private Class<? extends UDF> getUdfClassInternal()
      throws ClassNotFoundException {
    @SuppressWarnings("unchecked")
    Class<? extends UDF> clazz = (Class<? extends UDF>) Class.forName(
        udfClassName, true, Utilities.getSessionSpecifiedClassLoader());
    if (udfChecker != null && !udfChecker.isUdfAllowed(clazz)) {
      throw new SecurityException("UDF " + clazz.getCanonicalName() + " is not allowed");
    }
    return clazz;
  }
Gets the UDF class and checks it against the whitelist, if any.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFBridge.java