Design Patterns

Trends and Case Study

John HurstJune 2005

Overview

• Software Patterns• Design Patterns• Topics• Case Study / Examples

Introduction

• Christopher Alexander– A Pattern Language: Towns, Buildings, Construction

(1977)– A Timeless Way of Building (1979)

• “Gang of Four” (Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides)– Design Patterns (1995)

• Buschmann et al– Pattern Oriented Software Construction (1996)

Key Idea

• A Software Pattern is:– A solution– To a problem– In a context– With consequences

• A Software Pattern has a name and a vocabulary.

Pattern Languages

• Intent• Also Known As• Motivation• Applicability• Structure• Participants

• Collaborations• Consequences• Implementation• Known Uses• Related Patterns

OO Design Principles

• Code to interfaces• Prefer composition to inheritance• Encapsulate what varies

Benefits

• Improves thinking about OO design– New designs– Existing designs– Helps us realise the benefits of OO

• Reuse of solutions to common problems– Catalogs

• Communication tool– Common vocabulary

Example: Iterator

• Intent: – Provide a way to access the elements of

an aggregate object sequentially without exposing its underlying representation

Example: Decorator

• Intent: – Attach additional responsibilities to an object

dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

+Operation()

ConcreteComponent

+Operation()

Decorator-component

1

*

+Operation()

-addedState

ConcreteDecoratorA

+Operation()+AddedBehaviour()

ConcreteDecoratorB

component->Operation()

Decorator::Operation();AddedBehaviour()'

+Operation()

Component

Application of Decorator

• Apache Commons DBCP (Database Connection Pooling)

+createStatement()+prepareStatement()+prepareCall()+commit()+rollback()+close()

«interface»Connection

+createStatement()+prepareStatement()+prepareCall()+commit()+rollback()+close()

-_conn

DelegatingConnection

1

*

+close()

-_pool

PoolableConnection

+prepareStatement()+close()

-_pstmtPool

PoolingConnection

Patterns Landscape

• Architectural Patterns• Analysis Patterns• Design Patterns

“Platform Patterns”

• J2EE Core Patterns• MS Building Blocks

• Contrast with PEAA

Antipatterns

• Examples– Big Ball Of Mud– Copy and Paste Programming– Magic Container– Tower of Voodoo

Broken Patterns

• Double checked locking• “Deprecated” Patterns:

– Factory Method (due to confusion over what it means)

– Bridge– Flyweight– Interpreter

Specialised Fields

• Real-time• Concurrency• Enterprise• Messaging• Analysis

Refactoring to Patterns

• Remove duplicate code• Simplify logic• Communicate intention• Increase flexibility

Pattern Happy

Patterns with Programming Languages

• Strategy– C++ templates: static type checking– Java interfaces and objects: runtime

configuration– Ruby/Smalltalk blocks: idiomatic usage

Case Study: HHRDM

• Design Patterns• Architectural Patterns• Analysis Patterns

Design Patterns

• Abstract Factory• Template Method• Strategy• Façade• Proxy

Architectural Patterns

• Layering• Data Access Patterns• Domain Model• Service Façade

Analysis Patterns

• Temporal Patterns– Effectivity– Snapshot– Temporal Object– Temporal Property– Audit Trail– Bitemporal Model

References• Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides

Design Patterns: Elements of Reusable Object-Oriented Software – Addison-Wesley 1995

• Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, Michael Stal Pattern-Oriented Software Architecture Wiley 1996

• Martin Fowler Patterns of Enterprise Application Architecture – Addison-Wesley 2002

• Martin Fowler Refactoring – Addison-Wesley 2000• Martin Fowler Analysis Patterns – Addison Wesley 1996• Gregor Hohpe, Bobby Woolfe Patterns of Enterprise

Application Integration – Addison-Wesley 2003• Joshua Kerievsky Refactoring to Patterns – Addison-Wesley

2004• Deepak Alur, John Crupi, Dan Malks Core J2EE Patterns (2nd

Ed) – Addison-Wesley 2003

Web Sites

• Martin Fowler website http://www.martinfowler.com

• Portland Pattern Repository http://c2.com/cgi/wiki

• TheServerSide http://www.theserverside.com

• Microsoft Patterns and Practices http://msdn.microsoft.com/architecture/patterns/

Relationships and Similarities

+Request()

«interface»Subject

+Request()

Proxy

+Request()

RealSubject ...realSubject->Request()...

+Operation()

ConcreteComponent

+Operation()

Decorator-component

+Operation()

-addedState

ConcreteDecoratorA

+Operation()+AddedBehaviour()

ConcreteDecoratorB

component->Operation()

Decorator::Operation();AddedBehaviour()'

+Operation()

«interface»Component

Proxy Decorator

Many patterns look similar structurally, but have different intent

The Agile Manifesto

• Preferences:– Inidividuals and interactions over

processes and tools– Working software over comprehensive

documentation– Customer collaboration over contract

negotiation– Responding to change over following a

plan