Recover Code Lines from Probes

[Tom G]

Hi,

if I have a probe boolean array and the name of the class. Is there some simple manual way to map a hit probe to the corresponding source line?

Regards,

Tom

[Marc Hoffmann]

Hi Tom,

as “simple” as it is described here: https://www.jacoco.org/jacoco/trunk/doc/flow.html

In short: You need the underlying class files to interpret the probes. This is exactly what the JaCoCo analyzer does.

Regards,

-marc

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[Tom G]

Thanks for the quick response :)

Is it possible to recover it without relying on the class files?

E.g. if I only have the source code, theoretically, I know where the branches are (statically analyzing and identifying IFs, Switches, Exceptions, ...), then I also thereotically should know where the probes have been inserted right?

[Marc Hoffmann]

Is it possible to recover it without relying on the class files?

No, JaCoCo is based on class files.

E.g. if I only have the source code, theoretically, I know where the branches are (statically analyzing and identifying IFs, Switches, Exceptions, ...), then I also thereotically should know where the probes have been inserted right?

You can compile these class files given that you use the exact same build chain. Please see https://www.jacoco.org/jacoco/trunk/doc/classids.html

JaCoCo tracks execution with so called probes. Probes are additional byte code instructions inserted in the original class file which will note when they are executed and report this to the JaCoCo runtime. This process is called instrumentation. To keep the runtime overhead minimal, only a few probes are inserted at "strategic" places. These probe positions are determined by analyzing the control flow of all methods of a class. As a result every instrumented class produces a list of n boolean flags indicating whether the probe has been executed or not. A JaCoCo *.exec file simply stores a boolean array per class id.

At analysis time, for example for report generation, the *.exec file is used to get information about probe execution status. But as probes are stored in a plain boolean array there is no information like corresponding methods or lines. To retrieve this information we need the original class files and perform the exact same control flow analysis than at instrumentation time. Because this is a deterministic process we get the same probe positions. With this information we can now interfere the execution status of every single instruction and branch of a method. Using the debug information embedded in the class files we can also calculate line coverage.

If we would use just slightly different classes at analysis time than at runtime — e.g. different method ordering or additional branches — we would end-up with different probes. For example the probe at index i would be in method a() and not in method b(). Obviously this will create random coverage results. 

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