RFR: 8340133: Investigate if the java launcher could give hints about JShell [v3]

[Eirik Bjørsnøs]

On Wed, 16 Oct 2024 14:55:34 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 [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 single-file program
>> 
>> Where key java 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 as suggested: 'using main class of a JAR archive' and '<JarFile>.jar'/'<SourceFile>.java'

I like this change a lot.

The PR /JBS title does not describe the actual change proposed here. Perhaps something like "Add concise usage message to the java launcher" would be more descriptive?

src/java.base/share/classes/sun/launcher/resources/launcher.properties line 239:

> 237: \Where <application> is one of:\n\
> 238: \  <MainClass>                to execute the main method of a compiled class\n\
> 239: \  -jar <JarFile>.jar         to execute the main class of a JAR archive\n\

I know @magicus suggested `<JarFile>` for consistency with other options.

Perhaps bike shedding, but consider that JAR file names in the wild are predominately lower-case, not CamelCase. At least this is the convention the open source ecosystem.

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PR Review: https://git.openjdk.org/jdk/pull/21411#pullrequestreview-2373203062
PR Review Comment: https://git.openjdk.org/jdk/pull/21411#discussion_r1803530439