If I change the base class that a Java Exception class extends, do I need to update the serialVersionUID value?

Question

Consider the following Java exception classes:

public class FooException extends RuntimeException {
    // [...]
}

public class FooException extends BarException {
    private static final long serialVersionUID = -5322002268075295537L;

    // [...]
}

If I wish to update the inheritance hierarchy to remove BarException, such that FooException derives directly from RuntimeException, does this require a change to the serialVersionUID value?

// FooException with updated inheritance hierarchy
public class FooException extends RuntimeException {
    private static final long serialVersionUID = ???;

    // [...]
}

Answer 1

The Java 1.5 Serialization Specification suggests that removing classes from the inheritance hierarchy is a compatible change, so a change to serialVersionUID should not be required.

Any extra information in the serialization stream pertaining to BarException would be ignored when deserializing to the new FooException (which is derived directly from RuntimeException).

Answer 2

Yes. "Moving classes up or down the hierarchy" will cause incompatibility with previous serialized instances, as per the serialization spec.

Answer 3

Technically yes, but it depends on whether your system persists serialized objects and if you control how the new, refactored code is deployed.

If you don't do persistence and you will refresh the entire deployment with the new version of the code, I don't see the need to change the serialVersionUID.

Answer 4

Given that the specification is unclear enough to cause confusion and debate, with no clear answer emerging, the only option left is to trust empirical evidence.

Taking the examples from the question above, of FooException deriving from BarException deriving from RuntimeException, and then removing BarException from the inheritance chain, I put together a sample application to try serialization and de-serialization in various combinations.

I get the following results:

As long as I keep the serialVersionUID unchanged, I can successfully serialize and deserialize the original FooException as the updated FooException, and vice versa.

The following caveats apply:

• I am using JDK 1.5.0_07, and have not tried this on any other versions.
• FooException has members of type int and Exception, which are successfully deserialized.
• BarException adds no additional members to RuntimeException.