RFR: 8340114: Remove outdated SelectVersion() function from the launcher and update the code comments explaining the code flow [v5]

[David Holmes]

On Mon, 16 Sep 2024 06:16:56 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

>> src/java.base/share/native/libjli/java.c line 38:
>> 
>>> 36:  * One job of the launcher is to remove command line options which the
>>> 37:  * vm does not understand and will not process. These options include
>>> 38:  * options which select which style of vm is run (e.g. -client and
>> 
>> Aren't these the only options now?
>
> Right now, in mainline, the launcher code in `CheckJvmType` function additionally also checks for the presence of `-XXaltjvm` launcher option. Just like for `-server`, `-client` options, this `CheckJvmType` function also strips the `-XXaltjvm` option from the options that are passed along to the JVM.
> 
> Specifically, in mainline, right now the following appears to be functional:
> 
> 
> <JDK_24>/bin/java -XXaltjvm=<JDK_22>/lib/server/  -version
> 
> will load and print the Java 22 VM output:
> 
> 
> openjdk version "22" 2024-03-19
> OpenJDK Runtime Environment (build 22+36-2370)
> OpenJDK 64-Bit Server VM (build 22+36-2370, mixed mode, sharing)
> 
> 
> i.e. the launcher from mainline JDK launches the altjvm from JDK 22. `CheckJvmType` in the launcher code, is where this parsing and stripping of `-XXaltjvm` is currently happening. 
> 
> (There's also some hotspot VM side code which appears to be parsing a `-Dsun.java.launcher.is_altjvm=` option to detect this `-XXaltjvm` usage, but as far as I can see the launcher no where sets this `-Dsun.java.launcher.is_altjvm=` when `-XXaltjvm` is used. So, I think, this needs a separate investigation of its own)

The property is used by the gtest launcher - see comments in [JDK-8171508](https://bugs.openjdk.org/browse/JDK-8171508)

This is all a lot more complicated than I had thought it was.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20997#discussion_r1761027867