Requires optional permits <was> Re: Service provider module resolution

[Alan Bateman]

On 11/07/2012 10:19, Neil Bartlett wrote:
> You don't have to interact with the optional dependency purely with
> reflection. You could check for the availability of the class first
> with ClassLoader.loadClass() or Class.forName(), and if that doesn't
> throw an exception then you can proceed and use the class in the
> normal way. Of course this has to be done before calling any code that
> has the dependency, whether directly or indirectly e.g. via a
> supertype.
>
> I have sometimes seen this done in OSGi, alhough optionality at the
> level of static class/package dependencies is widely considered to be
> a bad practice.
>
> If you do this then you clearly need the dependency at compile time
> but it is not needed at runtime. This is one of several reasons why
> compile dependencies differ from runtime dependencies.
>
I realize you're not advocating this approach but it involves 
assumptions that may not be true everywhere. In particular it assumes 
that VM is not doing eager resolution as in theory you may get the CNF 
before the Class.forName executes. Precompilation and other install time 
optimizations have the potential to complicate things too. In any case, 
it's still all very fragile as the code compiles but may fail with CNF 
at runtime.

-Alan.