RFR: 8340133: Investigate if the java launcher could give hints about JShell [v2]

[Jan Lahoda]

> Currently, running `java` without any parameters will lead to an output that is a full `--help`, which is over 100 lines (on my computer at least), and it feels overwhelming. And many people might actually want to run JShell/REPL, not the `java` launcher, but it is difficult find out about JShell.
> 
> The proposal herein is to print a much shorter help, together with a pointer to JShell, when the launcher does not know what to do. I.e. there is nothing specified to start, and no option like `--help` is specified. In particular, on my machine, it prints:
> 
> $ java
> openjdk 24-internal 2025-03-18
> 
> Usage: java [options...] <what to execute> [arguments to main method...]
> 
> Where <what to execute> is one of:
>   <MainClass>                to execute the main method of a compiled class
>   -jar <jar-file.jar>        to execute the main class in a JAR archive
>   -m <module>[/<MainClass>]  to execute the main class of a module
>   <SourceFile.java>          to compile and execute a single-file program
> 
> Where key options include:
>   --class-path <class path>
>       a : separated list of directories and JAR archives to search for class files
>   --module-path <module path>
>       a : separated list of directories and JAR archives to search for modules
> 
> For more details about this launcher:   java --help
> For an interactive Java environment:    jshell
> 
> 
> Hopefully, this may be easier both for people trying to run something, and for people that are really looking for JShell.
> 
> What do you think?
> 
> Thanks!

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Adjusting the concise help based on review suggestions.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21411/files
  - new: https://git.openjdk.org/jdk/pull/21411/files/72939030..0a6c17f4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21411&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21411&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/21411.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21411/head:pull/21411

PR: https://git.openjdk.org/jdk/pull/21411