java - Deserialize an object after the underlying class structure got changed -

we serializing data objects , storing them in database, there hundreds of such objects in database since longtime. needed change structure of 1 of classes. deserializing object throwing errors.

we have not written custom serialization , deserialization earlier. use of defaultserialversionuid not helping here default 1l while serializing them.

is there better way deserialize them after underlying class structure got changed?

we serializing data objects , storing them in database ...

that was/is big mistake, imo. avoided having build , maintain tables corresponding data objects ... java object serialization not designed kind of thing, , kind of fragility 1 of downsides.

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@ejp , @sidoh's answers provide ideas digging out of current hole provided can hold of old versions of classes. there couple of approaches:

• roll code / schema state before changes broke compatibility ... , redo work in way avoids mistake.

• don't rollback, write ad-hoc converter loads readable objects readable form , updates stored versions.

the former may impractical; e.g. if else has changed, or if code / data in production.

the latter involves retrieving old , new versions of classes version control , building ad-hoc converter loads old classes, translates instances of new classes , saves. difficulty of building java application can simultaneously use 2 different versions of same classes in same jvm:

• you can instantiate 2 classloaders different versions on classpaths. problem jvm treat 2 sets of classes different types, , make static binding against both versions impossible. deal via reflection ... messy, if respective object apis extensive.

• you two-stage process. stage 1 load using old classes , serialize classes using (say) json write file (say). stage 2 read json, use create objects using new classes, , serialize them using object serialization.

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a third alternative write ad-hoc converter tweaks serialized objects. basically, need head around differences between old , new serialized forms , rewrites serialized objects reading / writing them using low-level api. variation of implement custom readobject , method in new version of class understands both old , new formats. however, tricky given old objects don't have custom version fields. should noted kind of stuff beyond scope of serialization spec ...

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but imo best alternative use opportunity stop using java serialization. read objects using old classes, , write them database regular sql tables, json or xml blobs, or using orm mapping.