16.52. Observation Framework

The ModeShape graph model also incorporates an observation framework that allows components to register and be notified when changes occur within the content owned by a graph.

Many event frameworks define the listeners and sources as interfaces. While this is often useful, it requires that the implementations properly address the thread-safe semantics of managing and calling the listeners. The ModeShape observation framework uses abstract or concrete classes to minimize the effort required for implementing ChangeObserver or Observable. These abstract classes provide implementations for a number of utility methods (such as the unregister() method on ChangeObserver) that also save effort and code.

However, one of the more important reasons for providing classes is that ChangeObserver uses weak references to track the Observable instances, and the ChangeObservers class uses weak references for the listeners. This means that an observer does not prevent Observable instances from being garbage collected, nor do observers prevent Observable instances from being garbage collected. These abstract class provide all this functionality for free.

Each connector is responsible for propagating ChangeRequest objects to the connector's Observer. The sequencing system uses Observers to monitor for changes in the graph content to determine which, if any, sequencers should be run. The JCR implementation also uses the observation framework to propagate those changes to JCR clients.