My understanding was that they may persist across upgrades because the
data source objects are serialized into a JNDI store. In general we can
*add* non-transient fields but we can't remove or change them.
I think also since we support the Referenceable interface, the object is
reconstructed in a compatible way using our own code, rather than
depending upon serialization's default mechanism. But that's where I'm
still a little muddled.
By the way, using the *exact* same compiler, I tried to gently modify a
DataSource following all the rules I could imagine, and because I didn't
know the serialVersionUID was accidentally made private, I kept getting
an incompatible class error or whatever it's called. I was doing
everything perfectly, and it was still breaking. Once I set the
serialVersionUID to be public, peace reigned.
David
Rick Hillegas wrote:
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.
Regards,
-Rick
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
