In distributed applications, lookup and exception handling for remote business components can be complex. When applications use business components directly, application code must change to reflect changes in business component APIs.

These problems can be solved by introducing an intermediate class called a business delegate, which decouples business components from the code that uses them. The Business Delegate pattern manages the complexity of distributed component lookup and exception handling, and may adapt the business component interface to a simpler interface for use by views.

See the Core J2EETM Patterns

Sample application business delegate class AdminRequestBD handles distributed lookup and catches and adapts exceptions in the sample application order processing center (OPC).

• The AdminRequestBD business delegate manages distributed component lookup and handles exceptions.

  The structure diagram in Figure 1 shows the ApplRequestProcessor servlet using AdminRequestBD to find and use distributed business components.

  Figure 1. AdminRequestBD locates and adapts other business components

  The code sample below shows the constructor for AdminRequestBD. It uses class ServiceLocator to acquire the remote home interface of session facade OPCAdminFacade . (See the Service Locator and Session Facade design patterns.) It uses the remote home interface to create a OPCAdminFacade remote component interface, which it maintains in a private field. The block that locates and creates the enterprise bean reference catches exceptions related to finding the home interface and to creating the component interface. Any exception that occurs is then wrapped in an AdminBDException , which effectively hides the implementation details of the business delegate from its clients.

              
     public AdminRequestBD() throws AdminBDException {
         try {
              OPCAdminFacadeHome home = (OPCAdminFacadeHome) ServiceLocator.getInstance().getRemoteHome(OPC_ADMIN_NAME, OPCAdminFacadeHome.class);
              opcAdminEJB = home.create();
          } catch (ServiceLocatorException sle) {            
              throw new AdminBDException(sle.getMessage());
          } catch (CreateException ce) {
              throw new AdminBDException(ce.getMessage());
          } catch (RemoteException re) {
              throw new AdminBDException(re.getMessage());
          }
      }
  
      
            



• The OPC request processor uses AdminRequestBD for simple access to business components components.

  OPC servlet class ApplRequestProcessor receives service requests from the admin client in the form of XML messages transmitted using HTTP. One of these request is for statistics about orders that have a given status.

  Method ApplRequestProcessor.getOrders receives part of an XML DOM tree representing a Web service request. It extracts the status code from the document and uses AdminRequestBD.getOrdersByStatus to retrieve a list of order information. The interface to that list is transfer object interface OrdersTO . (See the Transfer Object pattern.) The code in the request processor that retrieves the order information appears in the following code sample.

              
  public class ApplRequestProcessor extends HttpServlet {
  ...
      String getOrders(Element root) {
          try {
              AdminRequestBD bd = new AdminRequestBD();
              NodeList nl = root.getElementsByTagName("Status");
              String status = getValue(nl.item(0));
              OrdersTO orders = bd.getOrdersByStatus(status);
  ...
  }
            

  Because it has already created the reference to an OPCAdminFacadeEJB, the AdminRequestBD object can simply forward the call to the enterprise bean's method getOrdersByStatus, as follows:

              
      
  public class AdminRequestBD {
  ...
      public OrdersTO getOrdersByStatus(String status)
          throws AdminBDException {
  
          try {
              return opcAdminEJB.getOrdersByStatus(status);
          } catch (RemoteException re) {
              throw new AdminBDException(re.getMessage());
          } catch (OPCAdminFacadeException oafee) {
              throw new AdminBDException(oafee.getMessage());
          }
      }
  ...
  }
            

  Notice again that the method catches any exceptions that the enterprise bean may throw and re-throws an exception type that is specific to the business delegate's interface. This hides the business delegate's implementation details from the client.