Alex Theedom Java ee revisits design patterns

Java EE
revisits design patterns
Sep. 2-4
Kyiv, Desyatynna str. 12
@readlearncode
Alex Theedom Senior Java Developer

Who am I?
Alex Theedom, Senior Java Developer
@readlearncode

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

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

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?

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

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

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

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

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

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() {}
}
•Container/bean managed concurrency

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

•The Bad:
•overuse can cause problems
•lazy loading causes delays
•eager loading causes memory problems

•And the ugly:
•considered an anti-pattern
•only niche use
•smarter to use a stateless session bean

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

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

Factory Pattern
•Problem! Multiple concrete implementations
}
public class SoftDrinksMachine implements DrinksMachine {
}
@Inject
DrinksMachine drinksMachine;
•Which DrinksMachine to inject?
?!?

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

Factory Pattern
•Annotate respective classes
@Coffee
}
@SoftDrink
public class SoftDrinksMachine implements DrinksMachine {
}

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

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

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

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;
}

•The Good:
•easy to implement
•no boilerplate code
•works magically
•any object can be made injectable

•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

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

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

•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

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

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();
}
}

•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

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

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

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

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

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

•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

Entity
•Lightweight domain object which is persistable
•Annotated with javax.persistence.Entity
•Can represent relational data object/relational mapping
annotations

Entity
@Entity
public class Contact {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String firstName;
private String lastName;
}

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

Thank You
Sep. 2-4
Kyiv, Desyatynna str. 12
@readlearncode
Alex Theedom Senior Java Developer

Alex Theedom Java ee revisits design patterns

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