Fast class reloading

Igor Adam Klimer 150043 at
Tue Jun 1 01:13:20 PDT 2010

Thank you for the reply, David.

I think updating existing instances won't be a problem - because we don't care for them :) Once we tested a particular mutation, we no longer need it and won't reference it again, so I think it doesn't matter if that instance gets into a "dirty" state. We want to "discard" that instance, reload the class with a new mutation and test the new instance. Am I thinking about this right or am I missing something? (of course, this is definitely not a safe approach, but some sacrifices are need for the speed gain we want to achieve)

About the modified JVM: would you reckon that the idea of using a Java Agent to modify the JVM "on the fly" would work (if it even makes sense :)), or would the extent of changes needed to be made to the JVM would be so large (or just beyond the capabilities of a Java Agent) that a custom build would be required? I understand that maintaning a custom VM would be easier than "patching" via a Java Agent, but the latter would be more user friendly. BTW, thanks for the tip about building custom VM from the OpenJDK sources :)

Best regards,
Igor Klimer

> Hi Igor,
> Gut feeling: you need a modified JVM to do this. If you only 
> want to add new methods then a mechanism for expanding the 
> existing vtable and codecache etc of the loaded class might not 
> be too hard. Adding fields is much harder as all existing 
> objects need to be modified in place to have the new field (or 
> else you do some serious augmentation to allow missing fields to 
> be trapped and handled in a different manner). Dealing with 
> existing instances of a class is what makes general hot-swapping 
> very difficult.
> By the way you can build any kind of custom VM you want from the 
> OpenJDK sources.
> Cheers,
> David Holmes
> Igor Adam Klimer said the following on 06/01/10 10:37:
> >Dear all,
> >my name is Igor Klimer, I'm a 5th year student at the Wroclaw 
> University of Technology [1]. For my Master of Science thesis, 
> I'm working on improving the performance of a mutation testing 
> tool my colleagues are working on.
> >
> >Long story short - for each class you want to test, you end up 
> with hundreds (+/- it depends on the number of mutation 
> operators enabled) of "mutants" - slightly modified (by a 
> mutation operator) original class. This mutations (one mutation 
> at a time, in one place of the class at a time) range from 
> simple arithmetic operator changes ("-" becomes "+") to changing 
> method signature. The problem is that each of this mutation is 
> just another version of the same class, so I have to load each 
> of them via a separate ClassLoader - which has a very 
> significant impact on the overall testing process.
> >
> >So my main job is to make the class (re)loading faster or even 
> better (?) take it out of the equation. Unfortunately, Java 
> doesn't support [2] dynamic class reloading which makes this 
> problem tougher (and more interesting ;)). I've looked into a 
> number of possible solutions:
> >- HotSwap [3] offers great speed, but it can only modify the 
> body of a method. While that would speed up some/most of the 
> mutation operators, the *most* interesting ones, the object-
> oriented ones, would be impossible with this method
> >- many solutions involving proxy classes [4], etc. - while they 
> offer great flexibility, the speed penalty of modifying such a 
> number of classes would probably nullify any speed improvements 
> over normal classloading
> >- creating a custom JVM - while this solution seems the most 
> elegant one (and offering the greatest possibilities), I'm 
> afraid it would void any chances of thie mutation testing tool 
> becoming a popular tool/practice. Another reason against this 
> idea is that, AFAIK, Sun does not allow creating custom JVM's 
> based on their code. A possible solution (and usually preferred 
> for research purposes) is to use the JikesRVM [5] - which is 
> very "research-friendly". However, since speed and compatibility 
> is our goal, JikesRVM is also not acceptable.
> >
> >One solution I came up with is to use a Java Agent [3] to 
> modify some key elements of the JVM so that class reloading 
> would be possible (and didn't have the restrictions of HotSwap). 
> While I understand that it's harder than it sounds, I believe 
> (more like: I pray :)) it is doable - of course, this solution 
> would be highly customized to our needs/situation - for example, 
> we can be sure that once a class/mutant is tested it can be 
> safely discarded/reloaded, we're sure no other part of the 
> program is going to reference it. It might come out a bit 
> "hackish", but it's the best I could come up with.
> >
> >So my question to you, dear Hotspot developers, is what are 
> your thoughts on this? Just your "gut feeling" is ok - does my 
> idea seem feasible? Any advice where to start hacking the JVM? I 
> was also wondering if you could point to some more information 
> on the internals of class loading - I found [6], [7] and [8] a 
> very interesting read (although they are too brief, IMHO), the 
> source code and comments there are also helpful, but I was 
> wondering if there's anything more you'd recommend :)
> >
> >Any thoughts on this matter would be greatly appreciated.
> >
> >Best regards,
> >Igor Klimer
> >
> >[1]
> >[2]
> >[3] 
>>[4] M. Pukall et al, Flexible Runtime Program Adaptations in Java-A Comparision,
> >[5]
> >[6]
> >[7]
> >[8]

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