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

[Jaikiran Pai]

On Tue, 8 Oct 2024 15:28:17 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

> 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!

Hello Jan,

> Any more descriptive/specific suggestions than target?

I suspect you might already know this - internally, in the launcher implementation, this is called "launch mode" (represented as "Unknown", "Main class", "JAR file", "Module", "Source" modes). I however don't know if it will be appropriate to use `<launch mode>` here in the help text.

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PR Comment: https://git.openjdk.org/jdk/pull/21411#issuecomment-2401612115