RFR: 8317620: Build JDK tools with ModuleMainClass attribute

[Alan Bateman]

On Wed, 1 Nov 2023 19:58:07 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> Tool modules can be created via `jmod --main-class` option such that `ModuleMainClass` attribute will be added in `module-info.class` and the module's main class can be launched via `java -m <module-name>` without specifying the name of the main class.
> 
> In addition, for modules with `ModuleMainClass` attribute, jlink will pre-resolve the module graph such that when such module is launched at runtime (without `--add-modules` or `--limit-modules` option), the runtime can skip the module resolution and speed up the startup time.
> 
> This PR extends the build system to allow a module to specify the main class under `make/modules/$MODULE/Jmod.gmk` file.    Also JDK tools with a single entry point (or a primary entry point) are candidates to add `ModuleMainClass` attribute in `module-info.class` to benefit from the jlink optimization.   For example, `java -m jdk.jpackage` will be launched using the pre-resolved module graph.
> 
> Verified manually by running `java -m $MODULE` on the modules with main class.

make/modules/jdk.jdi/Jmod.gmk line 26:

> 24: #
> 25: 
> 26: JMOD_FLAGS_main_class := --main-class com.sun.tools.example.debug.tty.TTY

I'm not sure about this one.  Every few years there is a suggestion to deprecate or remove jdb. It's the example debugger from the original JPDA effort in the JDK 1.2 timeframe and hasn't really developed much since then. The main reason is stays around is that it is needed by our tests, and some people say it's useful to have something in production environments where the main stream debuggers can't go. I'm not opposed to "java -m jdk.jdi", it's just that this is a legacy/example tool that isn't the main focus of the jdk.jdi module.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16463#discussion_r1379699959