Design patterns are not only cool but represent the collective wisdom of many developers. Since the publication of Design Patterns: Elements of Reusable Object-Oriented Software by GoF many new concepts have extended the coverage of these design patterns, and now Java EE provide out of the box implementations of many of the most well known patterns. This talk will show how, by taking advantage of Java EE features such as CDI and the smart use of annotations, traditional design patterns can be implemented in a much cleaner and quicker way. Among the design patterns discuss there will be Singleton, Façade, Observer, Factory, Dependency Injection, Decorator and more.

1. Java EE
revisits design patterns
Alex Theedom @alextheedom

2. Who am I?
Alex Theedom, Senior Java Developer
@alextheedom

3. Available at

4. What’s on?
• Design patterns retrospective
• Map classical design patterns to Java EE 7
• Contexts and Dependency Injection
• Singleton, Factory, Façade, Decorator and Observer
• Final Conclusions
• Q&A

5. What are you doing?
• Java Enterprise Edition (J2EE, EE 6/7)
• Spring

6. The beginning
•Design Patterns: Elements of Reusable Object-Oriented Software
(E Gamma, R Helm, R Johnson and J Vlissides. 1994)
AKA Gang of Four AKA GoF
•Creational, behavioral and structural
•So what are design patterns in practice?

7. Enterprise Java and design
patterns
•JavaOne 2000 talk: Prototyping patterns for the J2EE
platform
•Core J2EE Patterns (D. Anur, J. Crupi and D. Malks)
•Out-of-box design pattern implementations with
Java EE 5

8. Java EE Programming Model
•Simplified programming model
•Annotations have replaced XML description files
•Convention over Configuration
•CDI hides object creation
•Resources are injected by type
•@Inject and disambiguation via custom qualifier
•POJO (JSR 299 managed bean)
•Otherwise @Produces

9. Singleton Pattern
•Creational pattern
•Single instance and instantiated once
•Must be thread safe
•Not normally destroyed during application life cycle
•Classic implementation: private constructor, double
locking, static initializer, enums …

10. Singleton Pattern
@DependsOn("DatabaseConnectionBean")
@Startup
@Singleton
public class Logger {
@PostConstruct
void constructExpensiveObject() {
// Expensive construction
}
}
@DependsOn("DatabaseConnectionBean")
@Startup
@Singleton
public class Logger {
@PostConstruct
void constructExpensiveObject() {
// Expensive construction
}
}
@Inject
Logger logger;
@Inject
Logger logger;

11. Singleton Pattern
•Conclusions so far
•Very different implementation
•Substantially less boilerplate code
•Enhancements via specialized annotations
There’s more…

12. Singleton Pattern
@Singleton
@ConcurrencyManagement(ConcurrencyManagementType.BEAN)
public class Logger {
@AccessTimeout(value = 30, unit=TimeUnit.SECONDS)
@Lock(LockType.WRITE)
public void addMessage(String message) {}
@Lock(LockType.READ)
public String getMessage() {}
}
@Singleton
@ConcurrencyManagement(ConcurrencyManagementType.BEAN)
public class Logger {
@AccessTimeout(value = 30, unit=TimeUnit.SECONDS)
@Lock(LockType.WRITE)
public void addMessage(String message) {}
@Lock(LockType.READ)
public String getMessage() {}
}
•Container/bean managed concurrency

13. The Good, Bad and the Ugly
•The Good:
•enhancements via specialized annotations
•startup behavioural characteristics
•fine grain control over concurrency and access
timeout
•substantially less boilerplate code

14. The Good, Bad and the Ugly
•The Bad:
•overuse can cause problems
•lazy loading causes delays
•eager loading causes memory problems

15. •And the ugly:
•considered an anti-pattern
•only niche use
•smarter to use a stateless session bean
The Good, Bad and the Ugly

16. Factory Pattern
•Creational pattern
•Interface for creating family of objects
•Clients are decoupled from the creation

17. Factory Pattern
•CDI framework is a factory !?!
public class CoffeeMachine implements DrinksMachine {
// Implementation code
}
public class CoffeeMachine implements DrinksMachine {
// Implementation code
}
•Use it like so:
@Inject
DrinksMachine drinksMachine;
@Inject
DrinksMachine drinksMachine;

18. Factory Pattern
•Problem! Multiple concrete implementations
public class CoffeeMachine implements DrinksMachine {
// Implementation code
}
public class SoftDrinksMachine implements DrinksMachine {
// Implementation code
}
public class CoffeeMachine implements DrinksMachine {
// Implementation code
}
public class SoftDrinksMachine implements DrinksMachine {
// Implementation code
}
@Inject
DrinksMachine drinksMachine;
@Inject
DrinksMachine drinksMachine;
•Which DrinksMachine to inject?
?!?

19. Factory Pattern
•Solution! Qualifiers
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.FIELD})
public @interface SoftDrink
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.FIELD})
public @interface SoftDrink
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.FIELD})
public @interface Coffee
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.FIELD})
public @interface Coffee

20. Factory Pattern
•Annotate respective classes
@Coffee
public class CoffeeMachine implements DrinksMachine {
// Implementation code
}
@Coffee
public class CoffeeMachine implements DrinksMachine {
// Implementation code
}
@SoftDrink
public class SoftDrinksMachine implements DrinksMachine {
// Implementation code
}
@SoftDrink
public class SoftDrinksMachine implements DrinksMachine {
// Implementation code
}

21. Factory Pattern
•Annotate injection points
@Inject @SoftDrink
DrinksMachine softDrinksMachine;
@Inject @SoftDrink
DrinksMachine softDrinksMachine;
@Inject @Coffee
DrinksMachine coffeeDrinksMachine;
@Inject @Coffee
DrinksMachine coffeeDrinksMachine;

22. Factory Pattern
•Remember
•Only JSR299 beans are ‘injectable’
•But I want to inject a Collection type or Object with a
parameterized constructor
-> let’s dive deeper

23. Factory Pattern
•Dive deeper
•Producer methods
•Use it like so:
@History // or @Named("History")
@Produces
public List<Book> getLibrary(){
// Generate a List of books called 'library'
return library;
}
@History // or @Named("History")
@Produces
public List<Book> getLibrary(){
// Generate a List of books called 'library'
return library;
}
@History
@Inject
List<Books> library;
@History
@Inject
List<Books> library;
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface History
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface History

24. Factory Pattern
•Scope
•Determines when method called
•Life of object: @RequestScoped -> @ApplicationScoped
@RequestScoped
@History
@Produces
public List<Book> getLibrary(){
// Generate a List of books called 'library'
return library;
}
@RequestScoped
@History
@Produces
public List<Book> getLibrary(){
// Generate a List of books called 'library'
return library;
}

25. The Good, Bad and the Ugly
•The Good:
•easy to implement
•no boilerplate code
•works magically
•any object can be made injectable
•automatic per class configuration
•disambiguation/filtration via qualifiers

26. The Good, Bad and the Ugly
•The Bad: named annotation is not type safe
@Named("History") -> @History
•and the ugly: object creation hidden, hard to follow
execution flow, IDE should help

27. Façade Pattern
•Hides the complex logic and provides the interface for
the client
•Typically used in APIs
•Classic implementation: Just create a method to hide
complexity

28. Façade Pattern
•Encapsulates complicated logic
•@Stateless, @Stateful
@Stateless
public class BankServiceFacade{
public void complexBusinessLogic(){
// Very very complex logic
}
}
@Stateless
public class BankServiceFacade{
public void complexBusinessLogic(){
// Very very complex logic
}
}
@Inject
public BankServiceFacade service;
@Inject
public BankServiceFacade service;

29. The Good, Bad and the Ugly
•The Good: simple, robust, gives you a range of services
•The Bad: over use introduces unnecessary layers, if you
don’t need it don’t introduce one
•and the ugly: there isn’t one really

30. Decorator Pattern
•Structural Pattern
•Dynamically adds logic to an object at runtime
•More flexible that inheritance
•Classic implementation: Both the decorator and target
object share the same interface. Decorator wraps the
target object and adds its own behaviour

31. Decorator Pattern
@Decorator
@Priority(Interceptor.Priority.APPLICATION)
public class PriceDiscountDecorator implements Product {
@Coffee
@Any
@Delegate
@Inject
private Product product;
public String generateLabel() {
product.setPrice(product.getPrice() * 0.5);
return product.generateLabel();
}
}
@Decorator
@Priority(Interceptor.Priority.APPLICATION)
public class PriceDiscountDecorator implements Product {
@Coffee
@Any
@Delegate
@Inject
private Product product;
public String generateLabel() {
product.setPrice(product.getPrice() * 0.5);
return product.generateLabel();
}
}

32. •The Good: very easy to change behaviour without
breaking legacy code
•The Bad: needs XML config (<CDI 1.1)
•and the ugly: overuse will introduce an execution flow
which is hard to follow
The Good, Bad and the Ugly

33. Observer Pattern
•Behavioural Pattern
•Publisher-Subscriber
•Classic implementation: Implement a Subscriber
interface, register with publisher and call a notify
method on subscribers

34. Observer Pattern
•Define an event class
public class MessageEvent {
private String message;
public MessageEvent(String message){
this.message = message;
}
public String getMessage(){
return message;
}
}
public class MessageEvent {
private String message;
public MessageEvent(String message){
this.message = message;
}
public String getMessage(){
return message;
}
}

35. Observer Pattern
•Notifies dependents of state change
@Stateless
public class MessageService {
@Inject
Event<MessageEvent> messageEvent;
public void fireMessage(){
messageEvent.fire(new MessageEvent("Hello World"));
}
}
@Stateless
public class MessageService {
@Inject
Event<MessageEvent> messageEvent;
public void fireMessage(){
messageEvent.fire(new MessageEvent("Hello World"));
}
}

36. Observer Pattern
•Dependent receives state change notification
@Stateless
public class MessageObserver {
public void listenToMessage(@Observes MessageEvent message){
System.out.println(message.getMessage());
}
}
@Stateless
public class MessageObserver {
public void listenToMessage(@Observes MessageEvent message){
System.out.println(message.getMessage());
}
}

37. Observer Pattern
•Qualifier to filter events
@WarningMessage
@Inject
Event<MessageEvent> messageEvent;
@WarningMessage
@Inject
Event<MessageEvent> messageEvent;
public void listenToMessage(
@Observes @WarningMessage MessageEvent message)
public void listenToMessage(
@Observes @WarningMessage MessageEvent message)

38. The Good, Bad and the Ugly
•The Good: very easy, no boilerplate code, less than JMS,
the container does the heavy lifting, light weight
•The Bad: confusing execution order but IDE will help
•and the ugly: nothing, its beautiful

39. Final Conclusion
•Efficiency savings
•Greater control over behaviour
•New features enhance implementation

40. Q & A

41. Java EE
revisits design patterns
Alex Theedom @alextheedom