@Override
  public String[] getRequiredFiles() {
    return new String[] {
        ObjectInspectorUtils.getWritableConstantValue(fileObjectInspector)
            .toString()
    };
  }
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