Lecture 06Using Design Patterns

Agenda Base Patterns

– Gateway, Mapper, Layerd Supertype, Separated Interface, Registry, Value Object, Plugin, Service Stub, Record Set

From Problem to Patterns– Using design patterns

Using Design Patterns Normally we don’t start with patterns

– We start with problems to solve– From Problem to Pattern

Must have clear objectives for the design– The patterns will come as they are needed

Establish Design Principles– This applies to your application

Remember the separation of concern

From Problem to Pattern How do I reuse common functionality of

my objects?–Layer Supertype

Layer SupertypeA type that acts as the supertype

for all types in its layer Super class that contains common

functionality in a layer How it works

– Use this pattern when you have common features from all objects in a layer

Layer Supertype When to use it

– When you have common features from all objects in a layer

Example– Domain objects can

have a common superclass for ID handling

class DomainObject...

private Long ID; public Long getID() { return ID; } public void setID(Long ID) { this.ID = ID; } public DomainObject(Long ID) { this.ID = ID; }

From Problem to Pattern How do I access an external service

without becoming too dependant on it?–Gateway

GatewayAn object that encapsulates access to an

external system or resource Wrap external APIs into an interface

– API is usually for accessing some external resource• Examples: JDBC, JDom, financial software

Gateway How It Works

– Create a simple API and use it access the external API through a Gateway

– All access is easily defined– Change in the resource does not require

changes in the client software– Gateways should be simple – complex logic

should not be in the clients of the Gateway– Gateways can be generated

Gateway When to Use It

– Gateway is useful when accessing external service

– Can be applied with Service Stub (504)– Clear benefit is that is makes it easy to swap

out one kind of resource for another

From Problem to Pattern How do I avoid creating unwanted

dependencies?–Separated Interface

Separated InterfaceDefines an interface in a separate package from its implementation

Decouples parts of a system– Controls the dependencies between packages– Implementation can easily be changed

How it works– Interface and implementation is placed in

separate packages– Client uses the interface– Implementation can be determined at

configuration time

Separated Interface Layered System

– Domain layer depends on Data Source layer– Data Source layer cannot access Domain layer

Data Source Layer

Domain Layer

JDBCCode

InterfaceRowCallBackHandler

processRow(ResultSet rs)

Concreate classRowCallBackHandler

processRow(ResultSet rs)

implements

Code reading SQL

Execution calls

Separated interface

Separated Interface Instantiating the implementation

– User of the interface should not know the implementation

– User of the interface creates the interface• The Arrray.sort developer creates Comparable• The Person developer implements Comparable

Solutions– Use a Factory and Plugin method– Use Dependency Injection

From Problem to Pattern How do I test my client code using a

service that I don’t have access to?–Service Stub

Service StubRemoves dependence upon problematic

services during testing Enterprise systems often need to access

external system– Can be out of developers control

Service Stub Service stub provides implementation for

development and testing purposes– Runs locally and in-memory– Implements the same interface of the gateway

used to access the real service When to Use It

– Service stub is useful when dependence on a particular service is hindering development or testing

– Called “Mock Object” in the extreme programming world

From Problem to Pattern How do I link to my implementation class

using configuration–Plugin

PluginLinks classes during configuration

rather than compilation Use plugin to provide specific implantation

– Plugins implement specific interface use by the client application code

– Decision at configuration time or run time– Use factory to load in the plugin– For example: on plugin for test, another for

production

Plugin

caller a plugin factory a plugin configuration

getPlugin

lookupPluginByType

newa plugin

A caller obtains a Plugin implementation of a separated interface

When to Use It– Use plugin when you have behavior that

requires different implementations based on runtime environment

From Problem to Pattern How can I keep common object available

within the application–Registry

RegistryA well-known object that other objects can use to find common objects and services

A registry is a global object How It Works

– Object that can easily be accessed at any time– Only one object available at any time– Provides services or information– Can have different scopes– Usually not mutable data– Example: System Settings, Loggers

Registry In this implementation, only one instance running

When to Use It– As a last resort

public class Registry{ private static Registry soleInstance = new Registry();

public static Registry getInstance() { return soleInstance; }

private Registry() { } ...}

Registry registry = Registry.getInstance();//registry = new Registry (); Does not work

From Problem to Pattern I need to set up communication between

two independent objects–Mapper

MapperAn object that sets up communiction

between two independent objects Create communication between two

systems but you still need to make them independent

Mapper How it Works

– A Mapper is an insulating layer between subsystems– It controls the details of communication between

them without either subsystem being aware of it– Mappers are fairly easy as they are well-defined– The tricky part is what system invokes them – third

party system or make the Mapper an Observer When to Use it

– When you want to decouple different parts of a system

From Problem to Pattern I need to define a simple immutable object

that does not have any identity –Value Object

Value ObjectA small simple object, like money or date

range, whose equality isn’t based on identity

Small and easily created objects that hold and represent some data

How it works– Not based on identity– Equality is based on comparing values of the object– Can be immutable (example is the Date class)

When to use it– When you’re basing equality on something other than

identify

Value Object Examples

– Date, Money

class Money...

private long amount; private Currency currency;

public Money(double amount, Currency currency) { this.currency = currency; this.amount = Math.round(amount * centFactor()); } ...

MailService

Mail Service We are building an web application

– One important service is sending messages in email

– We need to access the e-mail service

Mail Service During development we cannot have all

features fully developed– We program in steps– Code, test, fix, code…– This also applies over releases– Maybe we have to use hacks in Version 1.0

Lifestyle: REFACTOR

Refactoring Design, redesign, refactor

– Make the design as complete as possible– But be prepared to change design as you code– Unit tests become very important

Code Smell– Think of your code as a baby:

“If it smells, change it!”

Refactoring Refactoring is the process of improving

design in little steps at a time– Minimizes risks – calculated – Changes are controlled– Code can improve– Less likely to smell

The Danger Technical Dept

“I’ll fix it later”

The four most dangerous and expensive words in programming

Conway’s Second Law Technical Dept

“There’s never enough time to do something right, but there’s always enough time to do it over”

Mail Service We are building an web application

– One important service is sending messages in email

– We need to access the e-mail service

Mail Service We decide to use JavaMail API

– Problem is that this API is pretty low-level and complicated

– Lots of “noise” – not good to have the domain developers worry about that

What Design Pattern can we use here?

Mail Service Gateway We build a simple gateway to handle mail

– Domain developers don’t worry about the service

– We can easily change to a different mail API

GatewayAn object that encapsulates access to an

external system or resource Wrap external APIs into an interface

– API is usually for accessing some external resource• Examples: JDBC, JDom, financial software

MailSender (1/2) Class that sends the mail

– Method send that takes care of sending the mail

public class MailSender{ public void send(String from, String to, String subject, String body) { String smtpServer = "mail.ru.is"; try {

MailSender (2/2) Properties props = System.getProperties(); props.put("mail.smtp.host", smtpServer); Session session = Session.getDefaultInstance(props, null); Message msg = new MimeMessage(session); msg.setFrom(new InternetAddress(from)); msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to, false)); msg.setSubject(subject); msg.setText(body); msg.setSentDate(new Date()); Transport.send(msg); } catch (Exception ex) { ex.printStackTrace(); } }}

MailSender Name the problem with his class

public class MailSender { public void send(String from, String to, String subject, String body) { String smtpServer = "mail.ru.is"; try { Properties props = System.getProperties(); props.put("mail.smtp.host", smtpServer); Session session = Session.getDefaultInstance(props, null); Message msg = new MimeMessage(session); msg.setFrom(new InternetAddress(from)); msg.setRecipients(Message.RecipientType.TO,InternetAddress.parse(to, false)); msg.setSubject(subject); msg.setText(body); msg.setSentDate(new Date()); Transport.send(msg); } catch (Exception ex) { ex.printStackTrace(); } }}

MailSender Problem

– Many parameters instead of an object– Mail server is hard-coded– Exception handling is bad– Programming to implementation

public void send(String from, String to, String subject, String body) { String smtpServer = ”smtp.ru.is"; try { ... } catch (Exception ex) { ex.printStackTrace();}}}

MailService Interface for the domain developers

– Program-to-interfaces Principle

– So let MailSender implement this interface

public interface MailService{ public void send(String from, String to, String subject, String body);}

public class MailSender implements MailService{ public void send(String from, String to, String subject, String body) { ...

Testing Testing MailService and MainSender is

easypublic class TestMail{ public static void main(String[] args) { MailService mail = new MailSender(); mail.send("andri@ru.is", // from "andri@ru.is", // to "Hallo", // subject "So does this stuff work"); // body }}

TestingTestMail: sending mail.Done.

NOTE: mail.ru.is is not good for testing!

Testing What is the problem with clients like this?

public class TestMail{ public static void main(String[] args) { MailService mail = new MailSender(); mail.send("andri@ru.is", // from "andri@ru.is", // to "Hallo", // subject "So does this stuff work"); // body }}

Improvements Problem

– MailSender implementation class is exposed to the domain layer

Solution– Use the Plugin Pattern– Create a factory that will read a

configuration file and load the mail implementation class

– Client will use the MailService interface

PluginLinks classes during configuration

rather than compilation Use plugin to provide specific implantation

– Plugins implement specific interface use by the client application code

– Decision at configuration time or run time– Use factory to load in the plugin

• For example: one plugin for test, another for production

Factory with a Plugin Create a MailFactory class

– Loads mail.properties file– Creates the class specified in the properties file

and returns interface MailService– Clients use MailService and are not exposed to

particular implementation– It’s easy to change the properties file

Improvements Problem

– Can we make the loading of properties and class more generic?

– Other factories might need this functionality also Solution:

– Create a Layer Supertype– MailFactory extends Factory

Layer SupertypeA type that acts as the supertype

for all types in its layer Super class that contains common

functionality in a layer How it works

– Use this pattern when you have common features from all objects in a layer

Layer SupertypeA type that acts as the supertype

for all types in its layer Super class that contains common

functionality in a layer How it works

– Use this pattern when you have common features from all objects in a layer

Plugin Pattern

Factory Layer Supertype Has two methods

– loadPropertie and loadClass Exception handling

– Create a new exception class that we will use– FactoyException– Log the error

FactoryException Extends Exception

– Checked exception– Callers must catch this exception or explicitly

throw itpublic class FactoryException extends Exception{ public FactoryException(String message) { super(message); }

public FactoryException(String message, Throwable cause) { super(message, cause); }}

Factorypublic class Factory { Logger logger = Logger.getLogger(LogInfo.LOG_NAME);

protected Properties loadProperties(String filename) throws FactoryException { Properties props = new Properties(); try { props.load(new FileInputStream(new File(filename))); } catch (FileNotFoundException fnfex) { String msg = "Factoy: File '" + filename + "' not found."; logger.severe(msg); throw new FactoryException(msg, fnfex); } ... return props; }

Testing Fails Exception is thrown and message is

logged2.9.2007 16:49:34 is.ru.honn.rubook.factory.Factory loadClassSEVERE: Factoy: Class 'is.ru.honn.rubook.mail.MailServiceStubx' not found.

Testing Fails Problem

– MailService implementation classes have to handle FactoryException or pass it on

– Do we want clients to worry about some factory?

Solution– One solution is to catch FactoryException

and throw unchecked MailService exception

MailFactorypublic class MailFactory extends Factory{ public MailService getMailService() { MailService service; try { service = (MailService)loadClass( loadProperties("mail.properties"). getProperty("mail.service.class")); } catch(FactoryException fex) { throw new MailServiceException ("Unable to send e-mail", fex); } return service; }}

MailServiceException Extends RuntimeException

– Unchecked exception– Callers decide if they want to catch it

public class MailServiceException extends RuntimeException{ public MailServiceException(String message) { super(message); }

public MailServiceException(String message, Throwable cause) { super(message, cause); }}

Testing Using the MailFactory class

– We can catch the MailServiceException or ignore it

– Notice we have not only abstracted the Mail API but also the exception handling

public class TestMail{ public static void main(String[] args) { MailFactory mf = new MailFactory(); MailService mail = mf.getMailService(); mail.send("andri@ru.is", "andri@ru.is", "Hello", "Hello"); }}

Improvements Problem

– Exception handling in our original MailSender is bad

Solution– Use the MailServiceException public void send(MailMessage message)

{ try { ... } catch (Exception ex) { String msg = "Sending mail failed: " + ex.getMessage(); logger.severe(msg); throw new MailServiceException(msg, ex); }

SEVERE: Sending mail failed: Unknown SMTP host: mail.ru.is

Improvements Problem

– What if we don’t have access to the SMTP server at this time?

Solution– Use a Service Stub– Create the class MailServiceStub that will simply

log out the mail sent– Could also write in file

Service StubRemoves dependence upon problematic

services during testing Enterprise systems often need to access

external system– Can be out of developers control

MailServiceStubpublic class MailServiceStub implements MailService{ Logger logger = Logger.getLogger(LogInfo.LOG_NAME);

public void send(String from, String to, String subject, String body) { logger.info("Sending mail from '" + from + "' to '" + to + "' Subject: '" + subject); }}

2.9.2007 16:36:08 is.ru.honn.rubook.mail.MailServiceStub sendINFO: Sending mail from 'andri@ru.is' to 'andri@ru.is' Subject: 'Hello

mail.service.class=is.ru.honn.rubook.mail.MailServiceStub

mail.properties

Improvements Problem

– What if we need to add new parameter?

Solution– Use an object to group parameters– Easy to change without changing the interface

public interface MailService { public void send(String from, String to, String subject, String body);}

public interface MailService{ public void send(MailMessage message);}

MailMessage Typical Data Transfer Objectpublic class MailMessage { private String from; private String to; private String subject; private String body;

public MailMessage(String from, String to, String subject, String body) { this.from = from; this.to = to; this.subject = subject; this.body = body; } public String getFrom() { return from; } public void setFrom(String from) { this.from = from; }...

Improvements Problem

– The mail server in MailSender is still hardcoded Solution

– Place in the configuration file– Let the factory inject the name into the

Mail Servicepublic interface MailService{ public void setMailServer(String mailServer); public void send(MailMessage message);}

Injecting the Mail Server Name

New MailFactoy getMailService injects the name into the

servicepublic class MailFactory extends Factory{ public MailService getMailService() { ... loadProperties("mail.properties"); service = (MailService)loadClass(getProperties(). getProperty("mail.service.class")); service.setMailServer(getProperties(). getProperty("mail.server")); // injection return service; }}

mail.service.class=is.ru.honn.rubook.mail.MailSendermail.server=mail.ru.is

Improvements Problem

– loadProperties loads the file each time used Solution

– Load once then usepublic class Factory{ private Properties properties = new Properties();

protected Properties loadProperties(String filename) throws FactoryException { ... return properties; } public Properties getProperties() { return properties; } ...

Improvements Problem

– All mail server implementations must store server name and set function

– Common functionality in multiple classes Solution

– Create a Layered Supertype– Take care of the common functionality– Make the send method abstract

AbstractMailService Implements MailService

– Provides handling of the mail server propertypublic abstract class AbstractMailService implements MailService{ protected String mailServer;

// this is used by the factory to inject public void setMailServer(String mailServer) { this.mailServer = mailServer; }

public String getMailServer() { return mailServer; }}

MailSender Extends AbstractMailService

– Does not have to implement the MailServer interface

– Can use the getMailServer methodpublic class MailSender extends AbstractMailService{ public void send(MailMessage message) { try { Properties props = System.getProperties(); props.put("mail.smtp.host", getMailServer()); ...

Summary Base Patterns

• Gateway, Mapper, Layerd Supertype, Separated Interface, Registry, Value Object, Plugin, Service Stub, Record Set

We start with problems to solve– Then we find the patterns to use– Must have clear objectives for the design

Beware of code smell Refactoring is the process of improving design

in little steps at a time Example case study

– Mail service with a configurable factory