Substitution (aka aliases)

[Mandy Chung]

I have implemented the runtime support for substitution, aka aliases, 
that supports refactoring via aggregation and also allows a module to 
resolve against any one of a set of equivalent implementation modules 
[1].   The provides clause declares an alias name for a module:
    module foo {
         provides bar;
    }

Any module requiring bar can be satisfied by foo.

Webrev:
     http://cr.openjdk.java.net/~mchung/jigsaw/webrevs/aliases/

The main change is in the SimpleLibrary which now saves the module ids 
(views and aliases) for all installed modules in a file "%mids" that 
keeps a map from a ModuleId to its providing ModuleId (which I call it 
module id directory.  During installation and resolution, this module id 
directory is locked while being updated.

There are cases that will want to find the declaring modules or only the 
module-info (e.g. finding the service providers, find the modules with 
an entry point).  A few new methods are added in the Catalog class: 
gatherLocalDeclaringModuleIds, listLocalDeclaringModuleIds, 
listDeclaringModuleIds.

With this alias support, the default platform module is now "java.base" 
[2].  In addition, each platform module also provides an alias name (I'm 
open to any suggestion to any better names and they are what I came up 
with for now).

As noted in [2], whether aliases should have version numbers is an open 
issue which will be resolved separately from this prototype.  However, 
the compiler currently synthesizes a dependence on java.base with a 
version constraint of>= N version constraint in the module-info.class if 
the module doesn't declare an explicit dependence on java.base or not 
the java.base module itself.  Below shows the output from javap -v on 
module-info.class for foo above:

$ javap -v module-info.class
Classfile foo/module-info.class
....
SourceFile: "module-info.java"
Module: foo
ModuleRequires:
   1	// modules
     #7,10000	// requires synthesized java/base@>=8
...


As a query of "java.base@>=8" does not match a ModuleId "java.base" with 
no version, I workaround this issue in the Resolver implementation by 
allowing a synthesized dependence on "java.base" to match with an alias 
name.

Thanks
Mandy

[1] 
http://openjdk.java.net/projects/jigsaw/doc/draft-java-module-system-requirements-12#substitution
[2] http://cr.openjdk.java.net/~mr/jigsaw/notes/jigsaw-big-picture-01