how do I write a java.util.map to avro ? In GenericData I see entries for Record etc, but not for Map. Same question is there on http://apache-avro.679487.n3.nabble.com/How-to-write-an-AVRO-map-Something-like-GenericData-Map-td3407527.html , but I dont understand exactly how it was done.
Avro uses java.util.HashMap directly. You do not need to convert it to another type. Specifically, there is no GenericData class representing a map, so you simply put your HashMap directly into a GenericRecord.
All of the Avro to Java conversions are listed in the table in this answer. Note that GenericData classes are required for record, enum, array, and fixed types, but not for map.