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

[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!

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Commit messages:
 - Cleanup.
 - Adjusting/improving the concise help.
 - Attempting to make the java launcher message consice, and let it refer to JShell.

Changes: https://git.openjdk.org/jdk/pull/21411/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21411&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340133
  Stats: 54 lines in 6 files changed: 49 ins; 0 del; 5 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