is ClassLoader.loadClass() supposed to work on module-info classes?

David M. Lloyd david.lloyd at
Fri Dec 4 15:52:47 UTC 2015

On 12/04/2015 09:18 AM, Alan Bateman wrote:
> On 04/12/2015 12:59, David M. Lloyd wrote:
>> So you're saying that if I have C depending on A, then update A or I
>> split A and B in my distribution, I need to recompile C?  I don't
>> think that's a realistic proposition.  Do I then have to recompile
>> things that depend on C?  Or things that depend on those things?  How
>> would updates work in such a scenario?  Do I need to put the
>> dependency versions of every dependency in my version string so that I
>> can tell the difference between C compiled with the original A versus
>> C compiled with the new A/B?
>> It should not be necessary (and indeed it is not necessary today) to
>> do this kind of cascading recompilation; I don't think that is
>> something reasonable to ask of users.  If they have to recompile such
>> large swathes of their project any time anything has changed, the
>> point of modules has been at least partially defeated.
>> Java has a fairly clear ABI contract; a better approach would be for
>> users to know if they've changed compatibility when they compile A, or
>> when they split A into A and B, etc.  If the ABI is the same or is
>> forwards-compatible (i.e. only adding members), then why would a user
>> want to recompile everything else?  Is this not a red flag that the
>> encapsulation rules are unrealistic?
> FWIW, I also read your original mail as the wild west as it came across
> as arbitrary refactoring of other people's modules. So best to make
> things as clear as possible as it's just too easy to get into confusing
> discussions here.

Ah, yeah specifically I'm referring to splitting/merging or upgrading 
some given module in an existing system, possibly using aggregate 
modules for compatibility or alternatively updating the module 
descriptors of existing installed modules to accommodate the new layout 
(both cases seem reasonable to me, and we have done both).  The phrase 
in the original email from Alex that I was responding to was "We don't 
think the module declaration will be as fluid as you do"; however our 
real-world experience shows the opposite.

Really though you could say that splitting/merging or upgrading a 
dependency is almost-arbitrary refactoring, with the constraint being 
that the existing module's public ABI is maintained forward-compatibly. 
  I know there are third party tools for testing this (I've used them); 
I think that while not *strictly* necessary, bundling such a tool in the 
JDK (regardless of how the module system ultimately behaves) is going to 
be highly desirable going forward, if possible.  That's slightly 
tangential to the main idea here though, which is that an installation 
of modules may well be necessarily (and reasonably) fluid, and that 
fluidity extends to the descriptors of already-installed modules, 
because the run time graph of modules is something that is 
installation-dependent.  OSGi accommodates this by using packages to 
link - basically punting the link decisions to the run time.  We 
accommodate this by bundling the descriptor externally to the artifact 
so it can be easily updated by users and separately updated by tools 
(without modifying artifacts).  But in any event, this reality *has* to 
be accommodated in a robust module system.

> As regards refactoring then the current design supports many cases of
> merging and splitting, something for another thread I think.


More information about the jigsaw-dev mailing list