@SuppressWarnings("deprecation")
  public static MapJoinKey read(Output output, MapJoinObjectSerDeContext context,
      Writable writable) throws SerDeException, HiveException {
    AbstractSerDe serde = context.getSerDe();
    Object obj = serde.deserialize(writable);
    MapJoinKeyObject result = new MapJoinKeyObject();
    result.read(serde.getObjectInspector(), obj);
    return result;
  }
The base class for MapJoinKey.
Ideally, this should now be removed, some table wrappers have no key object.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinKey.java