private static Class<? extends GenericUDF> getGenericUDFClassFromExprDesc(ExprNodeDesc desc) {
    if (!(desc instanceof ExprNodeGenericFuncDesc)) {
      return null;
    }
    ExprNodeGenericFuncDesc genericFuncDesc = (ExprNodeGenericFuncDesc) desc;
    return genericFuncDesc.getGenericUDF().getClass();
  }
Get the UDF class from an exprNodeDesc. Returns null if the exprNodeDesc
does not contain a UDF class.
ql/src/java/org/apache/hadoop/hive/ql/exec/FunctionRegistry.java