Re: custom libgcj

[David Daney <ddaney at caviumnetworks dot com>]

Andrew Haley wrote:
> Bryce McKinlay wrote:
> > On Mon, Apr 20, 2009 at 12:41 PM, abhishek desai <abhi00@gmail.com> wrote:
> >
> > > I want to customize libgcj compilation to include only the garbage
> > > collector part and other required files for a simple java program to
> > > compile and execute in native using gcj. Then I want to add my own
> > > java files to libgcj. Is there some way this can be done? Which files
> > > can I edit for this ?
> > This would certainly be a very useful feature. Unfortunately, there is
> > no easy way to do it. You could edit Makefile.am files by hand to
> > remove classes you don't want, but you're likely to get missing symbol
> > errors at link time due to the way libgcj is compiled.
>
> I agree.
>
> I think the only way to do it is iterate.
>
> 1.  Link app statcially with libgcj and make sure it still works.
>
> 2.  Run app with -verbose:class to see what gets used.
>
> 3.  Remove stuff that isn't listed in -verbose:class.
>
> 4.  Fix link errors.
>
> 5.  If that isn't enough, start tracing through -verbose:class and edit
> the library source to prune dependencies.
>
> 6.  Goto 4...

That's right, unfortunately libgcj has many dependencies that are not 
obvious.  For example (from memory):

All programs have a ClassLoader...

ClassLoader->SecureClassLoader->URLClassLoader->(all known URL Handlers 
including https)->(Most of the crypto code)->(much of the xml 
code),(some javax.swing dialogs for certificate verification)->All of 
javax.swing

On a mips-linux system I was able to with a little work get the static 
image size of HelloWorld.java down to about 9MB (down from about 20MB). 
 To get much smaller than that would take some real work and create a 
library that is not really compatible with the reference implementation.

David Daney