How to write union when creating Avro file in Java

Question

I'm trying to create Avro file in Java (just testing code at the moment). Everything works fine, the code looks about like this:

GenericRecord record = new GenericData.Record(schema);

File file = new File("test.avro");
DatumWriter<GenericRecord> datumWriter = new GenericDatumWriter<GenericRecord>(schema);
DataFileWriter<GenericRecord> dataFileWriter = new DataFileWriter<GenericRecord>(datumWriter);
dataFileWriter.create(schema, file);
dataFileWriter.append(record);
dataFileWriter.close();

The problem I'm facing now is - what kind of Java object do I instantiate when I want to write Union? Not necessarily on the top level, possibly attach the union to a record being written. There are a few objects for complex types prepared, like GenericData.Record, GenericData.Array etc. For those that are not prepared, usually the right object is simply a standard Java object (java.util.Map implementing classes for "map" Avro type etc.).

But I cannot figure out what is the right object to instantiate for writing a Union.

This question refers to writing Avro file WITHOUT code generation. Any help is very much appreciated.

Answer 1

Here's what I did:

Suppose the schema is defined like this:

record MyStructure {
  ...
  record MySubtype {
    int p1;
  }
  union {null, MySubtype} myField = null;
  ...
}

And this is the Java code:

Schema schema; // the schema of the main structure
// ....
GenericRecord rec = new GenericData.Record(schema);
int i = schema.getField("myField").schema().getIndexNamed("MySubtype");
GenericRecord myField = new GenericData.Record(schema.getField("myField").schema().getTypes().get(i));
myField.put("p1", 100);
rec.put("myField", myField);