Many business processes involve complex manipulations of business classes. Business classes often participate in multiple business processes or workflows. Complex processes that involve multiple business objects can lead to tight coupling between those classes, with a resulting decrease in flexibility and design clarity. Complex relationships between low-level business components make clients difficult to write.

The Session Facade pattern defines a higher-level business component that contains and centralizes complex interactions between lower-level business components. A Session Facade is implemented as a session enterprise bean. It provides clients with a single interface for the functionality of an application or application subset. It also decouples lower-level business components from one another, making designs more flexible and comprehensible.

Fine-grained access through remote interfaces is inadvisable because it increases network traffic and latency. The "before" diagram in Figure 1 below shows a sequence diagram of a client accessing fine-grained business objects through a remote interface. The multiple fine-grained calls create a great deal of network traffic, and performance suffers because of the high latency of the remote calls.

Figure 1. Sequence diagram before and after adding Session Facade

Introducing a Session Facade, as shown in the "after" diagram in Figure 1, decreases network traffic and latency, because all access to fine-grained business objects is local. The Session Facade also acts as a Mediator [ GHJV95 ] between the business objects, decoupling their APIs from one another.

See the Core J2EETM Patterns

• The admin facade.

  The sample admin application of the Pet Store enterprise is responsible for management functionalities of the Pet Store order processing workflow. The admin application is a Swing client with Java^TM Web Start that communicates with the backend order processing center application to approve large purchase orders and view financial data. To interact with the order processing center application, the admin application uses the Session Facade pattern in the interface OPCAdminFacade , whose interface appears below.

              
  
  public interface OPCAdminFacade extends EJBObject {
  
      public OrdersTO getOrdersByStatus(String status)
          throws RemoteException, OPCAdminFacadeException;
  
      public Map getChartInfo(String request, 
                              Date start,
                              Date end, 
                              String requestedCategory)
          throws RemoteException, OPCAdminFacadeException;
  }
  
        
            

  The admin facade's implementation class is OPCAdminFacadeEJB . The admin application is a client of the OPCAdminFacade in the order processing application. A structure diagram of how the Session Facade interacts with other classes in the order processing application appears in Figure 2 below. The greyed-out class in the diagram indicates the transfer object created by the facade class; see the Transfer Object pattern for details.

  Figure 2. OPCAdminFacade provides an interface to complex interactions between components

  The key point to notice here, is that the client only interacts with the OPCAdminFacade, and the other Enterprise Beans are hidden behind the facade. So the client has a simple interface and is not exposed to the complexity of the application logic that occurs to process the request. One of OPCAdminFacade's methods, getOrdersByStatus, builds a summary of purchase information for display by the sample application's admin client. This method uses a ProcessManagerLocal stateless session bean and ManagerLocal local entity beans and iterates over a collection of PurchaseOrderLocal entity beans of the requested status. The method creates a serializable Collection of OrderDetails transfer objects that describe PurchaseOrderLocal beans found. (See the Transfer Object pattern for details). The method then returns the collection of objects to the client.

• The Java Pet Store website sample application shopping facade.
  • The Java Pet Store website application also uses the Session Facade pattern. ShoppingClientFacadeLocal is a local stateful session bean that centralizes the services that the Web site provides to shoppers. Its local interface appears below:

                    
    
    public interface ShoppingClientFacadeLocal extends EJBLocalObject {
    
        public ShoppingCartLocal getShoppingCart();
        public void setUserId(String userId);
        public String getUserId();
        public CustomerLocal getCustomer() throws FinderException;
        public CustomerLocal createCustomer(String userId);
    
    }
    
                
                  

    The shopping facade provides a single interface to the client for all client shopping functionality: the shopping cart, storing the user's id, and finding and creating the Customer data associated with the user. The facade is a stateful session bean because it stores information (such as the user id) that is specific to an individual user in the context of a session.