Delete field from old java class implementing Serializable
java, serialization
Solution
A few points:
When an object is deserialized, any fields not found in the byte-stream will be initialized to `null`. Therefore, when you add a new field, when a new version object is deserialized from an old version byte-stream, the new field will be initialized to `null`. If `null` is considered to be an invalid value, you can provide a `readObject` method to handle conversion. Old versions can still be deserialized from the new byte-stream- the new field is just ignored.
If a field is deleted, the situation is reversed: the old version class will now be missing a field. The missing field will be set to `null`. However, unlike the previous case, the old version can't add a `readObject` method (if you could add that method, then it would become the latest new version). Hence, deleting a field is considered to be incompatible.
In summary, the ability to create a `readObject` method in the new version class allows it to cope with the old version byte-stream when a new field is added. Unfortunately, the reverse is not possible.
It is important to note that, unless specifically defined, the `serialVersionUID` field will be automatically generated and will most likely change with practically all notable changes to a class. If two class versions have different `serialVersionUID`, an exception will be thrown when trying to carry out serialization/deserialization on a older or newer version byte-stream. If you don't manually set `serialVersionUID`, then no versions of your class will be compatible in terms of serialization.
P.S. If `null` happens to be a valid state for the deleted field (in the old version), then I guess you will be okay to delete fields. However, that is probably an edge case.
Problem
Suppose i have version of class `MyClass` where i have two fields `int count` and `String name`. And i have persisted the byte stream to file. After i delete the attribute `name` from the class then also the persisted byte stream is getting converted to object with no issues. But as per the `Serializable` docs `adding new attribute is compatible change but deleting attribute is incompatible change w.r.t. Serilaization`. I am confused can somebody please help me understanding this. Thanks!!!!