On Mon, 17 Oct 2016 12:59:25 +0200, Alan Bateman <alan.bate...@oracle.com>
wrote:
On 17/10/2016 08:32, Peter Levart wrote:
:
Do we need an --exclude-modules (in addition to --add-modules) option
on javac, java and jlink commands?
--exclude-modules would be different to --limit-modules. If some module
requires module M and there is no module M on the module path or it is
not observable because it was not mentioned in the --limit-modules
option, then an exception is raised. OTOH if some module X requires
module M and module M is mentioned in the --exclude-modules option,
then such requires is silently ignored in hope that module X will not
actually need types from module M.
The module declaration is intended to be authoritative and so we have to
trust module author when they declare that the module `requires M`. So
my view is that options such as --exclude-modules that would have the
effect of dropping requires puts us on the road to anarchy.
That said, I do see Robert's concern that there might be orphaned
`requires` clauses in some modules. My module started using the
preferences API but later the implementation changed to use something
else. I neglected to remove the `requires java.prefs` from the module
declaration and the result is that my module cannot compile against or
run on a run-time image that doesn't include this module. Static
analysis tools might help here, as might the IDE. We are used to IDEs
highlighting unused `import` statements and in time then I expect they
will do the same for apparently unused `requires` clauses in
module-info.java. If the usage is purely reflective then the module
author might need to put a comment on the `requires` clause to avoid
other maintainers from removing it (a bit like "// used by javadoc" in
comments today when an import is for an unqualified reference in the
javadoc).
Another part to Robert's mail is the case where something is making use
of types in modules that it doesn't depend on. Assuming these are static
references then they will be caught at compile-time. This is big
improvement compared to today's class path.
A more general comment is that module authors will need to learn a few
new things about compatibility and refactoring. One example is changing
`requires transitive M` to `requires M` is an incompatible change.
Another is splitting a module (several sub-cases) where the module
author will need to add `requires transitive` to avoid breaking
consumers. There are lots of opportunities here for authoritative books
and documentation to help module authors do this right.
-Alan
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I understand why *in concept* the --exclude-modules is an unwanted option.
The module-info clearly states "requires A.B", otherwise it should have
been marked as "optional" or simply removed.
Now that the user fully relies on the discipline of the library-builders:
users cannot control the modules, nor will the compilation fail in case of
an incorrect module-info.
It is really a matter of hoping that library-builders are aware of this
and maybe it will make libraries more popular based on the quality of the
module-info instead of the quality of the classes. As a user you probably
don't want to be forced to choose on these facts.
And for the smaller and medium application this will work, but for the
larger this can really become problematic.
Up until now the compiler was always about "is everything on the classpath
to compile the classes?". If there is more, we'll, it'll be ignored.
"More" was never a problem. And if it was a problem, the user could fix it.
Now we have the module-info, and it is actually a safety-belt for the
library-builder! Now he can never be blamed (almost): the module-info
contains at least all info to compile and run this library, maybe even
more for free.
But with a lot of libraries with their own safety-belts there can be (and
will be) conflicts and there's nothing you can do right now (apart from
dropping all safety-belts).
For the end-user all these small safety-belts doesn't feel very "safe". He
would feel much better if he had some of the control back (and yes, he's
very well aware of the possible consequences).
The introduction of the module-info comes with great powers, but that
comes with great responsibilities as well. I would like to see that the
compiler could help with controlling those required modules (which would
mean that "More" is considered to be a problem). Static analysis is IMHO
just a hint, ignorable, but to me it shouldn't be that way.
Robert
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