Java Technologies. Lecture III. Valdas Rapševičius. Vilnius University Faculty of Mathematics and Informatics

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1 Preparation of the material was supported by the project Increasing Internationality in Study Programs of the Department of Computer Science II, project number VP1 2.2 ŠMM-07-K , funded by The European Social Fund Agency and the Government of Lithuania. Java Technologies Lecture III Valdas Rapševičius Vilnius University Faculty of Mathematics and Informatics Valdas Rapševičius. Java Technologies 1

2 Session Outline To gain the good understanding of Gang of Four patterns Creational Structural Behavioral To familiarize you with the concept of evil of dependencies and the techniques on how it can be broken ServiceLocator pattern Dependency Injection (DI) pattern Valdas Rapševičius. Java Technologies 2

3 Design Patterns Introduced by Christopher Alexander in the 1970s Design pattern is a certain solutions that can be applied over and over again to the same or similar problems. In 1994 Erich Gamma, Richard Helm, John Vlissides, and Ralph Johnson published the now-famous book Design Patterns: Elements of Reusable Object- Oriented Software, which described a way of documenting patterns that has become the industry standard. These men are often referred to as the Gang of Four (GoF). The GoF patterns are categorized into three categories, as follows: Creational - support the creation of objects Structural - deal with relationships between portions of your application Behavioral - influence how state and behavior flow through the system Other patterns: Core J2EE Patterns (to be discussed in J2EE part) Other Java techniques Valdas Rapševičius. Java Technologies 3

4 GoF: Creational: Abstract Factory Client ProductA ProductB Factory +createproducta() : ProductA +createproductb() : ProductB ProductA1 Factory1 ProductB1 +createproducta() : ProductA +createproductb() : ProductB ProductA2 ProductB2 Factory2 +createproducta() : ProductA +createproductb() : ProductB Valdas Rapševičius. Java Technologies 4

5 GoF: Creational: Builder Client -builder «interface» Builder 1 +buildpart() +getproduct() : ProductA1 BuilderA1 ProductA1 +buildpart() +getproduct() : ProductA Valdas Rapševičius. Java Technologies 5

6 GoF: Creational: Factory Creator ProductA +create() : ProductA +otheroperation() Creator1 ProductA1 +creator() : ProductA Valdas Rapševičius. Java Technologies 6

7 GoF: Creational: Prototype Client -prototype Prototype 1 +clone() : Prototype Prototype1 Prototype2 +clone() : Prototype +clone() : Prototype Valdas Rapševičius. Java Technologies 7

8 GoF: Creational: Singleton Singleton -Singleton() +getinstance() : Singleton 1 -instance Valdas Rapševičius. Java Technologies 8

9 GoF: Structural: Adapter Client Adapter +request() Adapter1 -adaptee Adaptee +request() 1 +specificrequest() Valdas Rapševičius. Java Technologies 9

10 GoF: Structural: Bridge Client -impl Implementor 1 +operationimpl() Client1 Client2 Implementor1 Implementor2 +operationimpl() +operationimpl() Valdas Rapševičius. Java Technologies 10

11 GoF: Structural: Composite Client Component -child * Leaf Composite +add(in c : Composite) +remove(in i : int) +getchild(in i : int) : Composite Valdas Rapševičius. Java Technologies 11

12 GoF: Structural: Decorator Component -component 1 Component1 Decorator Decorator1 -addedstate : string Decorator2 +addedbehavior() Valdas Rapševičius. Java Technologies 12

13 GoF: Structural: Facade Client Facade Class1 Class3 Class2 Class Valdas Rapševičius. Java Technologies 13

14 GoF: Structural: FlyWeight Client FlyWeightFactory -flyweights FlyWeight * FlyWeight1 FlyWeight Valdas Rapševičius. Java Technologies 14

15 GoF: Structural: Proxy Client Proxy Proxy1 1 Proxy2 -proxy Valdas Rapševičius. Java Technologies 15

16 GoF: Behavioral: Chain of Responsibility Client CommandHandler -successor +handlerequest() 1 CommandHandler1 CommandHandler2 +handlerequest() +handlerequest() Valdas Rapševičius. Java Technologies 16

17 GoF: Behavioral: Command Client Invoker -command Command +invoke() 1 +execute() Command1 Command2 +execute() +execute() Receiver1 Receiver Valdas Rapševičius. Java Technologies 17

18 GoF: Behavioral: Interpreter Context Client Expression +interpret(in ctx : Context) -next 1 TerminalExpression NonTerminalExpression +interpret(in ctx : Context) +interpret(in ctx : Context) Valdas Rapševičius. Java Technologies 18

19 GoF: Behavioral: Iterator «interface» Aggregate +iterator() : Iterator Client «interface» Iterator +hasnext() : boolean +next() : Object SomeAggregator 1 SomeIterator +iterator() : SomeIterator -aggregate +hasnext() : boolean +next() : Object Valdas Rapševičius. Java Technologies 19

20 GoF: Behavioral: Mediator 1 -mediator Mediator -colleagues Colleague * ConcreteMediator Colleague01 Colleague Valdas Rapševičius. Java Technologies 20

21 GoF: Behavioral: Memento Originator Memento 1 Caretaker +setmemento(in m : Memento) +creatememento() +setstate() +getstate() : Object -memento Valdas Rapševičius. Java Technologies 21

22 GoF: Behavioral: Observer Subject +attach(in o : Observer) +detach(in o : Observer) +notify() +getstate() : Object -observers * -subject 1 +update() Observer ConcreteSubject ConcreteObserver +getstate() : Object +update() Valdas Rapševičius. Java Technologies 22

23 GoF: Behavioral: State StateContext +request() -handler 1 «interface» State +handle() ConcreteState1 ConcreteState2 +handle() +handle() Valdas Rapševičius. Java Technologies 23

24 GoF: Behavioral: Strategy StrategyContext -strategy Strategy +contextinterface() 1 +algorithminterface() ConcreteStrategy01 ConcreteStrategy02 ConcreteStrategy03 +algorithminterface() +algorithminterface() +algorithminterface() Valdas Rapševičius. Java Technologies 24

25 GoF: Behavioral: Template Algorithm +execute() #step01() #step02() ConcreteAlgorithm1 ConcreteAlgorithm2 +step01() +step02() +step01() +step02() Valdas Rapševičius. Java Technologies 25

26 GoF: Behavioral: Visitor Client «interface» Visitor +visit(in a : ElementA) +visit(in b : ElementB) Element +accept(in v : Visitor) ConcreteVisitor ElementA ElementB +visit(in a : ElementA) +visit(in b : ElementB) +accept(in v : Visitor) +accept(in v : Visitor) Valdas Rapševičius. Java Technologies 26

27 Fluent Builder pattern (1) public interface ObjBuilder<T> { T build(); public class Update { private final Author author; private final String updatetext; private Update(Builder b) { author = b.author; updatetext = b.updatetext; public static class Builder implements ObjBuilder<Update> { private Author author; private String updatetext; public Builder author(author author_) { author = author_; return this; public Builder updatetext(string updatetext_) { updatetext = updatetext_; return this; public Update build() { return new Update(this); Valdas Rapševičius. Java Technologies 27

28 Fluent Builder pattern (2) Update.Builder ub = new Update.Builder(); Update u = ub.author(myauthor).updatetext("hello").build(); // or Update u = new Update.Builder().updateText("Hello").build(); More about Fluent interface: Valdas Rapševičius. Java Technologies 28

29 Dependency problem class StudentFinder implements StudentFinder { private final StudentDao public Student findstudent(string name) { return class Course { private final StudentFinder finder; private final Collection<Student> students = new LinkedHashSet<>(); public void add(string name) { Student s = finder.findstudent(name); if (s!= null) students.add(s); Valdas Rapševičius. Java Technologies 29

30 Dependency problem (2) public class ProblemDemo { public static void main(string[] args) { StudentDao dao = new StudentDaoDatabase(); StudentFinder finder = new StudentFinder(dao); Course course = new Course(finder); course.add("martin Fowler"); course.add("bill Gates"); Valdas Rapševičius. Java Technologies 30

31 Service Locator (1): Simple class ServiceLocator { private static class Holder { public static final StudentDao dao = new StudentDaoDatabase(); public static final StudentFinder finder = new StudentFinder02(); public static StudentDao getdao() { return Holder.dao; public static StudentFinder getfinder() { return Holder.finder; class StudentFinder02 implements StudentFinder public Student findstudent(string name) { return ServiceLocator.getDao().get(name); Valdas Rapševičius. Java Technologies 31

32 Service Locator (2): Segregated Interface interface DaoLocator { StudentDao getdao(); interface FinderLocator { StudentFinder class ServiceLocator implements DaoLocator, FinderLocator { private static final ServiceLocator instance = new ServiceLocator(); private final StudentDao dao = new StudentDaoDatabase(); private final StudentFinder finder = new StudentFinder(); public static ServiceLocator getinstance() { return instance; DaoLocator daolocator = ServiceLocator.getInstance(); StudentDao dao = daolocator.getdao(); FinderLocator finderlocator = ServiceLocator.getInstance(); StudentFinder finder = finderlocator.getdao(); Valdas Rapševičius. Java Technologies 32

33 Service Locator (3): class ServiceLocator { private static final ServiceLocator instance = new ServiceLocator(); public static ServiceLocator getinstance() { return instance; private Map<String, Object> services = new HashMap<>(); public void load(string name, Object service) { services.put(name, service); public Object get(string name) { return services.get(name); ServiceLocator.getInstance().load("dao", new StudentDao()); StudentDao dao = (StudentDao) ServiceLocator.get("dao"); ServiceLocator.getInstance().load("finder", new StudentFinder()); StudentFinder finder = (StudentFinder) ServiceLocator.get("finder"); Valdas Rapševičius. Java Technologies 33

34 Dependency Injection (DI) Pattern Method to create needed dependencies or look them up somehow without doing it in the dependent code Often called Inversion of Control (IoC), known as Hollywood Principle IoC injects needed dependencies into the object instead Setters or Constructor Primary goal is reduction of dependencies in code an excellent goal in any case this is the central part of some J2EE, MVC and other frameworks More read Inversion of Control Containers and the Dependency Injection pattern by Martin Fowler Valdas Rapševičius. Java Technologies 34

35 DI frameworks Standards JSR-330 (Dependency Injection for Java) defines basic standard annotation and Provider interface JSR-299 (Java Contexts and Dependency Injection) is based on above, enhanced Reference implementation Guice Other frameworks to consider Spring PicoContainer Excalibur (former Avalon) Weld Valdas Rapševičius. Java Technologies 35

36 javax.inject Identifies injectable constructors, methods, and fields. public class Car { // Injectable public Car(Engine engine) {... // Injectable private Provider<Seat> seatprovider; // Injectable package-private void install(windshield windshield, Trunk trunk) Identifies qualifier annotations. Anyone can define a new String-based qualifier. public class Seat passengerseat; Valdas Rapševičius. Java Technologies 36

37 javax.inject Identifies scope annotations. A scope annotation applies to a class containing an injectable constructor and governs how the injector reuses instances of the Identifies a type that the injector only instantiates once. Not inherited. interface Provider<T> Provides instances of T. Typically implemented by an injector. For any type T that can be injected, you can also inject Provider<T>. class Car Car(Provider<Seat> seatprovider) { Seat driver = seatprovider.get(); Seat passenger = seatprovider.get(); Valdas Rapševičius. Java Technologies 37

38 DI example class RealService implements Service { private final CreditCardProcessor processor; private final TransactionLog RealService(CreditCardProcessor processor, TransactionLog transactionlog) { this.processor = processor; this.transactionlog = transactionlog;... public class BillingModule extends AbstractModule protected void configure() { bind(transactionlog.class).to(databasetransactionlog.class); bind(creditcardprocessor.class).to(paypalcreditcardprocessor.class); public static void main(string[] args) { Injector injector = Guice.createInjector(new BillingModule()); RealBillingService billingservice = injector.getinstance(realbillingservice.class); Valdas Rapševičius. Java Technologies 38

39 Session Conclusions During this session you should have learned how to organize your classes/objects to solve the repetitive and common problems by using patterns Need for patterns and proper understanding comes with time (experience) Consult numerous websites, books DI and IoC is a big heap in the programming world do not miss it out! Service Locator (aka Utility) is the first choice for many (beginners) Construct a complete application based on DI/IoC can be a challenge Valdas Rapševičius. Java Technologies 39