Add multiplicity declaration to use clause of service consuming module

[Alan Bateman]

On 26/03/2016 02:45, Stefan Dollase wrote:
> Hello,
>
> I read the article in [1], which describes the problem that jlink cannot
> figure out which service providing modules should be included into the
> custom modular run-time image. In [2] there is a statement about this:
> Previously, jlink included all service providing modules. Now, none of them
> is included. Since I am not sure whether this was already discussed, I
> would like to suggest a third approach:
JEP 282 [1] lists services as an open issue.

Yes, it was the case, at least for a brief period, that service 
providers were linked in automatically. It immediately leads to calls 
for ways to keep providers out and it gets very complicated once you 
have a command-line that it try to add and exclude modules at the same time.

So as things stand, jlink is an advanced tool and you do need to know 
about service providers when creating a custom runtime image. If you 
want specific crypto support or a specific dynamic language then you 
need specify the names of the modules to -addmods when creating the 
runtime image.

On extending the `uses` clause then this may be something to send to 
jpms-spec-comments for consideration if you feel it is important. One 
thing to be aware of is that the early exploratory phase of Project 
Jigsaw had `requires service S` and `requires optional service S`. The 
former could cause resolution to fail if there weren't any modules 
providing implementations of S. The latter is more like what we have 
today with `uses`.

At least for usages in the JDK, then we almost always used the latter in 
the prototypes at the time. One reason is the consumer modules often 
have a default implementation to use when there aren't any 
implementations deployed. The java.xml module has a default 
implementation of each of the XML parser types for example. There are a 
few cases, like the java.scripting or java.naming modules where there 
isn't any notion of the default implementation but it's not an error in 
either case to link or run without any service providers. The reason is 
that service provider modules may be loaded at runtime, maybe into 
module layers that a container creates.

-Alan

[1] http://openjdk.java.net/jeps/282