Spring's need for optional dependencies

[Peter Levart]

Hi Paul,

On 12/18/2015 05:14 PM, Paul Benedict wrote:
> Adding read edges at runtime is not a backward compatible solution. 
> Jigsaw should automatically allow you to read anything your Module 
> Descriptor gives you access to -- required or optional.

I was maybe not clear enough. So here's in the example. Suppose module A 
is a root module (the one mentioned in java -m option):

module A {
     requires B;
}

...searches for module B on -module-path and adds it to runtime 
configuration, failing to start the program if B is not found. If B is 
found it adds a read edge from A -> B.

module A {
     requires optional B;
}

... doesn't search for module B but still adds a read edge from A -> B 
if B happens to be included in the configuration by some other means. 
The 'other means' could be anything from:

- some other module that is already a part of configuration "requires B" 
(say a root application module directly or a transitive dependency)
- adding -addmods B command line option
- deployment descriptor of a .mwar application says so


I can certainly say that this is backward compatible in code. The code 
stays the same.

I'm all *for* intorducing optional dependencies concept into the 
deployment descriptor. It's just that I don't see -modulepath as an 
equivalent to -classpath in the sense that optional dependency should 
make module part of the configuration when it happens to be searchable.

Regards, Peter

>
> Cheers,
> Paul
>
> On Fri, Dec 18, 2015 at 10:02 AM, Peter Levart <peter.levart at gmail.com 
> <mailto:peter.levart at gmail.com>> wrote:
>
>
>
>     On 12/18/2015 04:49 PM, Peter Levart wrote:
>
>         You can check whether the optional module is included in a
>         runtime configuration or not with a simple Class.forName()
>         check even if you don't depend on the module (i.e. don't list
>         it in "requires" descriptor at runtime). The visibility of
>         classes is not restricted. It only depends on ClassLoader
>         hierarchy. When you successfully resolve some optional class
>         at runtime (with Class.forName), you then have to add a read
>         edge to it's module:
>
>             Class<?> optionalClass = Class.forName("...");
>         MySelfClass.getModule().addRead(optionalClass.getModule());
>
>         ...before invoking any code that uses this module.
>
>
>     Just had an idea!
>
>     Optional dependency at runtime could add a read edge to the module
>     if that module was included in the configuration, but would
>     otherwise not cause that module to be included in the
>     configuration by itself. How does that sound? Spring would work as
>     before - no .addRead() calls needed. Just
>     Class.forName("OptionalClass").
>
>     Regards, Peter
>
>