Bytecode generation, Source code mappings, JCov, Future

Alex Rau alex at
Wed Apr 16 14:17:19 PDT 2008

Hi all,

I've searched the web and asked at but my answer  
regarding the javac compiler couldn't be answered yet. so here we go  
with a rational first:

Rational: I'd like to achieve the following: The byte code created  
with javac should not only contain line number information in debug  
mode, but additionally I want to track exactly which statement  
(including column information as there can be multiple statements in  
one source code line) leads to a certain bytecode instruction or a  
set of instructions. I strongly need this kind of functionality in  
javac as the project I'm working on is settled in the testing area  
(mutation testing in java on bytecode level  which you can imagine as  
some kind of (beta) code coverage software with a different and IMHO  
superior technique - for more details if you  
are interested). The software must be able to report mutated  
*statements* which means that line informations are insufficient.

My Question is: in older JDK's there was something called JCov which  
enhanced byte code with additional information regarding which  
statements finally led to a/multiple bytecode instruction(s). I  
stumbled upon this while debugging javac when I wanted to learn it's  
design/code. I think it's mostly what I need - however the JCov  
switch is a hidden feature. It was "more" public in JDK 1.2 and was  
pretended to have been ported to the JVMPI interface later. However I  
did not find where the port should have been integrated - I'm no  
JVMPI (and JVMTI) guru. Honestly I doubt that something similar is in  
the JVM(P/T)I toolset...

So perhaps someone has more knowledge about JCov and what happened to  
it. Furthermore I'd like to push a discussion on whether this would  
be a candidate for the kitchen sink (no JLS changes, just internal)  
with the goal to implement the above described functionality  
*including a supported and public interface* (not a hidden feature  
anymore). The first benefit could then obviously be that javac could  
be mutation tested ;) Furthermore other Java developers would have  
solid information and documentation about this API instead of relying  
on mostly unknown and hidden features.

Best Regards,


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