To ease use of Avro objects with systems that rely on Java serialization, i.e. Apache Spark, this change updates generated classes to implement java.io.Externalizable.
This change is marked incompatible because the class hierarchy for SpecificFixed has changed. Updating the Avro library without recompiling the IPC classes (in my test this was for Flume) results in an AbstractMethodError.
(I couldn't add a Release Note since the jira is Closed, so I'm updating the description instead.)