overview of pojo programming - chris richardson

6/20/2006 Copyright (c) 2006 Chris Richardson. All rights reserved.

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Overview of POJO programming

A simpler, faster way to build long-lived applications

by

Chris [email protected]

http://www.chrisrichardson.net


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About Chris…Grew up in EnglandLive in OaklandTwenty years of software development experience

Building object-oriented software since 1986Using Java since 1996Using J2EE since 1999

Author of POJOs in ActionRun a consulting company that helps organizations build better software fasterChair of the eBIG Java SIG in Oakland (www.ebig.org)


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Overview

POJOs + lightweight frameworks:Simplify developmentAccelerate developmentMake applications immune to the volatility of enterprise Java technology

Focus on the “backend” frameworks:Business tierDatabase access tier


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Agenda

The trouble with traditional enterprise Java frameworksOverview of POJOsAssembling POJO applications with dependency injectionPersisting POJOs with HibernateMaking POJOs transactional with Spring


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Classic EJB architecture example


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Problems with intertwined business logic and infrastructure

Upgrading to new, better version of infrastructure framework is difficult/impossible:

Enterprise Java (1998-2006):Incompatible standards: EJB 1, EJB 2, EJB 3Many persistence options: EJB CMP 1/2, Hibernate 1/2/3, JDO 1/2, EJB 3 persistence

Makes development more difficultForced to think about business logic + infrastructure concerns simultaneouslyDevelopers need to know both


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…problems

Makes testing more difficultMust deploy code/tests in application serverSlows down the edit-compile-debug cycle

EJB 2 prevented OO developmentEJB application servers are

Complex Expensive (some)


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EJB as a cultIn 1999 I readily embraced EJBs and its development rituals:

writing DTOs and unused lifecycle methodsWaiting for EJBs to deploy

According to http://en.wikipedia.org/wiki/Cult

“a cult is a relatively small and cohesive group of people devoted to beliefs or practices that the surrounding culture or society considers to be far outside the mainstream”

But there is a better way….


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Agenda



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POJO = Plain Old Java Object

Java objects that don't implement any special interfaces or (perhaps) call infrastructure APIsCoined by Martin Fowler, Rebecca Parsons, and Josh MacKenzie to make them sound just as exciting as JavaBeans, Enterprise JavaBeansSimple idea with surprising benefits


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POJO application design

POJO facade

Domain model Database access


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POJO code example

Simple Java classesNo lookup code – uses dependency injection instead


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But POJOs are insufficient…⇒ Lightweight frameworks

Endow POJOs with enterprise featuresObject/relational mapping framework:

Persists POJOsJDO, Hibernate, JPA, …

Spring framework:Popular open-source frameworkDeclarative transaction managementDependency injectionRemoting, security, …


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Key point: non-invasive frameworks

Provide services without the application:Implementing interfacesCalling APIs

Configured using metadata:XMLJava 5 annotations

POJOs + non-invasive frameworks ⇒simple, faster development of applications that are immune to infrastructure changes


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Deployment optionsWeb container-only server

Tomcat or JettySimple yet sufficient for many applications

Full-blown serverWebLogic, JBoss, WebSphereRicher set of featuresEnhanced manageability and availabilityJTAJMS…


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Benefits of using POJOs

Separation of concernsBusiness logic is decoupled from infrastructureSwitch frameworks or upgrade more easilyNot everybody has to be an infrastructure framework expert

Simpler developmentThink about one thing at a timeBusiness logic, persistence, transaction management….

Faster developmentTesting without an application server (or a database)No deployment to slow you down

More maintainableModular object-oriented codeLoosely coupled design

Simpler, perhaps cheaper deploymentDeploy in a web-container only server


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Drawbacks of POJOs…

…none except that lightweight frameworks have their limitationsUse EJBs if you need:

Distributed transactions initiated by a remote clientSome application server-specific features…


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Agenda



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Dependency injectionApplication components depend on:

One another Infrastructure components

Using JNDI or the new operator:Introduces couplingComplexity

Solution:Pass dependencies to a componentSetter injectionConstructor injection


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Dependency injection example

public class MoneyTransferServiceImpl…

public MoneyTransferServiceImpl(AccountRepository

accountRepository, …){

this.accountRepository = accountRepository;

…}

public class HibernateAccountRepositoryimplements AccountRepository {

…}

You can implement dependency injection by hand but ….


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Spring lightweight container

Lightweight container = sophisticated factory for creating objectsSpring bean = object created and managed by SpringYou write XML that specifies how to:

Create objects Initialize them using dependency injection


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Spring code examplepublic class MoneyTransferServiceImpl…

public MoneyTransferServiceImpl(AccountRepository

accountRepository, …){

this.accountRepository = accountRepository;

…}

<bean name="MoneyTransferService"class="MoneyTransferServiceImpl">

<constructor-arg ref="AccountRepository"/>…

</bean>

<bean name="AccountRepository"class="HibernateAccountRepository">

…</bean>

public class HibernateAccountRepositoryimplements AccountRepository {

…}


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Spring 2 – dependency injection into entities

Domain model entities need to access repositories/DAOs/etcBut they are created by the application or by Hibernate – not SpringPassing repositories as method parameters from services clutters the codeSpring 2 provides AspectJ-based dependency injection into entitiesConstructors automatically invoke Spring

@Configurable("pendingOrder")public class PendingOrder {

private RestaurantRepository restaurantRepository;

public void setRestaurantRepository(RestaurantRepository

restaurantRepository) {this.restaurantRepository =

restaurantRepository;}

<aop:spring-configured />

<bean id="pendingOrder" lazy-init="true"><property name="restaurantRepository"

ref="RestaurantRepositoryImpl" />

</bean>


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Benefits of dependency injection

Simplifies codeNo calls to JNDI

Decouples components from:One anotherInfrastructure

Simplifies testingPass in a mock/stub during testing


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Mock object code example

Test the MoneyTransferServiceImplwithout calling the real AccountRepository


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Agenda



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POJO persistence

Use an object/relational framework:Metadata maps the domain model to the database schemaORM framework generates SQL statements

HibernateVery popular open-source project

JDOStandard from Sun – JSR 12 and JSR 243Multiple implementations: Kodo JDO, JPOX

EJB 3/Java Persistence API (JPA)


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Hibernate: code example

Provides transparent persistencePieces:

AccountHibernateBankingExample.hbm.xmlHibernateAccountPersistenceTestsHibernateAccountRepositoryHibernateAccountRepositoryTestsSpring beans

Only the repositories/DAOs call persistence framework APIs


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ORM framework features 1Declarative mapping

Map classes to tables; fields to columns; relationships to foreign keys and join tables

CRUD APIE.g. Hibernate Session, JPA EntityManager

Query languageRetrieve objects satisfying search criteria

Transaction managementManual transaction managementRarely call directly – used by Spring

Detached objectsDetach persistent objects from the DBEliminates use of DTOsSupports edit-style use cases


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ORM framework features 2Lazy loading

Provide the illusion that objects are in memoryBut loading all objects would be inefficient

⇒ load an object when it is first accessedEager loading

Loading objects one at a time can be inefficient⇒ load multiple objects per-select statement

CachingDatabase often the performance bottleneck⇒ cache objects in memory whenever you canEasy for readonly objectsOptimistic locking and cache invalidation for changing objects


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O/R mapping framework benefitsImproved productivity

High-level object-oriented APILess Java code to writeNo SQL to write

Improved performanceSophisticated cachingLazy loadingEager loading

Improved maintainabilityA lot less code to write

Improved portabilityORM framework generates database-specific SQL for you


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When and when not to use an ORM framework

Use when the application:Reads a few objects, modifies them, and writes them backDoesn’t use stored procedures (much)

Don’t use when:Simple data retrieval ⇒ no need for objectsLots of stored procedures ⇒ nothing to map toRelational-style bulk updates ⇒ let the database do thatSome database-specific features ⇒ not supported by ORM framework


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Agenda



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Making POJOs transactional

EJB 2 container-managed transactions are greatSpring provides declarative transactions for POJOsSimilar to CM transactions but

Runs outside of an application serverMore flexible exception handling


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Spring AOPAOP enables the modular implementation of crosscutting concernsSpring AOP = simple, effective AOP implementationLightweight container can wrap objects with proxiesProxy executes extra code:

Before original methodAfter original methodInstead of…

Spring uses proxies for:transaction managementsecuritytracing…


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Spring TransactionInterceptor


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Spring code example<bean

name="AccountManagementFacade“class="AccountManagementFacadeImpl"> …

</bean>

<bean id="transactionProxyCreator“class=“...BeanNameAutoProxyCreator"><property name="beanNames">

<list><idref

bean="AccountManagementFacade"/> </list>

</property><property name="interceptorNames">

<list><idref

bean="BankingTransactionInterceptor"/></list>

</property></bean>

<bean id="myTransactionManager"class="HibernateTransactionManager">

…</bean>

<bean id="BankingTransactionInterceptor"class="TransactionInterceptor"><property name="transactionManager"

ref="myTransactionManager"/></bean>


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Spring 2 – simplified XML<bean

name="AccountManagementFacade“class="AccountManagementFacadeImpl"> …

</bean>

<aop:config><aop:advisorpointcut="execution(* *..*Facade.*(..))"

advice-ref="txAdvice"/></aop:config>

<bean id="transactionManager"

class="HibernateTransactionManager">…</bean>

<tx:advice id="txAdvice"><tx:attributes>

<tx:method name="*"/></tx:attributes>

</tx:advice>


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Spring remotingRemoting

Spring HTTPHessian/BurlapRMI…

Server uses a <Xyz>Exporter bean

Service to exposeInterface to expose

Client uses a <Xyz>ProxyFactoryBean

URL to remote service

<bean name="/accountManagement" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter">

<property name="service" ref="TransferFacade"/>

<property name="serviceInterface“value="net.chrisrichardson…TransferFacade“

/></bean>

<bean id="httpInvokerProxy" class="org.springframework.remoting.httpinvoker.

HttpInvokerProxyFactoryBean"><property name="serviceUrl"

value="http://somehost:8080/accountManagement"/>

<property name="serviceInterface“value="net.chrisrichardson…TransferFacade“

/></bean>


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Spring SecurityAcegi Security

Open source projectExtension to Spring

MethodSecurityInterceptorVerifies that caller is authorized

Invoke methodAccess instances

<bean id=“transferSecurity" class="org.acegisecurity.intercept.method.aopalliance.

MethodSecurityInterceptor">…<property

name="objectDefinitionSource"><value>

net.chrisrichardson…TransferFacade.*=

ROLE_CUSTOMER, ROLE_CSR</value>

</property>

</bean>


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Deploying a Spring applicationOften packaged as a WARWeb.xml lists bean definition filesServletContextListenercreates Spring bean factoryWeb tier is either:

Injected with Spring beansCalls getBean()

<web-app>

<context-param><param-name>contextConfigLocation</param-name>

<param-value>/beans1.xml/beans2.xml</param-value>

</context-param>

<listener><listener-class>

org.springframework.web.context.ContextLoaderListener

</listener-class></listener>

..


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Summary

Simplify developmentAccelerate developmentImprove maintainabilityIncrease immunity to rapidly evolving infrastructure frameworks

POJOs

+ =

Non-invasive frameworks


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For more informationBuy my book ☺

Send email:[email protected]

Visit my website:

http://www.chrisrichardson.net

Please hand in your session evaluations


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Extra slides


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Thoughts about EJB 3 and POJOs☺ Better than EJB2☺ Supports POJOs☺ Reasonable ORM☺ Entity beans = JPA☺ Annotations are

concise☺ Has dependency

injection☺ It’s a standard

Less powerful than Spring, e.g. DI relies on JNDILess powerful than Hibernate, e.g. List<String>Session beans/MDBsmust be deployedComplexity of EJB lurking withinAnnotations couple your code to EJB3EJB’s poor track record as a standard


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Using Spring with EJBs

Simplify EJB client code with SpringSpring encapsulates JNDI lookupClient gets EJB reference from SpringBetter: Client is injected with EJB reference

Move business logic into Spring beansSession EJBs delegate to Spring beansUse Spring dependency injectionSimpler code, easier testing

Simplify DAOs with Spring JDBCEliminates error-prone boilerplate code


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Migrating to POJOs – part 1

2 year old application:Session EJBsEntity Bean-based domain modelSome JDBC DAOsBeginning development of version 2

Replaced entity beans with Hibernate:WAS vs. WLS portabilityTest business logic without persistenceTest persistence without a serverA much richer domain model


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Migrating to POJOs – part 2

Used Spring beans for V2 codeIncrementally replaced V1 session beans with Spring beans when:

Enhancing itV2 code needed to call V1 code

End result:Richer domain modelFaster developmentV2 code was deployable as a web app.

overview of pojo programming - chris richardson

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