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Thread: Modified Java Script Value > execution of a function only one time, not once per row

  1. #1

    Default Modified Java Script Value > execution of a function only one time, not once per row

    Hi all,

    I'm wondering if it's possible to execute a javascript function (snippet of code into a Modified Java Script Value) one time, before processing first row and not once per each row. In fact in the Mod Java Script Value Step I initialize a cryptographic key and a mac (Message authentication code) provider, but I'd like to reuse this provider instance and not to instantiate per each row.
    There is a chance to achieve this goal?

    Or maybe there are other strategies, maybe I could use a type of step to load into a buffer same data... Or I could write a Java class with "Singleton pattern"... Any hint would be appreciated.

    Thank you.
    Last edited by gianpieropiccolo; 04-04-2016 at 10:18 AM.

  2. #2
    Join Date
    Aug 2011



    Just add a new script (new tab), right click on the tab and choose "Start script".
    Then it will be executed once at transformation initialisation. Js variables created there
    are then accessible in your main script.
    That a good place to put custom function (even if they are ised on each rows!) and constant variables.

    If you need also to close ressources at the end of transformation, create another script tab, right click, and choose
    "End script". It will be called once after the step has processed every rows.

  3. #3


    Thank you very much!
    Thanks to your suggestion, I googled for "Startup script" in Pentaho and found other optimization hints here: (it includes your suggestion).


  4. #4
    Join Date
    Aug 2011



    And the best advice is dont use javascript unless your are forced to!
    It is really slowlier than other steps. Some times some java user expression can do the trick.
    For big business complicated logic, the best is:
    - write a java library that do the hard work. Create some classes (beans) with getters and setters that are pentaho-type-friendly
    (Long, Double, Boolean, String, Date, Byte)
    - use the user defined java class to wire the input/output rows of pentaho woth your library.

    You'll get something reaaallyyy efficient!

  5. #5


    Thank you for your advice. I thought that the javascript were compiled in java bytecode, some sort of pre-compilation in java source and then java compilation in bytecode. This idea came to me when I got an error about bytecode exceeding 64K limit. Maybe the javascript is embedded into a java class? What is the optimization level, indeed? what version of ECMA script is used?
    Last edited by gianpieropiccolo; 04-06-2016 at 06:50 AM.

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