Hey,
In my project I had a need to import a service (a DataSource in my
case), and I couldn't implement it using a ServiceComposite. So what I
did instead was to create a ServiceComposite that implements
ServiceImporter, and then create a class that implements ServiceImporter
that is then used for the importedBy() in assembly, and which delegates
to my ServiceComposite.
The funky thing that this accomplishes is that I can have a native
service in Qi4j which is "always on", and which can handle importing
external services which are only sometimes available. By implementing
isActive() in my ServiceComposite I can check at the time of usage of
the imported service if isActive() is true. If not the client can handle
that as it wishes.
Here's my assembly code:
module.addServices( DataSourceService.class ).identifiedBy( "datasource"
).visibleIn( Visibility.application );
module.importServices( DataSource.class ).
importedBy( ServiceInstanceImporter.class ).
setMetaInfo( "datasource" ).
identifiedBy( "streamflow" ).visibleIn( Visibility.application );
Currently I have hardcoded all datasource settings in the
DataSourceService, *but* if I change this to be meta-info on the
imported service I can use the import statement to do the actual
settings for the individual datasources, which is then sent to the
DataSourceService as ImportedServiceDescriptor!! In other words, the
single DataSourceService can handle any number of datasources, generally.
I think this could be a very useful pattern for importing services from
external sources in a way that there's an "always on" native service
which you can configure *how* to import services, and then use the
import-meta-info to describe *what* to import for the specific
instances. And then also, there is no limitation on the external
services, i.e. they don't have to be composites and they don't have to
implement an interface or anything like that.
WDYT? Does this sound useful? If so, I can move the
ServiceInstanceImporter to Qi4j from my project so that it's generally
available.
/Rickard
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