1. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-1 Advanced Java Programming Design issues Chris Wong cw@it.uts.edu.au Based on Sun J2EE design patterns, & Marinescu “EJB design patterns” (Wiley 2002)

2. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-2 Topics Design patterns Sun Design patterns Popular design patterns

3. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-3 Design Patterns Marinescu: “think of a pattern as a best practice solution to a common recurring problem.” Based on years of experience in developing applications The pattern bible: Gamma, E., R. Helm, R. Johnson, and J. Vlissides. 1995. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. See also: –TheServerSide.com. “Patterns Repository.” Available at: www.theserverside.com/patterns

4. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-4 Design Patterns We have already seen 2 design patterns in this course –Model-View-Controller –Session Facade

5. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-5 Sun Design Patterns-1 Sun have categorised several design patterns. http://java.sun.com/blueprints/ patterns/catalog.html

6. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-6 Sun Design patterns-2 Business Delegate –Reduce coupling between Web and EJB tiers Composite Entity – Model a network of related business entities Data Access Object (DAO) –Abstract and encapsulate data access mechanisms Fast Lane Reader –Improve read performance of tabular data Front Controller –Centralize application request processing Intercepting Filter –Pre- and post-process application requests

7. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-7 Sun Design patterns-3 Model-View-Controller –Decouple data representation, application behaviour, and presentation Service Locator –Simplify client access to enterprise business services Session Facade –Coordinate operations between multiple business objects in a workflow Transfer Object –Transfer business data between tiers Value List Handler –Efficiently iterate a virtual list View Helper –Simplify access to model state and data access logic

8. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-8 Popular Design patterns Most popular design patterns are combination of: –MVC + Data Transfer Object + Session Façade –Useful toolset to run is Apache Struts http://jakarta.apache.org/struts/ http://jakarta.apache.org/struts/ Another pattern that is common with asynchronous update operations is the: – Message façade pattern

9. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-9 Model View Controller http://java.sun.com/blueprints/patterns/MVC-detailed.html

10. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-10 Session Facade

11. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-11 Data Transfer Object Also called the Value Object Design transfers related information/objects as one object This improves performance and provides logical grouping of objects Provides a façade that encapsulates other objects. Usually implemented as a JavaBean ie: serializable

12. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-12 Data Transfer Object Variations: Data Transfer hash map – instead of getter/setter methods, use attributes keywords get/put data. Passes hash map to backend object (eg: Session EJB) Data Transfer rowset – like above, but pass a java.sql.Rowset object. Use only with BMP or JDBC backends.

13. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-13 Message Façade-1 Problem: sometimes you need to access multiple objects/EJB’s without blocking eg: airline reservation system and need high availability. Solution: Use asynchronous programming Message façade pattern –Uses Message Driven Beans to access Message Oriented Middleware like JMS –Servlet sends JMS message to destination. –MDB then calls appropriate session/entity EJB

14. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-14 Message Façade-2

15. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-15 Message Façade-3 Issue: How to communicate back to client success/failure? You need to devise alternative methods eg: confirmation/ receipt number displayed to client Alternative is to ensure all business logic on session façade bean, and message driven bean invokes session façade bean. Then if client uses the session façade, they will see updates automatically! Session JMS MDB servlet Entity Servlet does not need to wait when it sends a message to JMS

16. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-16 Other Design patterns EJB Home Factory pattern Often need to have multiple JNDI lookups of EJB homes. This pattern suggests creating a EJBHomeFactory class which caches bean homes. Good separation of business logic from management logic Performance can be 10x better. Can also be used in servlet

17. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-17 Other Design patterns Service Locator Pattern Often presentation tier needs to call business tier However, this means client code needs to know business tier details eg: EJB names. Also complex, can create network overhead, inefficient design ServiceLocator object can abstract JNDI, cache resources eg: InitialContext, EJB Homes Centralises all references to implementation, so you only need to change at one place.

18. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-18 Other Design patterns Business Delegate Pattern Often presentation tier needs to call business tier –However, this means client code needs to know business tier details eg: EJB names. This pattern abstracts business tier via client side BusinessDelegate helper object. Business delegate does all JNDI, EJB calls. Can cache (see EJBHomePattern). Also can wrap EJB/remoteExceptions. –Good seperation between client development and server development –Typically uses the Service Locator pattern for system calls. –Eg: Shopping Cart Bean

19. Faculty of Information Technology © Copyright UTS Faculty of Information Technology 2004Design-19 Design patterns conclusion We have only lightly touched on common design patterns. Should apply these common patterns at first. Try use Struts if you can. Don’t blindly follow patterns – use what is appropriate for you, even if this means not using EJB’s. (and some patterns provide alternatives to EJB’s) Recommended reading: SUN website: http://java.sun.com/blueprints/patternshttp://java.sun.com/blueprints/patterns Floyd Marinescu EJB™ Design Patterns (Wiley 2002) ISBN: 0-471- 20831-0 TheServerSide.com. “Patterns Repository.” Available at: www.theserverside.com/patterns www.theserverside.com/patterns Goodwill,J Mastering Jakarta Struts (Wiley 2002) ISBN: 0-471-21302-0