private void loadFromFile(String filePath) throws HiveException {
    set = new HashSet<String>();
    BufferedReader reader = getReaderFor(filePath);
    try {
      String line;
      while((line = reader.readLine()) != null) {
        set.add(line);
      }
    } catch (Exception e) {
      throw new HiveException(e);
    }
    finally {
      IOUtils.closeStream(reader);
    }
  }
IN_FILE(str, filename) returns true if 'str' appears in the file specified
by 'filename'.  A string is considered to be in the file if it that string
appears as a line in the file.

If either argument is NULL then NULL is returned.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFInFile.java