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

[Alex Buckley]

On 12/3/2015 2:04 PM, Peter Levart wrote:
> On 12/03/2015 08:46 PM, Alex Buckley wrote:
>> On 12/3/2015 10:04 AM, Peter Levart wrote:
>>> Loosely related to this debate, I would like to ask what's the story
>>> with modular jars that can run on JDK8 classpath and as modules on JDK9.
>>> How is one supposed to create such jar file as it must have:
>>>
>>> - classes compiled with JDK8 javac (or JDK9 javac with -source 8 -target
>>> 8 or -release 8)
>>> - module-info.class which can only be produced with JDK9 javac, but not
>>> with -source 8 -target 8 or -release 8
>>>
>>> Is one expected to invoke javac twice, 1st for sources (excluding
>>> module-info.java), 2nd just for module-info.java ?
>>
>> Yes (or get your build tool to do it).
>>
>> More generally, anyone creating a multi-release JAR file (you didn't
>> ask about them, and they are not necessary to achieve what you want
>> above, but I mention them for completeness) will invoke javac multiple
>> times. It's the cost of doing business.
>>
>>> Wouldn't it be nice
>>> for jar tool to be able to fabricate the module-info.class from scratch
>>> using just command-line options?
>>
>> No. How does a Java compiler know what's exported from a module, in
>> order to enforce accessibility, if the module declaration isn't
>> fabricated until packaging time?
>
> In this case (of building a modular jar file that is able to run on JDK8
> classpath), classes are compiled with javac -release 8, which can't run
> in "modular" mode and consequently can't enforce any accessibility. When
> module-info.java is then compiled separately, there are no classes to be
> compiled and so there's nothing to enforce accessibility on....

Sure, this special case of build-for-JDK-8-then-add-modular-info could 
be solved by passing the modular info to the jar tool. But the modular 
info is non-trivial -- a list of module dependencies plus a list of 
exported packages -- so in practice you'd stick the info in an @argfile 
and now you've reinvented module-info.java.

Alex