[jira] [Created] (FELIX-5749) Allow to use components that depend on optional imports

["Christian Schneider (JIRA)" <ji...@apache.org>]

Christian Schneider created FELIX-5749:
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             Summary: Allow to use components that depend on optional imports
                 Key: FELIX-5749
                 URL: https://issues.apache.org/jira/browse/FELIX-5749
             Project: Felix
          Issue Type: New Feature
    Affects Versions: scr-2.0.12
            Reporter: Christian Schneider


When desigining the scope of a bundle you sometimes have an optional part that could be externalized into its own bundle but you decide to keep it in your bundle to limit the number of bundles. In this case you have to use an optional import and make sure the code that depends on this import only runs when this import is wired. This code is often quite awkward and often also buggy.

We discussed on osgi-dev that you can make such code a lot simpler to write by using DS. 

This is how the code would look like:
You externalize the code that depends on the optional import into one or more components. These components offer a service interface that is not dependent on the optional import. Inside the component you can work freely with the optional packages. You have to make sure this component is disabled by default. Then you write a "starter" component that enables the component if the package is available.

I think scr could support such "optional" components without the disabled trick. We could load the component class and if it fails disable the component. If it works we enable it. 
So if the package is wired later and we get a refresh this approach would activate the component without any additional effort from the developer side.

----
Below I am copying a snippet from Ray that details what they did.

Given your component which has the optional import package (doesn't matter how it's used):

import com.liferay.demo.foo.Foo; // The optional package

@Component(
    enabled = false // disable by default so DS ignores it
)
public class OptionalPackageConsumer implements Foo {...}


Make sure the component is disabled by default. This will prevent SCR from classloading the component class.
Second, you construct a "starter" component who's job it is to check for the available package:

@Component
public class OptionalPackageConsumerStarter {
   @Activate
    void activate(ComponentContext componentContext) {
        try {
            Class.forName(com.liferay.demo.foo.Foo.class.getName());

            componentContext.enableComponent(OptionalPackageConsumer.class.getName());
        }
        catch (Throwable t) {
            _log.warn("Could not find {}", t.getMessage());
        }
    }
}




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