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Apache Felix Metatype Service

The OSGI Metatype specification provides a unified way to describe metadata about services and a Java interface to retrieve and manipulate these informations. These informations can optionally be localized, thus allowing the developer to provide human-readable metadata descriptions in multiple languages.

The specification defines a rich dynamic typing system to describe service attributes in a precise way; that's a useful functionality that makes it possible and easy to dynamically create user interfaces to configure services.

The simplest way to specify service metatypes is to add one or more XML files in the OSGI-INF/metatype folder.
That's for example the OSGI-INF/metatype/metatype.xml file included in Felix FileInstaller bundle.

<?xml version="1.0" encoding="UTF-8"?>
<metatype:MetaData xmlns:metatype="http://www.osgi.org/xmlns/metatype/v1.0.0">

  <OCD description="FileInstaller" name="org.apache.felix.fileinstall" id="org.apache.felix.fileinstall">
       
    <AD name="Poll directory"  id="felix.fileinstall.dir" required="true" type="String" default="load"/>   
    <AD name="Poll interval"  id="felix.fileinstall.poll" required="false" type="String" default="5000"/>
    <AD name="Debug"  id="felix.fileinstall.debug" required="false" type="String" default="-1"/>
    <AD name="Start new bundles?"  id="felix.fileinstall.bundles.new.start" required="false" type="String" default="true"/>
    
  </OCD>
  
    <Designate pid="org.apache.felix.fileinstall">
        <Object ocdref="org.apache.felix.fileinstall"/>
    </Designate>
  
</metatype:MetaData>

The main elements of this simple example are:

  • OCD, Object Class Definition; contains the information about a set of attributes
  • AD, Attribute Definition; describes an attribute (name, description, unique id, type, cardinality, default value, ...)
  • Designate, defines the association between a Object Class Definition and a pid; the pid can be a reference to a factory or a singleton configuration object

The following types are supported by the specification:

  • String
  • Long
  • Double
  • Float
  • Integer
  • Byte
  • Char
  • Boolean
  • Short

Localization

As mentioned before, it is possible to localize metatype informations. A good example in Felix code is contained in the Webconsole Bundle, that supports localization. 

<?xml version="1.0" encoding="UTF-8"?>

<metatype:MetaData xmlns:metatype="http://www.osgi.org/xmlns/metatype/v1.0.0" localization="OSGI-INF/metatype/metatype">

    <OCD id="org.apache.felix.webconsole.internal.servlet.OsgiManager" name="%manager.name" description="%manager.description">
        <AD id="manager.root" type="String" default="/system/console" name="%manager.root.name" description="%manager.root.description"/>
        <AD id="default.render" type="String" default="bundles" name="%default.render.name" description="%default.render.description"/>
        <AD id="realm" type="String" default="OSGi Management Console" name="%realm.name" description="%realm.description"/>
        <AD id="username" type="String" default="admin" name="%username.name" description="%username.description"/>
        <AD id="password" type="String" default="admin" name="%password.name" description="%password.description"/>
    </OCD>
    
    <Designate pid="org.apache.felix.webconsole.internal.servlet.OsgiManager">
        <Object ocdref="org.apache.felix.webconsole.internal.servlet.OsgiManager"/>
    </Designate>
    
</metatype:MetaData>

The key elements to write a localized metatype definition are:

  • the localization attribute, that points to the property file that localize this XML
  • the % notation that specifies a localizable stringme
  • one of more property files definining localized strings

The following example shows the localization property file of the Felix Webconsole

manager.name = Apache Felix OSGi Management Console
manager.description = Configuration of the Apache Felix OSGi Management Console.

manager.root.name = Root URI
manager.root.description = The root path to the OSGi Management Console.

realm.name = Realm
realm.description = The name of the HTTP Authentication Realm.

username.name = User Name
username.description = The name of the user allowed to access the OSGi \
 Management Console. To disable authentication clear this value.

password.name = Password
password.description = The password for the user allowed to access the OSGi \
 Management Console.
 
default.render.name = Default Page
default.render.description = The name of the default configuration page \
 when invoking the OSGi Management console.

Metatype Service API

Before using the Metatype Service API, it's worth noting that this specification is not part of the OSGi core specification, so it's not included in the standard Felix distribution. It is therefore necessary to download and install the additional Felix Metatype bundle.

Accessing the Metatype service is as simple as performing a lookup in the service registry:

  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, we have access to all the informations contained in the metatype XML definition. In particular, it is possible to access the ObjectClassDefinition (optionally specifying a locale)

  
  ObjectClassDefinition ocd = information.getObjectClassDefinition(pid, locale);

and, given the ObjectClassDefinition, retrieve all the attribute definitions

  
  AttributeDefinition[] attributes = ocd.getAttributeDefinitions(ObjectClassDefinition.ALL);
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