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https://issues.apache.org/jira/browse/AVRO-1562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14125063#comment-14125063
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Martin Kleppmann commented on AVRO-1562:
----------------------------------------
bq. I am not convinced it's worthwhile to get Avro reflect to correctly model
every Java datastructure.
+1. There is significant cost to every spec-level feature in Avro, because it
needs to be implemented in every supported language. We already have some
language implementations that don't support all features in the spec, but I'd
say we should be trying to keep the number of such incompatibilities as small
as possible.
> Add support for types extending Maps/Collections
> ------------------------------------------------
>
> Key: AVRO-1562
> URL: https://issues.apache.org/jira/browse/AVRO-1562
> Project: Avro
> Issue Type: Bug
> Affects Versions: 1.7.6
> Reporter: Sachin Goyal
> Attachments: custom_map_and_collections1.patch
>
>
> Consider the following code:
> {code}
> import java.io.ByteArrayOutputStream;
> import java.util.*;
> import org.apache.avro.Schema;
> import org.apache.avro.file.DataFileWriter;
> import org.apache.avro.reflect.ReflectData;
> import org.apache.avro.reflect.ReflectDatumWriter;
> public class AvroDerivingMaps
> {
> public static void main (String [] args) throws Exception
> {
> MapDerivedContainer orig = new MapDerivedContainer();
> ReflectData rdata = ReflectData.AllowNull.get();
> Schema schema = rdata.getSchema(MapDerivedContainer.class);
> System.out.println(schema);
>
> ReflectDatumWriter<MapDerivedContainer> datumWriter = new
> ReflectDatumWriter (MapDerivedContainer.class, rdata);
> DataFileWriter<MapDerivedContainer> fileWriter = new
> DataFileWriter<MapDerivedContainer> (datumWriter);
> ByteArrayOutputStream baos = new ByteArrayOutputStream();
> fileWriter.create(schema, baos);
> fileWriter.append(orig);
> fileWriter.close();
> }
> }
> class MapDerived extends HashMap<String, Integer>
> {
> Integer a = 1;
> String b = "b";
> }
> class MapDerivedContainer
> {
> MapDerived2 map = new MapDerived2();
> }
> class MapDerived2 extends MapDerived
> {
> String c = "c";
> }
> {code}
> \\
> \\
> It throws the following exception:
> {code:javascript}
> {"type":"record","name":"MapDerivedContainer","namespace":"avro","fields":[{"name":"map","type":["null",{"type":"record","name":"MapDerived2","fields":[{"name":"c","type":["null","string"],"default":null},{"name":"a","type":["null","int"],"default":null},{"name":"b","type":["null","string"],"default":null}]}],"default":null}]}
> {code}
> {color:brown}
> Exception in thread "main"
> org.apache.avro.file.DataFileWriter$AppendWriteException:
> org.apache.avro.UnresolvedUnionException:
> Caused by: org.apache.avro.UnresolvedUnionException: Not in union
> ["null",{"type":"record","name":"MapDerived2","namespace":"avro","fields":[{"name":"c","type":["null","string"],"default":null},{"name":"a","type":["null","int"],"default":null},{"name":"b","type":["null","string"],"default":null}]}]:
> {}
> at
> org.apache.avro.generic.GenericData.resolveUnion(GenericData.java:600)
> at
> org.apache.avro.generic.GenericDatumWriter.resolveUnion(GenericDatumWriter.java:151)
> at
> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:71)
> at
> org.apache.avro.reflect.ReflectDatumWriter.write(ReflectDatumWriter.java:145)
> at
> org.apache.avro.generic.GenericDatumWriter.writeField(GenericDatumWriter.java:114)
> at
> org.apache.avro.reflect.ReflectDatumWriter.writeField(ReflectDatumWriter.java:203)
> at
> org.apache.avro.generic.GenericDatumWriter.writeRecord(GenericDatumWriter.java:104)
> at
> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:66)
> at
> org.apache.avro.reflect.ReflectDatumWriter.write(ReflectDatumWriter.java:145)
> at
> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:58)
> at org.apache.avro.file.DataFileWriter.append(DataFileWriter.java:290)
> ... 1 more
> {color}
> \\
> \\
> It appears that ReflectData#createSchema() checks for "type instanceof
> ParameterizedType" and because of this, it skips handling of the map.
> The same is not true of GenericData#isMap() and GenericData#resolveUnion()
> fails because of this.
> The same may be true for classes extending ArrayList, Collection, Set etc.
> Also, note the schema for the class extending Map:
> {code:javascript}
> {
> "type":"record",
> "name":"MapDerived2",
> "fields":[
> {
> "name":"c",
> "type":[
> "null",
> "string"
> ],
> "default":null
> },
> {
> "name":"a",
> "type":[
> "null",
> "int"
> ],
> "default":null
> },
> {
> "name":"b",
> "type":[
> "null",
> "string"
> ],
> "default":null
> }
> ]
> }
> {code}
> This schema ignores the Map completely.
> Probably, for such a class, the schema should look like:
> {code:javascript}
> {
> "type":"record",
> "name":"MapDerived2",
> "fields":[
> {
> "name":"c",
> "type":[
> "null",
> "string"
> ],
> "default":null
> },
> .... // Other fields in the class extending the Map
> {
> "name":"BASE_MAP",
> "type":[
> "null",
> "map" ... // Normal map which the class extends (implements?)
> ],
> "default":null
> }
> }
> {code}
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