具有Avro的递归架构(SchemaBuilder) [英] Recursive schema with avro (SchemaBuilder)

问题描述

是否可以制作一个递归的avro模式，例如

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();

像这样使用它时，我得到一个StackOverflowError:

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();

注意:如果我使用以下方法创建架构，也会得到StackOverflowError

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

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

推荐答案

警告:我找到了一种写解决方案，但无法阅读:-\

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

我不确定是否真的了解，但是我设法使其适用于:

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

应该使用

1. ReflectDatumWriter代替SpecificDatumWriter

由于在编码时自动查找架构，我仍然找不到架构的问题.它查找具有自动派生的名称空间和名称的架构.在我的类是静态子类的情况下，应使用以下内容:

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() + "$";

要管理null，应使用以下架构

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();

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