Some suggested patches and improvements

[David M. Lloyd]

I've proposed five patches to the jpms-spec-experts list [1..5] for 
discussion.  The patches are as follows:

1. Layer primitive: addExports() - mirrors the existing 
Module.addExports() method for ModuleLayer.Controllers
2. Layer primitive: addUses() - mirrors the existing Module.addUses() 
method for ModuleLayer.Controllers
3. Layer primitive: addPackage() - allows ModuleLayer.Controllers to add 
packages to a module after it has been defined
4. Make run-time cycle checking optional
5. Add optional class loader isolation for modules on the module path

Apart from #3, each of these patches is small and very low-risk, and 
address a clear problem.  In the case of #3, I believe that the risk is 
very low, but am also willing to discuss modifying the patch to move the 
method to a "jdk.unsupported" class or in other ways.

I also am maintaining a mirror of the jigsaw/jake tree as a Git 
repository [6] which contains these patches where they can be viewed and 
fiddled with on a more interactive basis.

It is my belief that these small, low-cost patches will go a very long 
way towards mitigating many of the problems identified and shared by Red 
Hat, by other EG members, by other EC members, and by the community.

Please have a look and let me know if you have any feedback on any of 
these changes.

[1] 
http://mail.openjdk.java.net/pipermail/jpms-spec-experts/2017-May/000705.html
[2] 
http://mail.openjdk.java.net/pipermail/jpms-spec-experts/2017-May/000707.html
[3] 
http://mail.openjdk.java.net/pipermail/jpms-spec-experts/2017-May/000708.html
[4] 
http://mail.openjdk.java.net/pipermail/jpms-spec-experts/2017-May/000711.html
[5] 
http://mail.openjdk.java.net/pipermail/jpms-spec-experts/2017-May/000710.html
[6] https://github.com/dmlloyd/openjdk-modules

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- DML