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As part of debugging an application, I noticed that Field.getDeclaredFields() returns some synthetic fields, including a serialVersionUID field in a class extending an interface, although none extend Serializable.

Why does the compiler add such fields?

UPDATE

In fact, there is also a $VRc synthetic field created.

Jérôme Verstrynge
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2 Answers2

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The Java compiler/runtime will not automatically create a serialVersionUID field. I suspect that you are using some form of bytecode enchancement framework under the hood that is being instructed to add the synthetic fields either at runtime, or during compilation.

The $VRc field is produced by the Emma instrumentation framework, so that would be the reason for at least one of the synthetic fields.

The serialVersionUID field is also added by Emma, when the instr.do_suid_compensation property is set to true.

Vineet Reynolds
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  • You are right, I am indeed using Emma for test coverage. When I disactivate it, the fields are not there anymore. – Jérôme Verstrynge Sep 12 '11 at 14:49
  • Yes, even serialVersionUID is also [generated by Emma](http://emma.sourceforge.net/reference/ch03s02.html#prop-ref.instr.do_suid_compensation) if a certain property was set. – Vineet Reynolds Sep 12 '11 at 14:49
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This field is essential for Java serialization. In short: it allows the JVM to discover that the class that was serialized (e.g. saved on disk) has been changed afterwards and cannot be safely deserialized back to object.

Have a look at Version Control chapter in the document quoted above, it explains how serialVersionUID is used.

UPDATE: just noticed your class does not implement Serializable. Are you sure none of super classes or implemented interfaces aren't extending Serializable?

Tomasz Nurkiewicz
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    I think the question is more about why the field got added when none of the classes in the inheritence hierarchy implement the `Serializable` interface, not what the purpose of the field is. – John Topley Sep 12 '11 at 14:23
  • Yes, 100% sure. It is similar to class A implements B and B is declared as an interface. A and B don't declare any fields themselves. That's it.. (and no, this is not about the meaning/use of serial version UIDs) – Jérôme Verstrynge Sep 12 '11 at 14:29