private static void applyDefaultNonNullConfVars(Configuration conf) {
    for (ConfVars var : ConfVars.values()) {
      String defaultValue = var.getDefaultValue();
      if (defaultValue == null) {
        // Don't override ConfVars with null values
        continue;
      }
      conf.set(var.varname, defaultValue);
    }
  }
Overlays ConfVar properties with non-null values
common/src/java/org/apache/hadoop/hive/conf/HiveConf.java