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
