@Override
    public <O extends OutputShim> void write(final KryoShim<?, O> kryo, final O output, final VertexWritable vertexWritable) {
        kryo.writeObject(output, vertexWritable.get().graph());
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
spark-gremlin/src/main/java/org/apache/tinkerpop/gremlin/spark/structure/io/gryo/VertexWritableSerializer.java