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

[Jiangli Zhou]

On Fri, 14 Mar 2025 18:14:04 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> 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.
>
> 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.

I think for common cases, there would be no C/C++ `main()` function provided from user code with Java applications. The JDK `java` launcher's `main()` is the C/C++ entry point. So we do need to provide a static library with the C/C++ `main` function for linking the executable.

With the customized launcher cases, the C/C++ `main()` is implemented by the customized launcher. Then, the JDK provided `main` entry point is not needed.

@magicus, yes, this involves with the optional static library in the jmods, as you pointed out. I think for C/C++ `main`, supporting that is necessary.

An alternative is to require Java application implement C/C++ `main()`. That doesn't seem to be an applaudable approach.

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PR Review Comment: https://git.openjdk.org/leyden/pull/46#discussion_r1996151267