'Ello. I suspect we're in agreement, but I think there may've been a slight miscommunication somewhere...
On 2017-07-22T12:39:41 +0100 Neil Bartlett <njbartl...@gmail.com> wrote: > > I'm not talking at all about multiple versions of the same jar. I'm talking > about multiple providers of the same service, possibly even from different > vendors. This happens just as easily under Service Loader as under OSGi. Yes, as am I. I used the example of multiple versions of the same jar resulting in multiple conflicting registrations of services trying to use the same name because having multiple versions of the same jar is the primary "error" case where this can happen. If we ignore the case where two unrelated jars or bundles publish services that just happen to pick conflicting names, then that just leaves us with the case where two versions of the *same* jar attempt to publish services with conflicting names. So, with that in mind: In Java < 9, having multiple versions of the same jar on the classpath is discouraged and considered to be bad practice, so this situation is _possible_ but _somewhat less likely_ to occur with ServiceLoader. In Java 9, there are VM-level checks in place to prevent conflicting modular jars from being loaded, so this particular error case can't happen. In OSGi however, as I said, it's possible if not encouraged to have two different versions of a bundle installed, so this error case *can* occur. That's not a disadvantage of OSGi at all, quite the opposite, but it does mean that classes that do that kind of centralized registration of services have to be able to cope with it. > In your original email you asked: why does the consumer of PNG have to care > which PNG provider to choose. Well isn't that just a fact of life? I need a > plumber to fix my toilet... there are ten plumbers in my town, which should > I choose? Why do I have to care about plumbers?! > > Having choices can suck, but not as much as having no choice. Again, I agree. However, I gave the example of the ImageIO interface as a general example of a problem: You have multiple providers, those providers are essentially hidden from you, and you only have a mindlessly simple selection criteria such as "PNG" to pick a provider. Under traditional Java, the problem is slightly diminished in some sense because of the primitive nature of ServiceLoader, the generally static nature of module deployments, and in Java 9, the VM-level ModuleLayer checks. Under OSGi, the situation is more complex, and I was asking how experienced OSGi people tended to deal with the situation. Nothing more! -- Mark Raynsford | http://www.io7m.com
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