Multiple Exported Services of the Same Type

The OSGi Service Platform states that a bundle may export multiple instances of a service. While this is quite rare, the BaseBundleActivator does support this need.

BaseBundleActivator provides APIs for handling multiple exported services of the same type:
- Configuration API
  - void addExportedService(String, Object, Dictionary)
  - void removeExportedService(String, Object)
  - void removeExportedServices(String)
  - void setExportedServiceProperties(String, Object, Dictionary)
- Query API
  - Object[] getExportedServices(String)
  - Dictionary getExportedServiceProperties(String, Object)
In the above methods, the Object parameter is always the exported service.
When there are multiple exported services of the same type, SAT often needs to be told on which exported service to operate.
It is common therefore for a bundle activator to maintain a reference to each of the exported services of the same type so that they may be passed as parameters to the above methods.

To export multiple services of the same type, simply call the configuration method addExportedService(String, Object, Dictionary) multiple times. The following example shows how to export multiple TimerService objects and register them with properties:

protected void activate() {
  Timer timer;

  Dictionary properties = new Hashtable();
  properties.put("cn", "simona");
  properties.put("o", "OTI");
  properties.put("c", "US");

  timer = new Timer(60000);
  timer.startup();
  setMinuteTimer(timer);
  addExportedService(TimerService.SERVICE_NAME, timer, properties);

  timer = new Timer(3600000);
  timer.startup();
  setHourTimer(timer);
  addExportedService(TimerService.SERVICE_NAME, timer, properties);
}

Exported services can be accessed after the fact using the query method getExportedServices(). For example:

protected void deactivate() {
  Timer[] timers = (Timer[]) getExportedServices(TimerService.SERVICE_NAME);
  Timer timer;

  for (int i = 0; i < timers.length; i++) {
    timer = timers [ i ];
    timer.shutdown();
  }
}

The method getExportedServiceProperties(String, Object) is used to query the properties that were registered with an exported service.
The method setExportedServiceProperties(String, Object, Dictionary) is used to set the properties of an exported service.

The following example shows how to update the properties that were registered with an exported service:

Timer timer = getMinuteTimer();
Dictionary properties = getExportedServiceProperties(TimerService.SERVICE_NAME, timer);
properties.put("o", "IBM");
setExportedServiceProperties(TimerService.SERVICE_NAME, timer, properties);

Modifying the properties of an exported service is considered a service modification by the OSGi framework.