Jasper:  The Saga Continues.

Posted by Uncle Bob on 02/04/2007

It’s Sunday. Justin came in second in his wrestling tournament. And I downloaded the source code for Tomcat to figure out how the Jasper compiler works. sigh

I have an hour or so before Ann Marie and I drive to Angela’s house to visit our three granddaughters, then pick up some things at Sam’s club, and then I fly to Pennsylvania to kick off an Agile transition for a new customer.

Last night, after the tournament, I started studying how Jasper works. At the high level, Jasper is a fairly standard compiler that parses it’s input and creates an intermediate representation of nodes. It generates java output by walking those nodes with code-generating visitors.

At the low level Jasper was written by slinging a lot of brute force code around. There are long if/else chains, and funky variables like t1, t2, t3, and t4. For some reason the authors felt they should do their own parsing rather than using antlr or javacc; but that’s fine with me. JSP is a pretty simple syntax after all.

What I’ve learned so far is that there are quite a few ways for the compiler to ignore the taglib directive. I don’t understand yet what all those reasons are, I can just see the if statements that silently bypass that processing. Some are based on a variable named isDirective. This variable is passed down a long chain of calls from the upper levels of the compiler, down to the lower reaches of the generators. I still haven’t figured out why it gets set, but I think it might have something to do with a special compiler pass or something.

Anyway, I won’t bore you with the details right now. I’ll just see if I can learn anything more about how this thing works.

But I will say that it’s a shame that I have to dig through compiler code to figure out why Jasper generates code differently in the tomcat environment and the command-line environment.

Two hours later… While digging through the compiler code, I noticed that there are lots of nice debug logging messages. If I could turn on debug logging, I could get a sense about what’s happening inside the compiler.

So now all I have to do is turn on debug logging. That ought to be easy, right?...

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• • Criminal Records over 4 years later:

    • • This variable is passed down a long chain of calls from the upper levels of the compiler, down to the lower reaches of the generators. I still haven’t figured out why it gets set, but I think it might have something to do with a special compiler pass or something.