProxy codebase.
>>>>> Am I correct?
>>>>> And if so: is it desired behavior?
>>>>> Regards,
>>>>> --
>>>>> Michał Kłeczek
>>>>> XPro Quality Matters
>>>>> http://www.xpro.biz

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