OSGi Configuration Admin and MetaType Service

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I've been using the OSGi Configuration Admin to implement some basic configuration capability in our program. I now started looking into the MetaType Service specification because I need type information for each configuration property.

It's unclear to me how these two services interact. The Configuration Admin deals with essentially untyped Key/Value pairs. The MetaType Service knows names and types (among other things) of configuration properties, but not their values. My goal is to dynamically generate a configuration/preferences dialog for all components which have a configuration and corresponding metatype information. According to the MetaType Service spec, the service was conceived to cover this exact use case. So I reckon it shouldn't be too difficult

I can retrieve the metatype information with the following the sample code:

ServiceReference metatypeRef = bundleContext.getServiceReference(MetaTypeService.class.getName());
MetaTypeService service = (MetaTypeService) bundleContext.getService(metatypeRef);
MetaTypeInformation information = service.getMetaTypeInformation(myBundle);

After retrieving the MetaTypeInformation object for the required bundle, I have access to all the information contained in the metatype XML definition. In particular, it is possible to access the ObjectClassDefinition:

ObjectClassDefinition ocd = information.getObjectClassDefinition(pid, null);
AttributeDefinition[] attributes = ocd.getAttributeDefinitions(ObjectClassDefinition.ALL);

My questions are:

  • Given the AttributeDefinition; how can I retrieve the actual value of the underlying property? I know its name, but not its value.
  • How can I enumerate the metatype information for all components in all bundles that are currently present (active and inactive)? I know how to list all configurations through the Configuration Admin interface. It there perhaps a way to get to the MetaTypeInformation from the Configuration?
osgi
1 Answers

The Configuration Admin and MetaType services are separate, but related, specifications. There is no hard link between them, which I think is probably one of the main points that will help to answer your questions.

Essentially Configuration Admin is a store of configuration records. Each configuration record has a unique persistent identifier (PID) and if the configuration record is for a factory configuration then it will also have a Factory PID. The configuration record then also contains a number of key value pairs, where the key is always a String, and the value is one of a limited set of types.

Metatype, on the other hand, is a tool for providing configuration definitions. These describe the expected layout of keys and values in a configuration, including things like the type of the value associated with a given key, a minimum/maximum size for the value, an enumerated list of allowable values, and potentially a default value. Each key/value definition is held in an Attribute Definition, and these are grouped together in an Object Class Definition, which is associated with a PID.

The important difference is that Metatype knows nothing about what the configuration actually is at runtime (it's just information about what shape the configuration should be). Similarly Configuration Admin knows nothing about what shape the configuration should be, it just knows what the values currently are.

Therefore:

Given the AttributeDefinition; how can I retrieve the actual value of the underlying property? I know its name, but not its value.

You need to identify the PID associated with the ObjectClassDefinition containing the Attribute Definition, and then to use this to find the relevant configuration dictionary in Configuration Admin. If the OCD is for a factory PID then you will need to identify which of the configurations for that factory PID you want to look at.

How can I enumerate the metatype information for all components in all bundles that are currently present (active and inactive)? I know how to list all configurations through the Configuration Admin interface. It there perhaps a way to get to the MetaTypeInformation from the Configuration?

The MetaTypeService is a service in the OSGi Service Registry that you can use to ask for the MetaTypeInformation for a given bundle. If you ask for the Metatype Information for each bundle in turn then you will have the information that you are looking for. There is no hard link between Configuration Admin and Metatype, so a Configuration object has no way of knowing whether a meta type exists for it.

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