Recursive schema with avro (SchemaBuilder)

问题

Is it possible to make an avro schema which is recursive, like

Schema schema = SchemaBuilder
    .record("RecursiveItem")
    .namespace("com.example")
    .fields()
    .name("subItem")
    .type("RecursiveItem")
    .withDefault(null) // not sure about that too...
    .endRecord();

I get a StackOverflowError when using it like that:

static class RecursiveItem {
  RecursiveItem subItem;
}

RecursiveItem item1 = new RecursiveItem();
RecursiveItem item2 = new RecursiveItem();
item1.subItem = item2;

final DatumWriter<RecursiveItem> writer = new SpecificDatumWriter<>(schema);

// note: I actually want a binary output, but I started with some json code I found
ByteArrayOutputStream stream = new ByteArrayOutputStream();
final JsonEncoder encoder = EncoderFactory.get().jsonEncoder(schema, stream);
writer.write(rec1, encoder);
String json = stream.toString();

Note: I also get StackOverflowError if I make the schema using:

Schema schema = ReflectData.get().getSchema(RecursiveItem.class);

回答1:

Warning: I found a solution to write, but cannot read it bask :-\

I am not sure to really understand, but I manage to make it works with:

1. ReflectDatumWriter should be used instead of SpecificDatumWriter

2. I still had issue with schema not found due to automated lookup of schema upon encoding. It looks for a schema with a namespace+name derived automatically. And in my case where the class is a static subclass, the following should be used:

   String cls = RecursiveItem.class.getSimpleName();
   String pck = RecursiveItem.class.getPackage().getName();
   if (RecursiveItem.class.getEnclosingClass() != null) // nested class
     pck = RecursiveItem.class.getEnclosingClass().getName() + "$";
   

3. To manage the null, the following schema should be used

   Schema schema0 = SchemaBuilder
     .record(cls)
     .namespace(pck)
     .fields()
     .name("subItem")
     .type().unionOf().nullType().and().type("RecursiveItem").endUnion()
     .nullDefault()
     .endRecord();
   

来源：https://stackoverflow.com/questions/56681506/recursive-schema-with-avro-schemabuilder