java -modulepath

[Alan Bateman]

I've started to look at supporting "exploded modules" as a prerequisite 
to changing the jdk build so that it compiles as modules rather than the 
post-processing step as it is now. The idea is that we be able to 
compile and run the modules on the module path without requiring them to 
be installed in a module library. Aside from the JDK build it would also 
be nice to be able to compile and run in the development environment 
without needing to install.

As an initial (and very limited) prototype I've added a -modulepath (and 
-mp) option to the launcher to specify a modulepath. That allows us to do:

javac -d modules/ -modulepath modules/
java -modulepath modules/ -m foo

When running with the -modulepath option then configuration is generated 
at startup rather than loading it from the module library. That is very 
expensive now and something to look at it. Implementation wise it's just 
a simple Library implementation that uses the usual module library as 
its parent.

This prototype brings up a couple of issues that need examination. 
Performance and whether the configuration can be cached for example. 
Another one is native libraries as up until this point the javac 
modulepath has been for classes only. This is somewhat related to how 
module with native libraries are supported in the module library [1]. 
Another issue is that we have code in various places of the JDK that 
currently assume a system class loader so care needs to be taken when 
computing the configuration at startup. This is something that Mandy and 
Sean ran into in a different context [2].

For now this is just a prototype, the webrev is here but doesn't require 
review:
    http://cr.openjdk.java.net/~alanb/jigsaw-mp-prototype1/webrev/
I have also some hotspot changes to allow for the case that the jdk.boot 
module isn't installed in the module library, as would arise during the 
build.

-Alan

[1] http://openjdk.java.net/projects/jigsaw/doc/topics/nativecode.html
[2] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2011-March/001186.html