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THe OSGi Componendium Configuration Admin Service specifies a service, which allows for easy management of configuration data for configurable components. Basicaly configuration is a list of name-value pairs. Configuration is managed by management applications by asking the Configuration Admin Service for such configuration. After updating the configuration, it is sent back to the Configuration Admin Service. The Configuration Admin Service is like a central hub, which cares for persisting this configuration and also for distributing the configuration to interested parties. One class of such parties are the components to be configured. These are registered as {{ManagedService}} services. There is also a notion of {{ManagedServiceFactory}}, which allows for multiple configurations of the same kind to be applied.

For more information, I suggest you read Chapter 104, Configuration Admin Service Specification, of the OSGi Compendium Services Specification book. IMHO this is worth reading.

For a starter this page sets out to describe how you can create a component, which is interested in some configuration. As such this page is at its very beginning just highlighting the simplest of all cases: A single component being interested in its configuration

{code

:title

=PrettyPrinterActivator.java

}
...
private ServiceRegistration ppcService;
public void start(BundleContext context) {
    Dictionary props = new Hashtable();
    props.put("service.pid", "org.sample.PrettyPrinterConfigurator");
    ppcService = context.registerService(ManagedService.class.getName(),
        new PrettyPrinterConfigurator(), props);
}
public void stop(BundleContext context) {
    if (ppcService != null) {
        ppcService.unregister();
        ppcService = null;
    }
}
...
{code}

That's more or less it. You may now go on to use your favourite tool to create and edit the configuration for the {{PrettyPrinterConfigurator}}, for example something like this:

{code

}
Configuration config = configurationAdmin.getConfiguration(
    "org.sample.PrettyPrinterConfigurator");
Dictionary props = config.getProperties();

// if null, the configuration is new
if (props == null) {
    props = new Hashtable();
}

// set some properties
props.put(..., ...);

// update the configuration
config.update(props);
{code}

After the call to {{update}} the Configuration Admin service persists the new configuration data and sends an update to the {{ManagedService}} registered with the service PID {{org.sample.PrettyPrinterConfigurator}}, which happens to be our {{PrettyPrinterConfigurator}} class as expected.


h2. Apache Felix Implementation Details

The Apache Felix implementation of the Configuration Admin Service specification has a few specialities, which may be of interest when deploying it. These are described here.


h3. Configuration Properties

The Apache Felix implementation is configurable with Framework properties. Here is a short table listing the properties. Please refer to the later sections for a description of these properties.

|| Property || Type || Default Value || Description ||
| {{felix.cm.loglevel}} | int | {2} | Logging level to use in the absence of an OSGi LogService. See the _Logging_ section below. |
| {{felix.cm.dir}} | String | {{BundleContext.getDataFile("config")}} | Location of the Configuration Admin configuration files. See the _Configuration Files_ section below. |

h3. Logging

Logging goes to the OSGi LogService if such a service is registered int the OSGi framework. If no OSGi LogService is registered, the log output is directed to the Java platform standard error output ({{System.err}}).

To limit the output in the absence of an OSGi LogService, the {{felix.cm.loglevel}} framework property may be set to an integer value limiting the level of the log messages actually written: Only messages whose level is lower than or equal to the limit is actually written. All other messages are discarded.

The log levels correspond to the predefined log levels of the OSGi Log Service Specification as listed in the following table:

|| Level Number || LogService Constant || Remark ||
| 1 | LOG_ERROR | Used for error messages |
| 2 | LOG_WARNING | Used for warning messages. This is the default value of the {{felix.cm.loglevel}} property if it is not set or if the value cannot be converted to an integer. |
| 3 | LOG_INFO | Used for informational messages |
| 4 | LOG_DEBUG | Used for debug messages |

*Note*: The {{felix.cm.loglevel}} property is ignored if an OSGi LogService is actually used for logging because it is then the responsibility of the LogService to limit the actual log output.


h3. Configuration Files

By default the Apache Felix Configuration Admin Implementation stores the configuration data in the platform filesystem. The location of the configuration data can be configured with the {{felix.cm.dir}} framework property.

The resolution of the location using the {{felix.cm.dir}} and the {{BundleContext}} is implemented according to the following algorithm.

# If the {{felix.cm.dir}} property is not set, a directory named {{config}} is used inside the persistent storage area of the Apache Felix Configuration Admin Service bundle is used. This is the default behaviour.
# If the {{felix.cm.dir}} property is not set and the framework does not support persistent storage area for bundles in the filesystem, the {{config}} directory is used in the current working directory as specified in the {{user.dir}} system property is assumed.
# Otherwise the {{felix.cm.dir}} property value is used as the directory name to take the configuration data.

The result of these steps may be a relative file. In this case and if the framework provides access to persistent storage area, the directory name is resolved as being inside the persistent storage area. Otherwise the directory name is resolved to an absolute path calling the File.getAbsoluteFile() method.

If a non-directory file exists as the location found in the previous step or the named directory (including any parent directories) cannot be created, the configuration data cannot be stored in the filesystem. Generally this will result in failure to store configuration data at all, except if there is a {{org.apache.felix.cm.PersistenceManager}} service registered, which is then used.