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executing java expression from a text file!

Mike Mass

Joined: Aug 13, 2004
Posts: 12
Hi all!

This might be far fetched, but i really need to know if its possible to read a java expression from a text file and execute it.
Example : width = imgwidth - 100;
Where the variables r defined and known by the class that read this line.

Thanks in advance!
Kai Witte
Ranch Hand

Joined: Jul 17, 2004
Posts: 356

that is possible. In the general case it is quite hard. For a simple construct like your example there is an easy way, however:
  • read the file as a properties file using java.util.Properties
  • get the property "width"
  • replace imgwidth by x using String#replaceAll
  • use jfunction to evaluate the resulting String: width=Terms.evaluate(resultingString, imgwidth);

  • Kai

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    Michael Lloyd Lee

    Joined: Sep 07, 2005
    Posts: 22

    Please please please use code tags!<br /> <br /><a href="" target="_blank" rel="nofollow">Java API</a> - <a href="" target="_blank" rel="nofollow">Java Tutorials</a>
    Ulf Dittmer

    Joined: Mar 22, 2005
    Posts: 42959
    There are a number of libraries that allow you to do that. Google for JEP or JEL. JEP builds an expression tree and interprets it using values, which makes it a bit slow. JEL compiles the expression, which is much faster, but can be tricky to handle to situations where you have multiple classloaders. For the ultimate Java integration, look for Javassist, which lets you build your own Java classes from Strings of code - very cool.
    Mike Mass

    Joined: Aug 13, 2004
    Posts: 12
    I would like ot thank each of you for your contribution, particularly Mr Dittmer. JEP is the answer to what i have in mind, since i will integrate it to an ant task, speed is not an issue.

    Thanks again guys,

    [ September 08, 2005: Message edited by: Mike Mass ]
    It is sorta covered in the JavaRanch Style Guide.
    subject: executing java expression from a text file!
    It's not a secret anymore!