JavaEE Design Patterns

8/14/2019 JavaEE Design Patterns

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Design PatternsDesign Patterns((source: Core J2EE Patternssource: Core J2EE Patternswritten by Alur, Crupi, Malks)written by Alur, Crupi, Malks)


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Agenda

Presentation tier design patterns Business tier design patterns

Integration tier design patterns


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J2EE Pattern Catalog

Core J2EE Patterns


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Three Tiers

PresentationTier

BusinessTier

IntegrationTier


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Presentation-Tier Patterns

Intercepting Filter

Front Controller

View Helper

Composite View

Service to Worker


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Business-Tier Patterns

Business Delegate Service Locator

Session Facade

Data Transfer Object (DTO) (was Value Object)

Data Transfer Object Assembler

(was Value Object Assembler)

Composite Entity

Value List Handler


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Integration-Tier Patterns

Connector

Data Access Object

Service Activator


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Presentation-TierPresentation-TierDesign PatternsDesign Patterns


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Presentation Tier Processing

Client BusinessService

ControlLogic Display

InterceptingFilter

Pre/Post-Processor


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Intercepting Filter: Forces

Each service request and responserequires common pre-processing and

post-processing logging, authentication, caching, compression,

data transformation

Adding and removing these pre andpost processing components should beflexible deployment time installation/configuration


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Intercepting Filter: Solution

Create pluggable and chainable filters toprocess common services such that

Filters intercept incoming and outgoingrequests and responses

Flexible to be added and removed withoutrequiring changes to other part of the

application Examples

Servlet filters for HTTP requests/responses

Message handlers for SOAPrequests/responses


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Intercepting Filter:

Class Diagram

Consumer

Service request interceptedby FilterManager

FilterManager

FilterChain

Target_Resource

Filter Two

Filter One


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Intercepting Filter PatternSequence Diagram

Consumer SecurityFilterCompression

FilterLoggingFilter FrontController

IncomingRequest

Apply

Forward request

Complete RequestProcessing

Forward request

Apply

Response

ApplyForward response

ApplyOutgoingresponse


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Intercepting Filter PatternSample code for writing Servlet 2.3 Filter

Sample Deployment DescriptorPublic final class SecurityFilter implements Filter{public void doFilter(ServletRequest req,ServletResponse res,FilterChain chain)

throws IOException, ServletException{// Perform security checks here.....

// Complete the filter processing by either passing the control// to the next servlet filter in chain or to the target URI.chain.doFilter(modified_req, modified_res);

}}

SecurityFilterControllerServlet


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Presentation Tier Processing

Client BusinessService

ControlLogic Display

InterceptingFilter

FrontController

Pre/Post-Processor


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Front Controller: Forces There is a need for centralized controller

for view selection and navigation (Model2) based on user entered data

business logic processing

client type

Common system services are typicallyrendered to each request, so having asingle point of entry is desirable Example: Authentication, authorization, Logging

Can leverage filter pattern


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Front Controller: Solution Use a controlleras an centralized point

of contact for all requests Promote code reuse for invoking common

system services

Can have multiple front controllers, eachmapping to a set of distinct services

Works with other patterns Filter, Command, Dispatcher, View Helper


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Front Controller:

Implementation Strategy

Consumer

Sends Service request

Controller

ServletController

JSPController


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Front Controller Sample CodeServlet-based Implementation

Sample Deployment DescriptorPublic class EmployeeController extends HttpServlet{

//Initializes the servletpublic void init(ServletConfig config) throws ServletException{

super.init(config);}

//Destroys the servletpublic void destroy(){}

//Handles the HTTP GET Requestsprotected void doGet(HttpServletRequest request,

HttpServletResponse response) throws ServletException, java.io.IOException{

processRequest (request, response);}

//Handles the HTTP POST Requestsprotected void doPost(HttpServletRequest request,

HttpServletResponse response) throws ServletException, IOException{

processRequest (request, response);

}


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Front Controller Sample CodeServlet Front Controller with Command

PatternSample Deployment Descriptor

//Processes requests for HTTP Posts and Getsprotected void processRequest(HttpServletRequest request,

HttpServletResponse response) throws ServletException, IOException{String resultPage;

// Create a RequestHelper object that represent the client request specific informationRequestHelper reqHelper = new RequestHelper(request);

/********************************************************************* Create a Command object. Command object is an implementation of the Command* Pattern. Behind the scenes, implementation of getCommand() method would be like* Command command = CommandFactory.create(request.getParameter("op"));

********************************************************************/ Command command= reqHelper.getCommand();

// Command performs the actual operationresultPage = command.execute(request, response);

// Dispatch control to the view

dispatch(request, response, resultPage);}


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Front Controller Sample CodeServlet Front Strategy with Dispatch

PatternSample Deployment Descriptor//Implement the dispatch methodprotected void dispatch(HttpServletRequest request,

HttpServletResponse response, String page)throws ServletException, IOException {

RequestDispatcher dispatcher

= getServletContext().getRequestDispatcher(page);dispatcher.forward(request, response);

}}


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Business-TierBusiness-TierDesign PatternsDesign Patterns


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Business Delegate Pattern:

Forces Business service interface (Business

service APIs) change as business

requirements evolve Coupling between the presentation tier

components and business service tier

(business services) should be kept tominimum

It is desirable to reduce network traffic

between client and business services


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Solution Use a Business Delegate to

Reduce coupling between presentation-tier

and business service components Hide the underlying implementation details of

the business service components

Cache references to business services

components

Cache data

Translate low level exceptions to applicationlevel exceptions


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Class Diagram

BusinessDelegate

Uses

BusinessService

LookupService

1 1..*

Lookup / create


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Business Delegate Pattern

Sequence DiagramClient

BusinessDelegate

LookupService

BusinessService

1. Create1.1 Get service

1.1.1 Lookup

1.1.2 Return businessservice

2. Invoke

2.1 Invoke

2.2 Invoke


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Service Locator Pattern:

Forces Service lookup and creation involves

complex interfaces and network

operations JNDI operation is complex

ex) PortableRemoteObject.narrow(.., ..)

Service lookup and creation operationsare resource intensive and redundant Getting JNDI context


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Solution Use a Service Locator to

Abstract naming service usage

Shield complexity of service lookup andcreation

Enable optimize service lookup and creationfunctions

Usually called within Business Delegateobject


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Class DiagramClient

Component

Uses

ServiceLocator

BusinessService

1 1..*

Lookup / create

Uses

Uses

Creates

Uses

Uses

InitialContext

Lookup

ServiceFactory


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Sequence DiagramClient ServiceLocator InitialContext ServiceFactory BusinessService

1. Get Instance 1.1 Create

2. Get Service

2.1. Lookup

2.1.1 Create

2.1.2 Return

EJBHome

2.2 Get Service

2.2.1 Creates2.3 Return

service


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Implementation Strategies Implementation strategies for Service

Locator EJB Service Locator Strategy JMS Queue Service Locator Strategy

JMS Topic Service Locator Strategy

Combined EJB and JMS Service LocatorStrategy


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Service Locator PatternSample code using EJB Service Locator

public class ServiceLocator{

private static ServiceLocator me;InitialContext context = null;

private ServiceLocator() throws ServiceLocatorException{try{context = new InitialContext();

}catch(NamingException ex){throw new ServiceLocatorException(...);

}}

// Returns the instance of ServiceLocator class (singleton)public static ServiceLocator getInstance() throws ServiceLocatorException{

if (me == null){me = new ServiceLocator();

}return me;

}


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Service Locator Pattern: Sample code

using EJB Service Locator Strategy// Convert the given string into EJB Handle and then to EJB Objectpublic EJBObject getService(String Id) throws ServiceLocatorException{

if (Id == null){throw new ServiceLocatorException(...);

}

try{byte[] bytes = new String(Id).getBytes();InputStream io = new ByteArrayInputStream(bytes);ObjectInputStream is = new ObjectInputStream(io);javax.ejb.Handle handle = (javax.ejb.Handle)is.readObject();return handle.getEJBObject();

}catch(Exception ex){

throw new ServiceLocatorException(...);}

}

// Returns the string Id that represents the given EJBObject's handle// in serialized format

public String getId(EJBObject session) throws ServiceLocatorException{

...}


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Service Locator PatternSample code using EJB Service Locator

Strategy// Converts the serialized string into EJBHandle and then to EJBObjectpublic EJBHome getHome(String name, Class homeClass)

throws ServiceLocatorException{try{

Object objRef = context.lookup(name);

EJBHome home= (EJBHome)PortableRemoteObject.narrow(objRef,homeClass);

return home;}catch(NamingException ex){

throw new ServiceLocatorException(...);}

}

// Other methods pertaining to getting service using a string ID or// getting a string ID based on the given service...

}


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Service Locator PatternClient code using EJB Service

Locatorpublic class SampleServiceLocatorClient{

public static void main(String[] args){ServiceLocator objServiceLocator = ServiceLocator.getInstance();try{

ResourceSessionHome objResourceSessionHome

= (ResourceSessionHome)objServiceLocator.getHome(myExamples.resourcesession.ResourceSessionHome.class);}catch(ServiceLocatorException ex){

// Client handles exception...

}}

}


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JavaEE Design Patterns

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