How can I serialize a Java class via Hazelcast DataSerializable when it contains generic types?

Question

Can you please let me know what am I doing wrong here? I'm really struggling implementing a more efficient serializer for Hazelcast. I have the following Java class:

public class Tuple<X, Y> implements DataSerializable
{ 
    public X x;
    public Y y;

    private Tuple()
    {
    }

    public Tuple(X x, Y y)
    { 
        this.x = x; 
        this.y = y; 
    }

    @Override
    public void readData(ObjectDataInput in)
            throws IOException {
        Instance.log("read data");
        this.x = (X)in.readObject();
        this.y = (Y)in.readObject();
    }

    @Override
    public void writeData(ObjectDataOutput out)
            throws IOException {
        Instance.log("write data");
        out.writeObject(this.x);
        out.writeObject(this.y);
    }

}   

and I'm using it like so:

HazelcastInstance hazelcast = Hazelcast.newHazelcastInstance();
IMap<String, Set<Tuple<Integer, Double>>> myMap = Hazelcast.getMap("myMap");

however, whenever I'm attempting to use this map (except implementing Serializable, that scenario works fine) I'm getting back this exception:

com.hazelcast.nio.serialization.HazelcastSerializationException: java.io.NotSerializableException: Tuple

What am I doing wrong here? Appreciate any idea, thanks a million!

Update

Having tried the suggestion (thanks noctarius) I now have this:

public class TupleSetStreamSerializer
        implements StreamSerializer<Set<Tuple>> {

    @Override
    public int getTypeId () {
        return SerializationIdentifiers.TupleSerializationFactoryId;
    }

    @Override
    public void write(ObjectDataOutput out, final Set<Tuple> set )
            throws IOException {
        int size = set == null ? 0 : set.size();
        Instance.log("Serialize set");
        out.writeInt(size);
        if (size>0) {
            Iterator iterator = set.iterator();
            while (iterator.hasNext()) {
                out.writeObject(iterator.next());
            }
        }
    }

    @Override
    public Set<Tuple> read( ObjectDataInput in )
            throws IOException {

        Instance.log("Deserialize set");
        int size = in.readInt();
        Set result = null;
        if (size > 0) {
            result = new HashSet<Tuple>();
            for (int i = 0; i < size; i++) {
                result.add(in.readObject());
            }
        }
        return result;
    }

    @Override
    public void destroy () {
    }
}

and this also

public class TupleStreamSerializer
        implements StreamSerializer<Tuple> {

    @Override
    public int getTypeId () {
        return SerializationIdentifiers.TupleSerializationFactoryId;
    }

    @Override
    public void write(ObjectDataOutput out, Tuple tup )
            throws IOException {
        out.writeObject(tup.x);
        out.writeObject(tup.y);
    }

    @Override
    public Tuple read( ObjectDataInput in )
        throws IOException {
        Object x = in.readObject();
        Object y = in.readObject();
        return new Tuple(x,y);
    }

    @Override
    public void destroy () {
    }
}

with a configuration like this (attempted commenting various variants and all)

<hazelcast>
    <serialization>
        <serializers>
            <serializer type-class="Tuple">TupleStreamSerializer</serializer>
            <serializer type-class="Set<Tuple>">TupleSetStreamSerializer</serializer>
        </serializers>
    </serialization>
</hazelcast>

I still seem to be missing something? Thanks a lot!

Answer 1

You have to write a special StreamSerializer for the Set interface. Whenever Hazelcast starts to serialize a Serializable marked class it looses serialization control. There are a few workarounds but all of them are not really nice. The best way is to intercept serialization of Set using Serializable and offer a custom serializer: http://docs.hazelcast.org/docs/3.5/manual/html-single/index.html#streamserializer

You'll find an example for the LinkedList type inside the Hazelcast core code: https://github.com/hazelcast/hazelcast/blob/master/hazelcast/src/main/java/com/hazelcast/internal/serialization/impl/LinkedListStreamSerializer.java