1 KC Web & Java – 29 november 2005 – Design Patterns – The Template Method AJAX! KC Web & Java...

1KC Web & Java – 29 november 2005 – Design Patterns – The Template Method

AJAX!

KC Web & Java

29 november 2005


1-0


J2SE Design Patterns:

Het Template Pattern

Lucas Jellema

KC Web & Java – 10 Minute Talks,

Donderdag 29 november 2005


Introducing Design Patterns

Recurring algorithms, construction, designs Best practices Associated with various technologies

ERD Database Design J2EE Architecture

• MVC, Business Delegate, Façade, …

Non IT environments J2SE – Java Programming

• Most famous: …. Singleton


Great Book: Head First Design Patterns

Observer Decorator Factory Singleton Command Adapter and Façade Iterator and Composite State Proxy Compound


The Template Patternaka Don’t Call us, We will call You!

TeaMaker• getCup• boilWater• steepTeaBag• pourInCup• optional addSugar• optional addLemon

CoffeeMaker• getCup• boilWater• brewCoffee• pourInCup• optional addSugar• optional addMilk

CoffeeVerkeerdMaker• getCup• boilWater• brewCoffee• pourInCup• addMilk• optional addSugar

• getCup• boilWater• brew WHATEVER• pourInCup• addCondiments

Generic pattern


Template Method Patterntemplate method: prepareRecipe


The Template Pattern

The Template (Method) Pattern Defines the skeleton of an algorithm in a method,

deferring some steps or pieces to sub-classes Lets subclasses (re)define part of an algorithm without

changing its structure(abstract) superclass

algorithm

final

abstracthook

(concrete) subclass (concrete) subclass


Template Method Patterntemplate method: prepareRecipe


The Template Pattern

The Template (Method) Pattern Three modes

• Forcing subclasses to implement certain steps (abstract or interface)

• Allowing subclasses to complement certain steps (through overridable hooks)

• Allowing subclasses to redefine steps (through overridable algorithm methods)

Non-overridable steps are defined final in the superclass


Template Method: BeverageMachine.prepareRecipe()


package nl.amis.shop.mypackage;

public class CoffeeShop { public CoffeeShop() { }

public void drinkBeverage( BeverageMachine bm) { bm.prepareRecipe(); System.out.println("Enjoy your drink\n"); }

public static void main(String[] args) { CoffeeShop coffeeShop = new CoffeeShop(); coffeeShop.drinkBeverage(new CoffeeMachine()); coffeeShop.drinkBeverage(new TeaMachine()); coffeeShop.drinkBeverage(new CoffeeWrongMachine()); }//main}

Using the Template MethodMy Little Coffeshop


Examples of the Template (Method) PatternDon’t Call Us – We Will Call You

Spring JDBC Arrays.sort(Object[] a)

Not a Superclass/Subclass situation To use sort on an array, the array elements must implement the

Comparable interface • by defining the compareTo() method

java.io.InputStream – abstract read() method that subclasses must implement

Swing JFrame – paint() method: a hook with an empty default

implementation Applet – start(), stop(), destroy(): hooks

ADF DataAction override onCommit, add onMyEvent


Examples of the Template (Method) PatternOutside the Java Arena

Oracle Forms event-triggers Pre-Insert, Post-Insert – hooks surrounding the Insert

algorithm even On-Insert that allows overriding of the core of the

the Insert algoritm • but even then the other hooks will exist

Database DML Event Triggers Before insert statement, before insert row After insert row, After insert statement


Architectuur

Service

DAO

DomainObject

DomainObject

DomainObject

JNDI


Example

import java.sql.*;import javax.sql.*;public class EmpDao { public List getAllEmployees() { Connection con = null; PreparedStatement pstmt = null; ResultSet rs = null; List emps = new ArrayList(); try { con = getConnection(); pstmt = con.prepareStatement ("select * from emp"); rs = pstmt.executeQuery(); while (rs.next()) { Employee e = new Employee(); e.setId (rs.getLong(1)); e.setName (rs.getString(2)); // ... emps.add(e); } } catch (SQLException e) { // handle exception } finally { try { rs.close(); pstmt.close(); con.close(); } catch (SQLException e1) { // no action needed } } return emps; }}

private Connection getConnection() throws SQLException{ try { Context ic = new InitialContext(); DataSource ds = (DataSource) ic.lookup ("java:comp/env/jdbc/myDatabase"); return ds.getConnection(); } catch (NamingException e) { // handle exception return null; }}

private Connection getConnection() throws SQLException{ try { DriverManager.registerDriver (new oracle.jdbc.driver.OracleDriver()); return DriverManager.getConnection ("jdbc:oracle:thin:@localhost:1521:orcl“ ,"scott", "tiger"); } catch (SQLException sqle) { // handle exception return null; }}


Template Pattern

Operation largely follows a standard algorithm At certain steps, specialization or customization is

required Several implementations

Abstract ‘hook’ methods that sub-class may override Parametrize behaviour and have invoker provide the

details• Such as the SQL Query

Spring JDBC Templates Implement all JDBC wiring Parametrize the query and the result-handling


Example of Spring JDBC Template

public interface empDao {

public List getAllEmployees ();

}

public class EmployeeJdbcDao extends JdbcDaoSupport implements EmpDao {

public List getAllEmployees() {

JdbcTemplate jt = getJdbcTemplate();

return jt.queryForList (“select * from emp”);

}

}

<bean id="dataSourceDBDirect"

class="org.springframework.jdbc.datasource.DriverManagerDataSource"

destroy-method="close">

<property name="driverClassName" value="oracle.jdbc.driver.OracleDriver" />

<property name="url" value="jdbc:oracle:thin:@localhost:1521:BAARSJES1" />

<property name="username" value="scott" />

<property name="password" value="tiger" />

</bean>

<bean id="employeeDAO" class="nl.amis.demo.dao.jdbc.EmployeeJdbcDao" >

<property name="dataSource">

<ref local="dataSourceDBDirect" />

</property>

</bean>


Conclusions

Template Method Pattern Instead of overriding and reimplementing the algorithm in a

subclass The algorithms has predefined call outs

• To abstract method that subclasses MUST implement

• To concrete, overridable methods to change parts of the algorithm – but not the overall structure!

• To hooks (optionally implemented by subclasses to complement the steps of the algoritm

Easy to apply Recognized from duplication of algorithm across classes

You frequently override a method and duplicate most of its implementation

Read – and enjoy – Head First Design Patterns

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