Can't serialize generic class with generic field in Kryo

Question

I am running into a strange issue with Kryo. I can serialize a generic class (such as List) just fine. I can even serialize a generic class that has no generic fields. However, if I serialize a generic class with generic fields it is failing. An example should show the problem.

Here is the class I want to serialize:

import java.util.ArrayList;

public class GenericClass<T>
{
    private final ArrayList<T> list;

    public GenericClass()
    {
        list = new ArrayList<T>();
    }
}

And here is the code that tests it. The second call to writeClassAndObject() throws the exception. The ArrayList write is simply to prove that works just fine.

public void testWhySoDumb()
{
    File s = Environment.getExternalStorageDirectory();
    String dir = s.getAbsolutePath() + "/pagetest";
    String fileLocation = dir + "/filetest";
    new File(dir).mkdirs();
    Output output = null;

    Kryo kryo = new Kryo();
    kryo.setAsmEnabled(true);
    kryo.setDefaultSerializer(CompatibleFieldSerializer.class);

    // Test writing ArrayList (this works)
    List<Integer> arrayList = new ArrayList<Integer>();

    try
    {
        output = new Output(new FileOutputStream(fileLocation));
        kryo.writeClassAndObject(output, arrayList);
    }
    catch (Exception e)
    {
        System.out.println(e.getMessage()); // No problem here
    }
    finally
    {
        if (output != null)
        {
            output.close();
        }
    }

    // Test writing GenericClass (this does NOT work)
    GenericClass<Integer> genericClass = new GenericClass<Integer>();

    try
    {
        output = new Output(new FileOutputStream(fileLocation));
        kryo.writeClassAndObject(output, genericClass); // throws exception!!! <------
    }
    catch (Exception e)
    {
        System.out.println(e.getMessage()); 
    }
    finally
    {
        if (output != null)
        {
            output.close();
        }
    }
}

And I am getting the following exception:

com.esotericsoftware.kryo.KryoException: java.lang.IllegalArgumentException: type cannot be null.
Serialization trace:
list (com.myapp.page.history.GenericClass)
at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:82)
at com.esotericsoftware.kryo.serializers.CompatibleFieldSerializer.write(CompatibleFieldSerializer.java:42)
at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:624)
at com.myapp.page.history.TestGenerics.testWhySoDumb(TestGenerics.java:57)
at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:191)
at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:176)
at android.test.InstrumentationTestRunner.onStart(InstrumentationTestRunner.java:555)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1837)
Caused by: java.lang.IllegalArgumentException: type cannot be null.
at com.esotericsoftware.kryo.Kryo.isFinal(Kryo.java:1135)
at com.esotericsoftware.kryo.serializers.CollectionSerializer.setGenerics(CollectionSerializer.java:60)
at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:60)
... 15 more

What is very odd is that if I change the type of "list" from "ArrayList" to just "T", it works fine. Kryo doesn't seem to know what to do with the ArrayList field. I am using Kryo version 3.0.0.

Thanks!

Answer 1

I don't know if you still have this problem. I would probably think that this is a version problem or something specific to Android because this should work with the current version.

Kryo kryo = new Kryo();
Output output = new Output(new FileOutputStream("fileLocation"));
kryo.writeObject(output, new GenericClass<String>());