RFR: 8347755: Support static library in jmod [v6]

[Magnus Ihse Bursie]

On Fri, 14 Mar 2025 17:48:30 GMT, Jiangli Zhou <jiangli at openjdk.org> wrote:

>> Henry Jen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add hotspot static libs to java.base-static-jmod
>
> make/CreateJmods.gmk line 104:
> 
>> 102:   endif
>> 103: 
>> 104:   # Copy hotspot libs, currently only libjvm.a
> 
> I just realized that we also need the launcher's `main.o` in the static `java.base.jmod`. Creating a `liblauncher.a` with the `main.o` and including the `liblauncher.a` in `java.base.jmod` seems to be cleaner than including `main.o` directly in the jmod.
> 
> When jlinking the final image for the application, if user doesn't provide customized launcher, `liblauncher.a` is used with other needed JDK/VM static libraries for linking the executable using the native linker (e.g. `ld`). If customized launcher is used, the object file(s) or the static library for the customized launcher is then used when linking the executable. `liblauncher.a` is not used in that case.

Should we really, though? That includes a `main` function. If the user wants to use the static libs from the jmod to create their own application, that will cause a symbol conflict.

Sorry @jianglizhou, I did not properly read the second part of your suggestion, to create a `liblauncher.o` out of `main.o`. Well, tbh, this is just JDK-8351367 all over again. If we should include "optional" libraries in the jmods, we need to figure out first how to handle them. How should they be marked/classified as optional or required? How should the user tell jlink to include or exclude them? If we do get a good answer to that question, then we can introduce an optional `liblauncher.a` as well alongside `libjsig.a`. But until then, we should just stay clear of "optional" static libraries.

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PR Review Comment: https://git.openjdk.org/leyden/pull/46#discussion_r1996034651
PR Review Comment: https://git.openjdk.org/leyden/pull/46#discussion_r1996053235