[AVRO-1865] GenericData.Array class missing no arg constructor for Kryo serialization

• Type: Bug
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.8.1
• Component/s: java
• Labels: beginner patch
• Assignee: Alexander Kasper
• Reporter: Alexander Kasper
• Created: 20/Jun/16 15:35
• Updated: 18/Aug/17 12:57

Description

When trying to serialize Java classes generated by Avro that contain a field as follows:
array<MyClass> myclasses = []

serialization fails with the following error message
com.esotericsoftware.kryo.KryoException: Class cannot be created (missing no-arg constructor)

Minimum example to recreate this:
Avro IDL definition:

@namespace("com.adello")
protocol KryoTest {
	record TestRecord {
		array<int> values = [];
	}
}

This gets compiled using java -jar avro-tools-1.8.1.jar idl2schemata kryo.avdl .
Resulting Avro schema:

{
  "type" : "record",
  "name" : "TestRecord",
  "namespace" : "com.adello",
  "fields" : [ {
    "name" : "values",
    "type" : {
      "type" : "array",
      "items" : "int"
    },
    "default" : [ ]
  } ]
}

Generate Java class using java -jar avro-tools-1.8.1.jar compile schema TestRecord.avsc

Kryo serialization test code:

Kryo kryo = new Kryo();
try {
            Output output = new Output(new FileOutputStream("test.bin"));
            TestRecord o = TestRecord.newBuilder().build();
            System.out.println("Before serialization: ");
            System.out.println(o.toString());
            kryo.writeObject(output, o);
            output.close();

            Input input = new Input(new FileInputStream("test.bin"));
            TestRecord i = kryo.readObject(input, TestRecord.class);
            input.close();
            System.out.println("After deserialization: ");
            System.out.println(i.toString());

            System.out.println(Objects.deepEquals(o, i));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

This is a common enough issue with Kryo that they have a dedicated section on their website on it: http://docs.datatorrent.com/troubleshooting/#application-throwing-following-kryo-exception

The attached patch adds a no args constructor for the inner class Array of GenericData with zero entries and a dummy schema. My personal tests for serialization with Kryo were successful when using it. Since I do not have complete insight into Avro I'd like to know if this could be a breaking change and how to test it if so.