Small VM change for booting from a module

[Mark Reinhold]

> Date: Thu, 14 May 2009 13:37:11 -0400
> From: karen.kinnear at sun.com

> It appears that meta_index_path and meta_index_dir are by default
> NULL. I'm not sure what happens on different OSs when classLoader.cpp:
> setup_meta_index calls fopen(meta_index_path, "r") and passes in a
> null. You might be safer initializing those values in
> os::set_boot_path.

Good point, but I have no values to which to initialize them, and am
unlikely to have such values even in the long run.  I've instead added
a check to ClassLoader::setup_meta_index so it doesn't open the file if
meta_index_path is null.

> I'm assuming you want to deal with Class Data Sharing compatibility
> after J1.

Yes!

>           If it helps to model usage of the new property, today CDS
> requires that the bootclasspath contain individual jars, not lists of
> directories, to reduce having to check timestamp and file size for all
> .class files, and reduce startup costs.

Thanks for pointing that out -- I'll keep it in mind.

> If you want to know what the vm is using for the sysclasspath you can
> run with -XX:+TraceClassLoading -XX:+Verbose, if you are running
> fastdebug.

Right, already found those.

> Also, there is a comment in ClassLoader.cpp you might want to check
> into:
> ... doesn't reorder the bootclasspath which would break
> java.lang.Package (see PackageInfo).

Yes, I already ran across that one too.  My present tiny change shouldn't
affect this.  It turns out that in order to support multi-module packages
on the boot class path I'll eventually have to expose this method through
a new JVM_ interface, but that's for another day.

> Date: Thu, 14 May 2009 14:03:09 -0400
> From: karen.kinnear at sun.com

> There does exist a -Xbootclasspath: flag already if you just want to use
> that, or -Xbootclasspath/p: if you want to prepend or -Xbootclasspath/a:
> to append, a different location.

I thought about doing that as a short-term hack, but since the VM will
ultimately have to do something module-specific I'd prefer to go with
what I have now even if it is, at the moment, technically equivalent to
-Xbootclasspath.

Updated webrev: http://cr.openjdk.java.net/~mr/vm-module-boot/
(only change is to classLoader.cpp)

Thanks,
- Mark