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

[Henry Jen]

On Fri, 14 Mar 2025 19:20:40 GMT, Jiangli Zhou <jiangli at openjdk.org> wrote:

>> 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.

I would prefer that thin wrapper is to be provided by the end tools(like jlink) which use the jmod to link an executable.

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PR Review Comment: https://git.openjdk.org/leyden/pull/46#discussion_r1996336751