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new a33eca5 Add readme for config admin
a33eca5 is described below
commit a33eca5bce0414b97e27f7a5416edb455e3d05c5
Author: Carsten Ziegeler <[email protected]>
AuthorDate: Tue Mar 3 19:38:56 2020 +0100
Add readme for config admin
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+# Apache Felix Configuration Admin Service
+
+The OSGi Componendium Configuration Admin Service specifies a service, which
allows for easy management of configuration data for configurable components.
Basically a configuration is a list of name-value pairs. A cnfiguration 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 pers [...]
+
+For more information, its suggested you read [Chapter 104, Configuration Admin
Service
Specification](https://osgi.org/specification/osgi.cmpn/7.0.0/service.cm.html),
of the OSGi Compendium Services Specification book.
+
+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.
+
+If you use [OSGi Declarative
Services](https://osgi.org/specification/osgi.cmpn/7.0.0/service.component.html),
that implementation takes care of all the low level configuration handling and
you don't need to interact with Configuration Admin directly.
+
+## Project Info
+
+The Apache Felix Configuration Admin Service provided the reference
implementation for the OSGi specification. The latest release of this
implementation implements specification version 1.6 of the [OSGi R7 Compendium
release](https://osgi.org/specification/osgi.cmpn/7.0.0/service.cm.html).
+
+## `ManagedService` Example
+
+Consider you have requirement for some configuration, say the line length of a
pretty printer. You want to have this configurable through configuration admin.
+
+You need the following parts:
+
+ * A service PID identifying the configuration
+ * A `ManagedService` to receive the configuration
+ * Name(s) for the configuration property/ies
+
+The PID is just a string, which must be globally unique. Assuming a simple
case where your PrettyPrinter configurator receives the configuration has a
unique class name, you may well use that name. So lets assume, our
`ManagedService` is called `org.sample.PrettyPrinterConfigurator` and that name
is also used as the PID. For more information on the Service PID, refer to
Section 104.3, The Persistent Identity of the OSGi Compendium Services
Specification.
+
+The class would be:
+
+ package org.sample;
+ class PrettyPrinterConfigurator implements ManagedService {
+ public void update(Dictionary props)
+ throws ConfigurationException {
+ if (props == null) {
+ // no configuration from configuration admin
+ // or old configuration has been deleted
+ } else {
+ // apply configuration from config admin
+ }
+ }
+ }
+
+
+Now, in your bundle activator's start() method you register the
`PrettyPrinterConfigurator` as a `ManagedService`:
+
+ ...
+ private ServiceRegistration ppcService;
+ public void start(BundleContext context) {
+ Dictionary<String, Object> 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;
+ }
+ }
+ ...
+
+
+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:
+
+
+ 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);
+
+
+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.
+
+
+## ManagedServiceFactory example
+
+Registering a service as ManagedServiceFactory means that it will be able to
receive several different configuration dictionaries; that's particularly
useful when we want to create a Service Factory, that is, a service responsible
for creating multiple instances of a specific service.
+
+A ManagedServiceFactory service needs to implement the ManagedServiceFactory
interface, as showed in the example.
+
+
+ public class SmsSenderFactory implements ManagedServiceFactory
+ {
+ Map existingServices = new HashMap();
+
+ public void updated(String pid, Dictionary dictionary) throws
ConfigurationException
+ {
+ // invoked when a new configuration dictionary is assigned
+ // to service 'pid'.
+ if (existingServices.containsKey(pid)) //the service already
exists
+ {
+ MyService service = (MyService) existingServices.get(pid);
+ service.configure(dictionary);
+ }
+ else //configuration dictionary for a new service
+ {
+ MyService service = createNewServiceInstance();
+ service.configure(dictionary);
+ existingServices.put(pid, service);
+ }
+ }
+
+ public void deleted(String pid)
+ {
+ // invoked when the service 'pid' is deleted
+ existingServices.remove(pid);
+ }
+
+ public String getName()
+ {
+ return "test.smssenderfactory";
+ }
+ }
+
+
+The example above shows that, differently from the ManagedService, the
ManagedServiceFactory is designed to manage multiple instances of a service. In
fact, the `update` method accept a `pid` and a `Dictionary` as arguments, thus
allowing to associate a certain configuration dictionary to a particular
service instance (identified by the `pid`).
+
+Note also that the ManagedServiceFactory interface requires to implement
(besides the `getName` method) a `delete` method: this method is invoked when
the Configuration Admin Service asks the ManagedServiceFactory to delete a
specific service instance.
+
+The registration of a `ManagedServiceFactory` follows the same steps of the
`ManagedService` example:
+
+
+ private ServiceRegistration factoryService;
+ public void start(BundleContext context) {
+ Dictionary props = new Hashtable();
+ props.put("service.pid", "test.smssenderfactory");
+ factoryService =
context.registerService(ManagedServiceFactory.class.getName(),
+ new SmsSenderFactory(), props);
+ }
+ public void stop(BundleContext context) {
+ if (factoryService != null) {
+ factoryService.unregister();
+ factoryService = null;
+ }
+ }
+ ...
+
+
+Finally, using the ConfigurationAdmin interface, it is possible to send new or
updated configuration dictionaries to the newly created ManagedServiceFactory:
+
+
+ public class Activator implements BundleActivator
+ {
+ private List configurationList = new ArrayList();
+
+ public void start(BundleContext bundleContext) throws Exception
+ {
+ ServiceReference configurationAdminReference =
+
bundleContext.getServiceReference(ConfigurationAdmin.class.getName());
+
+ if (configurationAdminReference != null)
+ {
+ ConfigurationAdmin confAdmin = (ConfigurationAdmin)
bundleContext.getService(configurationAdminReference);
+
+ Configuration configuration =
confAdmin.createFactoryConfiguration("test.smssenderfactory", null);
+ Dictionary properties = createServiceProperties();
+ configuration.update(properties);
+
+ //Store in the configurationList the configuration object, the
dictionary object
+ //or configuration.getPid() for future use
+ configurationList.add(configuration);
+ }
+ }
+ }
+
+
+
+## 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.
+
+
+### 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. |
+
+### 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.
+
+
+### 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.
+
+1. 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.
+1. 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.
+1. 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.
