Bytecode generation, Source code mappings, JCov, Future

Jonathan Gibbons Jonathan.Gibbons at Sun.COM
Wed Apr 16 15:12:06 PDT 2008


The features for jcov support are still in the compiler.  The switch is 
-Xjcov, it causes a CharacterRangeTable attribute to be added to the 
class file.  The format of the attribute is not great -- in particular, 
it uses the old "packed line/offset" format for coordinates, which may 
not work for very long lines (over 1024 characters) that sometimes occur 
in mechanically generated code. Internally, javac now uses a simple 
character offset from the beginning of the file to define a source 
position. It would arguably be good to evolve the CharacterRangeTable 
attribute (in a compatible way) to using character offsets.

Jcov is still around, though it has never really got the care and 
attention it merited.  Every now and then, there is interest in making 
it more available. It generally comes down to lack of demand and 
internal resources. :-(

-- Jon

Alex Rau wrote:
> 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,
>  Alex

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