Review request: JDK-8159596 Add java --dry-run

[David Holmes]

On 3/07/2016 8:09 AM, Mandy Chung wrote:
>
>> On Jul 2, 2016, at 2:02 PM, Peter Levart <peter.levart at gmail.com> wrote:
>>
>> What about extending the behavior of -version option so that when it is specified together with other startup options like -mp, -m, -cp, MainClass, ... it would behave just like --dry-run has been designed to work?
>
> I don't see any issue with a new —-dry-run option that gives a clear intended usage.
>
> I don’t like recommend the existing -version option to be used for dry run.  Although java -version starts the module system, it just assumes the default root module.

I asked earlier if loading the main class impacted the way the module 
system was initialized - in which case the difference here and with 
-version is obviously significant.

David
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>> Would such combinations of options be hard to parse/detect/validate ?
>
> It’s just an existing behavior that can be changed.  But I don’t think making -version for dry run is the right thing to do and requiring java -version must specify the main entry point (at least -m if launching a modular app).
>
>> For example, what to do if someone specifies -version in combination with -mp, -addmods, but no -m ? Would we still try to resolve the modules and print the version or fail and with what?
>
> The launcher should fail if there is no main class or main module specified even before it creates the VM.   java -version assumes the default root module and not requiring any main entry point specified (that’s one difference between java -version and —-dry-run)
>
> -—dry-run opens more opportunity to add diagnostics information.  For example you can run
>    java -Xdiag:resolver —-dry-run <all other options>
>
> that will print the resolver traces for diagnostic.
>    java -XshowSettings:args —-dry-run <all other options> @argfiles
>
> This could print the launcher/VM arguments, as Paul suggested.
>
>> Specifying just -mp by itself currently prints usage help.
>
> This is a bug in the existing launcher argument processing that ARG_CHECK should return false instead of true.  I checked with Kumar and he would have to do archeology why it is implemented that way for long time.  I have a patch fixing the argument processing on the module-system options that should not print usage help.
>
>> Having an explicit --dry-run option at least does not have those dilemmas and defines the behavior explicitly when some startup options are missing - the same behavior as without --dry-run.
>>
>
> Exactly.
>
>> In order to put more weight into --dry-run, I suggest extending its behavior to report to the stdout, the result of resolving the modules when successfull.
>
> Are you aware of -Xdiag:resolver?
>
> Mandy
>