Maybe you know, maybe no – it’s about five years as I’m involved in the Apache Struts project. About two years ago I became a Struts 2 Lead – whatever it means in non-profit organisation working on an opensource project where everyone is equal 🙂 Nevertheless I like what I’m doing – code and refactor 🙂 And as there is a plenty of code in the project and most of them need to be improved – I have what to do for the next few years 😀

Recently I’ve started working on a new plugin – the MVEL plugin – which will allow to use MVEL instead of OGNL to handle expressions. In theory the task wasn’t too difficult, Struts 2 already provides many extension points where you can hook in with your code. Replacing expression engine require to implement two interfaces – very simple and straightforward.  After I did that, I have started testing many possible expression syntaxes which are already present in unit tests.

And then problems show off :/ The main problem is conversion, to give you a hint check the below expression which simple means array in both – MVEL and OGNL


but the problem is how both engines handle such statement (pseudo code):

  • MVEL:
    new Object[]{Integer, Integer, Integer}
  • OGNL:
    new ArrayList(){Integer, Integer, Integer}

As you can see the difference is slight but important – the wrapping object is totally different. And then goes conversion logic, as I wanted to get string on output, built-in conversion mechanism do this:

  • MVEL: [1]
  • OGNL: [1,2,3]

Not so good 😉 The problem is that the conversion logic is too narrowed to OGNL. After some internal brainstorming I’ve came up with a plan – to allow other expression engines to work with Struts 2, first I must decouple conversion logic and OGNL 🙂 So the plan is:

  • extract conversion mechanism to allow replacement of it (another extension point)
  • simplify current logic related to converters created by users
  • implement more common logic to allow handle wide list of results
  • implement MVEL specific conversion
  • implement MVEL plugin 🙂

As you see the plan is simple, but as it is with plans – it is good to prepare them, but to follow them is a different story 😉

Finally, I found something interesting in the latest newsletter from TED – something about attention 🙂