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Re: [equinox-dev] Question about DI/ DS and Application model


Eventually someone has to decide which implementation of IMetadataRepositoryManager is going to be used. I think in the case of an application it is quite reasonable for the application to make this decision directly (by looking up the service, perhaps with some filter that helps to select the manager to use). By moving the lookup of IMetadataRepositoryManager into a DS component it just hides the fact that it is a simple service lookup and doesn't seem to offer any advantage.

I think because both the service declaration, the implementation, and the client are all in the same bundle it's not a particularly interesting case. However I could imagine in more complex cases something like your solution 3 would be interesting. An executable extension factory could allow the services required by an executable extension to be injected into it rather than having the extension reach out.

You'll see another package "solution3" in the bundle where I was playing around with another approach. I'm not sure it's any better than your solution 1 but you can take a look.

John



Pascal Rapicault/Ottawa/IBM@IBMCA
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09/16/2009 04:00 PM

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[equinox-dev] Question about DI/ DS and Application model





Today I have done some more DI exploration using DS to see how it fits with the constructs we have in eclipse and I'm struggling to integrate in a nice way with the application model (I mean without using static) and I'm looking to know how others are doing this?
The one line summary of my experiment is: I have a class that does some work (named RepositoryDumper), it needs a service (RepoMgr). I want now to create an eclipse application that invokes the RepositoryDumper and I would like to not have to acquire the RepoMgr service manually.

Here is what I have been exploring with:
Solution 1:
I have an application declared in the plugin.xml. I have created a DS component that instantiates RepositoryDumper. However the question now is how does the application (remember that an eclipse application extension needs to provides n class) can get a hold of the RepositoryDumper instance that got created by DS:
- 1.1: Ugly -> Store the instance RepositoryDumper in the Activator of the plug-in
- 1.2: Get the RepositoryDumper be registered as a Service and have the application get this service. I don't like this because now RepositoryDumper is visible to everybody just so I can get access to it

Solution 2:
This solution assumes that the declarative approach to the eclipse application model is the hindrance and works around it by registering an ApplicationDescriptor (org.osgi.service.application). To do so I create a DS component that instantiates the RepositoryDumper and also register an ApplicationDescriptor as a service. This has the nice attribute that everything gets injected and that the application is only available to run if all the necessary pieces are available. However it requires a lot of code since one has to implement ApplicationDescriptor and ApplicationHandle, and I don't think this application would even be launchable using the -application argument.

Solution 3:
This solution is an hybrid between 1 and 2 using the IExecutableExtensionFactory.
There is a DS component that creates the RepositoryDumper and register a service, let's call it X. Then let's make the class specified in application extension (in the plugin.xml) implements IExecutableExtensionFactory and have it get the service X. This solution allows to have the application construction be completely done by injection however given that the application is contributed through extension registry it still is visible even though not ready to run.

How are others doing this? Is this a real problem or is it just me? Should I just not worry about that and use static fields?
Btw, the code is available /cvsroot/rt org.eclipse.equinox/incubator/p2/bundles/org.eclipse.equinox.p2.diagnostic Only solution 1 and 2 are available.

Thx for your attention and feedback

PaScaL
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