Avro19Test.java

import org.apache.avro.Schema;
import org.apache.avro.io.ResolvingDecoder;

import java.io.File;
import java.io.IOException;

public class Avro19Test {
    public static void main(String[] argc) throws IOException {
        Schema reader = new Schema.Parser().parse(new File("reader"));
        Schema writer = new Schema.Parser().parse(new File("writer"));
        ResolvingDecoder.resolve(writer, reader);
    }
}

reader

{
  "fields": [ {
    "default": null,
      "name": "r1",
      "type": [ "null", {
        "items": {
          "fields": [ {  "name": "num1", "type": "int" } ],
          "name": "a1",
          "namespace": "n1",
          "type": "record"
        },
        "type": "array"
      } ]
  } ],
  "name": "f1",
  "type": "record"
}

result.txt

checking 1.9.0
Exception in thread "main" java.lang.ClassCastException: org.apache.avro.Resolver$ReaderUnion cannot be cast to org.apache.avro.Resolver$Container
	at org.apache.avro.io.parsing.ResolvingGrammarGenerator.generate(ResolvingGrammarGenerator.java:95)
	at org.apache.avro.io.parsing.ResolvingGrammarGenerator.generate(ResolvingGrammarGenerator.java:110)
	at org.apache.avro.io.parsing.ResolvingGrammarGenerator.generate(ResolvingGrammarGenerator.java:141)
	at org.apache.avro.io.parsing.ResolvingGrammarGenerator.generate(ResolvingGrammarGenerator.java:65)
	at org.apache.avro.io.ResolvingDecoder.resolve(ResolvingDecoder.java:85)
	at Avro19Test.main(Avro19Test.java:11)
status=1
checking 1.8.2
status=0

run.sh

#!/bin/bash

mkdir -p 1.8.2 1.9.0
wget --quiet 'https://repo1.maven.org/maven2/org/apache/avro/avro-tools/1.8.2/avro-tools-1.8.2.jar' -O 1.8.2/avro-tools.jar
wget --quiet 'https://repo1.maven.org/maven2/org/apache/avro/avro-tools/1.9.0/avro-tools-1.9.0.jar' -O 1.9.0/avro-tools.jar
echo checking 1.9.0
javac -cp 1.9.0/* Avro19Test.java
java -cp 1.9.0/*:. Avro19Test
echo status=$?
echo checking 1.8.2
javac -cp 1.8.2/* Avro19Test.java
java -cp 1.8.2/*:. Avro19Test
echo status=$?

writer

{
  "fields": [ {
    "default": null,
      "name": "r1",
      "type": [ "null", {
        "items": {
          "fields": [ {  "name": "num1", "type": "int" } ],
          "name": "a1",
          "namespace": "n2",
          "type": "record"
        },
        "type": "array"
      } ]
  } ],
  "name": "f1",
  "type": "record"
}