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From Rick Hillegas <Richard.Hille...@Sun.COM>
Subject Re: serialization of Derby DataSources
Date Fri, 21 Apr 2006 20:33:57 GMT
Thanks, Lance. I agree. We seem to have a muddle if someone adds a new 
non-transient field to one of these classes: either a) the engineer 
changes the serialVersionUID, giving rise to the problem you mention or 
b) the serialVersionUID isn't changed and deserialization fails because 
the new field is missing from the persisted stream. Hopefully we don't 
mean for these objects to persist across Derby upgrades. Hard to tell 
from the code.


Lance J. Andersen wrote:

> Hi Rick,
> once the serialVerisonUID is there, you should not remove it as chaos 
> can break out if the IDs start to differ. IMHO would leave them alone.
> One example is you have say someone using say derby version x with a 
> an ID of 1 and then persisted the object... now u remove the ID in 
> derby y and the compiler generates say -2 for the ID , you will 
> encounter problems when you try and grab the persisted version as the 
> IDs no longer match.
> Rick Hillegas wrote:
>> Thanks, David. I'm afraid I'm still muddled. I think I understand the 
>> basic purpose of serialVersionUID: It's a compiler-generated checksum 
>> of the source which serialization uses as a sanity check. By 
>> explicitly setting this field, the engineer promises to keep the 
>> following contract: Although the class behavior may change between 
>> versions, the  non-transient fields won't.
>> But I'm still not grasping the serialization issue we're addressing 
>> here. How do we get into a situation where there are two different 
>> versions of one of these classes? Is anyone persisting these classes 
>> across upgrades of the Derby code?
>> Perhaps all that's being addressed here is the following 
>> recommendation from the javadoc of java.io.Serializable: "However, it 
>> is /strongly recommended/ that all serializable classes explicitly 
>> declare serialVersionUID values, since the default serialVersionUID 
>> computation is highly sensitive to class details that may vary 
>> depending on compiler implementations..." I don't think we have this 
>> problem, though: at release time we produce a standard, vetted 
>> version of Derby for which the compiler is constant.
>> Thanks for helping me puzzle through this.
>> Regards,
>> -Rick
>> David W. Van Couvering wrote:
>>> I had to look into this when I was playing around with a classloader 
>>> for code sharing.
>>> Basically, by setting the serialVersionUID, you are telling the VM 
>>> that you guarantee that the newer version of the class is compatible 
>>> with the old version (in terms of serialization).
>>> If you don't set this, then you will get an exception saying the 
>>> class is not compatible if the VM determines that version UID 
>>> (basically a hash) is different.  There is documentation explaining 
>>> how this UID is determined, and I struggled to get it right, but 
>>> finally I had to set the serialVersionUID.
>>> Note that you have to set the serial version UID on the *second* and 
>>> subsequent versions of the class, it's not required for the first 
>>> version of the class.  Basically, you run serialver on the first 
>>> version of the class, and then use this to set serialVersionUID in 
>>> the second version.
>>> I wrote some tests to verify serialization compatibility between 
>>> versions of classes but never got to the point of checking them in. 
>>> They may be valuable, and could be added to our compatibility tests, 
>>> so if you'd like I can poke around and find them.
>>> One bug I uncovered in my tests was that for one of the data sources 
>>> the serialversion UID was not public, so I was getting failures.  
>>> Now I can't remember if I checked in that fix or not.
>>> David
>>> Rick Hillegas wrote:
>>>> I'm confused about the presence of serialVersionUIDs in the 
>>>> DataSources exposed by our network client (e.g., 
>>>> ClientConnectionPoolDataSource). I think I understand why these 
>>>> classes are serializable (JNDI wants to serialize them). But I 
>>>> don't understand why we are forcibly setting the serialization id. 
>>>> I don't see any documentation explaining the serialization problem 
>>>> this addresses, stating the implications for engineers editting 
>>>> these classes, or describing our expectations at version upgrade.
>>>> Can someone shed some light on this?
>>>> Thanks,
>>>> -Rick

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