How do I handle versioning of my Externalizable classes? Do I have to write a version number to the stream?

Just as with classes that implement Serializable, the serialVersionUID field is checked when reading in Externalizable objects via an ObjectInputStream. If serialVersionUID is not explicitly defined in your Externalizable class, then one will be computed for you upon deserialization. See the FAQ at http://www.jguru.com/jguru/faq/view.jsp?EID=5063 for more details.