RFR: 8340133: Add concise usage message to the java executable [v6]

[Roger Riggs]

On Fri, 25 Oct 2024 12:45:25 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` executable, 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 executable 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:
>> 
>> openjdk 24-internal 2025-03-18
>> 
>> Usage: java [java options...] <application> [application arguments...]
>> 
>> Where <application> is one of:
>>   <mainclass>                to execute the main method of a compiled class
>>   -jar <jarfile>.jar         to execute the main class of a JAR archive
>>   -m <module>[/<mainclass>]  to execute the main class of a module
>>   <sourcefile>.java          to compile and execute a source-file program
>> 
>> Where key java options include:
>>   --class-path <class path>
>>     where <class path> is a list of directories and JAR archives to search for class files, separated by ":"
>>   --module-path <module path>
>>     where <module path> is a list of directories and JAR archives to search for modules, separated by ":"
>> 
>> For additional help on usage:           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 two additional commits since the last revision:
> 
>  - Using correct pplaceholders.
>  - Adjusting text as suggested.

lgtm

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Marked as reviewed by rriggs (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21411#pullrequestreview-2395480750