Bytecode generation, Source code mappings, JCov, Future

Alex Rau alex at
Wed Apr 16 16:47:23 PDT 2008

On 17.04.2008, at 00:12, Jonathan Gibbons wrote:

> 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. :-(

What exactly is the purpose of jcov in current JDK's ? The coverage  
feature itself has been superseded by JVMTI while the jcov  
implementation still exists, is that correct ? Legacy coverage tools  
(at least I know) rely anyway on line information exclusively and are  
therefore not interested in column information or offsets. So what  
use case is left for JCov ? Am I correct, that it just exists more or  
less as an artificial piece of code from past times without a real  
purpose ?

Well. Actually I think the implementation deserves more attention  
from the compiler's point of view. JVMTI and JVMPI focus on making  
the state of the VM (during runtime) more visible. JCov in it's  
current implementation makes the compiler operation (in a different  
way via byte code attributes) visible and that's perfectly fine in my  
opinion and useful for tools which focus on static information on  
class files (independently what happens during runtime).

It's however open whether the compiler operation information is  
really required to be stored in the byte code. Another option would  
be to create a single file or one file for each class which  
exclusively contains the information. These files could be enhanced  
with other information from the compiler execution as well (what ever  
that could be) - some kind of standard compiler tracing output.

Due to my personal interest I'd love to work on that feature. If it  
all goes well and it's getting along in a way which is useful for  
general purpose - perfect.

> 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.

I agree.  I think - given that javac itself  uses (as you mentioned)  
a single offset value it would make sense to stick with it and use  
this value for generating appropriate byte code including these  
offset attributes. Tools using these values in general have access to  
the source code anyway as most often the code is pretty printed in  
HTML or something else. So mapping a single file offset to a certain  
line + offset combination would be easily possible during parsing the  
source code for such tools.

So in case the usefulness of properly tracing above mentioned  
information is common sense - what would be the best way ? Here some  

- interleaved output with byte code vs. separate output (file) interface
- replace/reuse/adapt jcov code vs. keep jcov as it is and implement  
new stuff from scratch in parallel (compatibility?)
- absolute file offsets as you mentioned vs. something else ?



> -- 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 - http:// 
>> 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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