ClassCastException When Writing a Map After Previously Reading a Map with Different Generic Type

[jlmorton]

While reading an object with a generic type map, Kryo will create a MapSerializer and set the generic type on the serializer. When the FieldSerializer encounters a reference to the same map, it will again set the generic types of the MapSerializer, but will return without re-setting the generic types.

After this, a subsequent request to serialize a map with different generic types different will lead to a ClassCastException, as demonstrated in the test case below:

@Test
public void testSerializeMapAfterDeserialization() throws IOException {
    Kryo kryo = new Kryo();

    byte[] serializedMapHolder = serialize(kryo, new MapHolder());
    deserialize(kryo, serializedMapHolder);

    Map<Integer, List<String>> stringMap = new HashMap<>();
    stringMap.put(1, new ArrayList<>());
    serialize(kryo, stringMap);
}

static class MapHolder {
    private Map<Integer, String> mapOne = new HashMap<>();
    private Map<Integer, String> mapTwo = this.mapOne;
}

private byte[] serialize(Kryo kryo, Object object) throws IOException {
    try (ByteArrayOutputStream outputStream = new ByteArrayOutputStream(); Output output = new Output(outputStream)) {
        kryo.writeClassAndObject(output, object);
        output.flush();
        return outputStream.toByteArray();
    }
}

private Object deserialize(Kryo kryo, byte[] serializedBytes) throws IOException {
    try (ByteArrayInputStream inputStream = new ByteArrayInputStream(serializedBytes); Input input = new Input(inputStream)) {
        return kryo.readClassAndObject(input);
    }
}

This test case results in the following error:

java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.String

at com.esotericsoftware.kryo.serializers.DefaultSerializers$StringSerializer.write(DefaultSerializers.java:184)
at com.esotericsoftware.kryo.Kryo.writeObjectOrNull(Kryo.java:603)
at com.esotericsoftware.kryo.serializers.MapSerializer.write(MapSerializer.java:109)
at com.esotericsoftware.kryo.serializers.MapSerializer.write(MapSerializer.java:39)
at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:625)
at com.test.MapSerializerTest.serialize(MapSerializerTest.java:36)
at com.test.MapSerializerTest.testSerializeMapAfterDeserialization(MapSerializerTest.java:26)

[magro]

Resolved with #433