modules and bootclasspath

[Jonathan Gibbons]

In an alternate reality, we would have this discussion in the context of 
a change to the compiler spec. But as it is, we get to do it here.

I'm coming up with a series of questions regarding the interaction of 
bootclasspath options and modules in JDK 7. More than a little, these 
answers depend on what the anticipated contents of JDK 7 might be, with 
respect to modules, libraries and rt.jar or its replacements.

For javac, the bootclasspath options include
    -bootclasspath
    -Xbootclasspath
    -Xbootclasspath/p:
    -Xbootclasspath/a:
    -extdirs
   
There's also -J-Xbootclasspath, which technically speaking is a not a 
javac option but an indirection to a JRE option. It's also a very 
important mechanism in the way we bootstrap the JDK build. There is an 
implicit assumption in the build that once we have built JDK, we can use 
it to bootstrap itself ("make SKIP_BOOT_CYCLE=false").

There are a number of points along a number of axes for which we need to 
determine if this is a supported combination.

1: bootclasspath specified
    -- no
    -- yes (meaning any of the bootclasspath related options)

2: javac source option
    -- not given
    -- given, less than 7 (i.e. modules not supported in language)
    -- given, greater than or equal to 7 (i.e. modules supported in 
language)

3: client code: module access modifier
    -- no
    -- yes
   
4: client code: module-info.java files
    -- no
    -- yes, but no explicit requirement on platform module
    -- yes, with explicit (but possibly indirect) requirement on 
platform module


Here are some issues to take into account when considering some of the 
combinations:

-- how much can JDK 7 be used as "a better JDK 6" for non-modular code?
-- do we allow or forbid any use of bootclasspath with any modular code?
-- does it make sense to allow bootclasspath as a sort of implicit 
override of the default platform requirement?
-- when composing the internal virtual classpath(s) in javac, can we 
consider the bootclasspath options as affecting those components 
identified with the platform module, however specified (implicit or 
explicit?)

-- Jon