Distribution and persistence as aspects

aspect ServerSide { abstract pointcut facadeMainExecution(); abstract Remote initFacadeInstance(); abstract String getSystemName(); abstract String getServerName(); Those abstract methods and pointcut are implemented by system specific aspects. The abstract aspect also defines and around advice that uses the abstract methods to export the facade instance and make it remotely available. Copyright c © 2000 John Wiley & Sons, Ltd. Softw. Pract. Exper. 2000; 00:1–6 Prepared using speauth.cls DISTRIBUTION AND PERSISTENCE AS ASPECTS 11 void around(): facadeMainExecution() { try { Remote facade = initFacadeInstance(); String systemName = getSystemName(); UnicastRemoteObject.exportObject(facade); java.rmi.Naming.rebind("/"+ systemName, facade); } catch (RemoteException rmiEx) { ... } } catch (MalformedURLException rmiEx) { ... } } } In fact, this advice replaces the entire original main method, and therefore, the original behavior if despised. However, usually, the main method of a facade class might not even exist or it may only has tests over the system. It is very unusual a main method to be executed while the system is being executed. Defining a concrete aspect As previously mentioned, RMI remote objects must implement a remote interface. Hence, the concrete server-side aspect has to modify the facade class (HWFacade) to implement a corresponding remote interface (IRemoteFacade), which extends the RMI remote interface (java.rmi.Remote). Since the IRemoteFacade interface is only needed to implement distribution, its relationship with the system facade was implemented by the distribution aspect. This is done by using AspectJ’s declare parents construct: aspect HWServerSide extends ServerSide { declare parents: HWFacade implements IRemoteFacade; The IRemoteFacade interface, which is distribution specific code, was part of the pure Java version of the system, so we did not have to implement it again. In the AspectJ version of the system, this interface is specific and auxiliary to the distribution aspects, so it was grouped with the other auxiliary types. Besides extending RMI’s Remote interface, this interface contains the signatures of the facade public methods, however, adding the java.rmi.RemoteException in their throws clauses. This exception is used by RMI in order to indicate several kinds of configuration problems and remote communication failures. The concrete aspect must implement the abstract members of the abstract aspect in order to specialize it to a specific system. The methods Remote initFacadeInstance() { return HWFacade.getInstance(); } String getSystemName() { return "HealthWatcherSystem"; } String getServerName() { return /* server IP or DNS address */ }