这有票和公关
https://issues.apache.org/jira/browse/kafka-4794
目前，我设法使用反射（joor）获得offsetreader：

private OffsetStorageReader getOffsetStorageReader() {

    try {
        Object innerContext = on(context).get("this$0");
        Object ctx = on(innerContext).get("ctx");

        Object herder = on(ctx).get("herder");
        String connectorName = on(ctx).get("connectorName");
        Object worker = on(herder).get("worker");
        Object internalKeyConverter = on(worker).get("internalKeyConverter");
        Object internalValueConverter = on(worker).get("internalValueConverter");
        Object offsetBackingStore = on(worker).get("offsetBackingStore");

        return (OffsetStorageReader)
            on(Class.forName("org.apache.kafka.connect.storage.OffsetStorageReaderImpl"))
                .create(offsetBackingStore, connectorName, internalKeyConverter, internalValueConverter)
                .get();

    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}