Service Providers and versioning

[Jesse Glick]

On 04/18/2012 01:13 PM, Alan Bateman wrote:
> there isn't anything in Jigsaw to aid with the selection when there are several service providers installed

It may be useful to point out that there are two related but distinct features Jigsaw currently forgoes:

1. Declaring a dependency on a service matching some attribute - in other words, rejecting an attempt to resolve the module when some service providers are present but 
not the "right" one. While the app module could of course just emit a graceful message if it does not find the desired signature algorithm, you might want a DSL in module 
metadata for specifying such dependencies ahead of time. (For example, OSGi 4.3 "capabilities" can be annotated with attributes you can match on.) This could be 
interpreted not just by the module system runtime, but by native packaging schemes too - so that "sudo apt-get install signing-app" would prompt to install also 
java-ecdsa-8.deb (or whatever), rather than installing every known provider or none of them.

2. Directly loading ECDSASignature. While you can simply iterate every Signature implementation and check impl.getAlgorithm().equals("ECDSA") until you find the one you 
want, this adds startup overhead and pollutes the PermGen with unused providers. ServiceLoader's API cannot do any better, which is why other frameworks exist which can 
inspect service metadata without triggering class loading, such as sezpoz.java.net and Eclipse extension points.