What is serialVersionUID in java, normally in exception class?

serialVersionUID is a field to define the version of a particular class while seriializing & deseriializing.. consider a scenario where you have a class Employee which has 3 fields which has been in production for some time (meaning there may exist many serialized versions of employee objects), when you update the class to include (say a 4th field) then all the previous class (which are serialized) cannot be casted or de-serialized to the new one & you'll get an exception.

to avoid this issue, you can use the serialVersionUID field to tell JVM that the new class is in fact of a different version (by changing the serialVersionUID).

@Farmor & @Tom Jefferys said pretty much the same thing, but with an example things look a lot simpler.

It's used for serialization, it should be declared in any class that implements Serializable.

It's effectively a version number the JVM can use to check if a serialized class matches the class definition you're trying to deserialize it into.

There's more information on it here: http://download.oracle.com/javase/1,5.0/docs/api/java/io/Serializable.html

It's to determine if they have serializable and deserializable compatibility if they have the same serialVersionUID and both implements Serializable then they are compatible.

And it's not limited to exceptions only as you will notice eclipse is prone to put a serialVersionUID early in your normal java class if it implements Serializable.

Edited: Updated to include @Spychos correct comment about the Serializable interface.