RFR: 8340133: Add concise usage message to the java executable [v10]

[Alan Bateman]

On Fri, 29 Nov 2024 07:25:23 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:
>> 
>> Usage: java [java options...] <application> [application arguments...]
>> 
>> Where <application> is one of:
>>   <mainclass>                to execute the main method of a compiled main 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 ":"
>>   -version
>>     to print product version to the error stream and exit
>> 
>> For additional help on usage:           java --help
>> For an interactive Java environment:    jshell
>> 
>> 
>> The full `java -help` with these changes is as follows:
>> 
>> Usage: java [options] <mainclass> [args...]
>>            (to execute a class)
>>    or  java [options] -jar <jarfile>.jar [args...]
>>            (to execute a jar file)
>>    or  java [options] -m <module>[/<mainclass>] [args...]
>>        java [options] --module <module>[/<mainclass>] [args...]
>>            (to execute the main class in a module)
>>    or  java [options] <sourcefile>.java [args]
>>            (to execute a source-file program)
>> 
>>  Arguments following the main class, source file, -jar <jarfile>.jar,
>>  -m or --module <module>/<mainclass> are passed as the arguments to
>>  main class.
>> 
>>  where options include:
>> 
>>     -cp <class search path of directories and zip/jar files>
>>     -classpath <class search path of directories and zip/jar files>
>>     --class-path <class search path of directories and zip/jar files>
>>                   A ":"-separated list of directories, JAR archives,
>>                   and ZIP archives to s...
>
> Jan Lahoda has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Dropping printing of the version.
>  - Dropping the version from the concise help

I think this looks okay, it doesn't preclude us changing it in the future or add the version in the concise or full usage in the future.

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Marked as reviewed by alanb (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21411#pullrequestreview-2470342068