1. Unit 7: Design Patterns and Frameworks
 Web presentation layer patterns
 The Model-View-Controller (MVC) architectural pattern
 Catalog of MVC-based patterns
 Other patterns
 Web presentation-business integration patterns
 Business layer architectural patterns
 MVC Web frameworks
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2. The MVC Architectural Pattern
 Divides an interactive application into three components/levels:
 Model:
 Contains the functional core of the application
 Encapsulates the appropriate data, and exports procedures that
perform application-specific processing
 View:
 Displays information to the user
 Obtains the data from the model
 Different views present the model’s information in different ways
 Controller:
 Accepts user input as events
 Translates events to service requests for the model or display
requests for the view
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3. The “Classical” MVC Pattern
Observer
*
update()
1
Model
data
attach( ob : Observer )
View
detach( ob : Observer ) Controller
notify() 1 1
getData() initialize( m : Model )
initialize( m : Model, v : View )
service() makeController()
handleEvent()
activate()
update()
display()
update()
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4. Catalog of MVC-Based Web Presentation Patterns
 Controller Patterns:
 Page Controller
 Front Controller
 Application Controller
 Intercepting Filter
 View Patterns
 View Helper
 Composite View
 Transform View
 Composite Patterns:
 Dispatcher View
 Service to Worker
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5. Page Controller
 Each Page Controller component act as the controller for a dynamic
page on the Web site.
 The basic responsibilities of a Page Controller are:
 Decode the URL and extract any form data
 Invoke model components to process the request data
 Determine which view should display the result page and forward the
model information to it
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6. Page Controller: How It Works
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7. Page Controller: When to Use It
 Page Controller works particularly well in a site where most
of the controller logic is pretty simple:
: User : System
service(parameters)
result
 Variant: Page Controller and View implemented by the same
Server Page: (Model 1)
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8. Front Controller (Model 2)
 Problem: The system requires a centralized access point for
request handling. Without a central access point, control code that
is common across multiple requests is duplicated in numerous
places. When control code is intermingled with view-creation code,
the application is less modular and cohesive.
 Forces:
 You want to avoid duplicate control logic.
 You want to apply common logic to multiple requests.
 You want to separate system processing logic from the view.
 You want to centralize controlled access points into your system
 Solution: Use a Front Controller as the initial point of contact for
handling all related requests. The Front Controller centralizes
control logic that might otherwise be duplicated, and manages the
key request handling activities .
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9. Front Controller: Class Diagram
1
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10. Front Controller: Sequence Diagram
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11. Front Controller + Application Controller
 An Application Controller has two main responsibilities: deciding which
domain logic to run and deciding the view with which display the
response.
 It is useful for designing complex use cases with definite rules about the
order in which pages should be visited and different views depending on
the state of objects.
 Separating the Application Controller from the Front Controller allows
improving the modularity of the system, while enhancing its reusability.
 The Application Controller component is not a Server Page
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12. Front Controller + Application Controller (cont.)
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13. Application Controller with Mapper
 Application Controller: Uses Mapper to resolve an incoming request to the
appropriate action and view, to which it delegates or dispatches.
 Mapper: Uses a Map to translate an incoming request into the appropriate
action and view. A Mapper acts as a factory
 Map: Acts as a dictionary or registry of references to target resources.
 Target: A resource that helps fulfill a particular request (view, command)
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14. Application Controller with Mapper (cont.)
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15. Application Controller with Mapper (cont.)
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16. Intercepting Filter
 Problem: You want to intercept and manipulate a request and a response
before and after the request is processed in order to determine if:
 The client has a valid session
 The request path violates any constraint
 You support the browser type of the client
 The client has used a special encoding to send the data
 The request stream is encrypted or compressed
 Forces
 You want centralized, common processing across requests
 You want pre and postprocessing components loosely coupled with core
request-handling services.
 You want pre and postprocessing components independent of each other and
self contained to facilitate reuse.
 Solution: Use an Intercepting Filter as a pluggable filter to pre and
postprocess requests and responses. A Filter Manager combines loosely
coupled filters in a chain, delegating control to the appropriate filter.
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17. Intercepting Filter: Structure, Participants and
Responsibilities
 FilterManager: Manages filter processing. It creates the FilterChain with
the appropriate filters, in the correct order, and initiates processing.
 FilterChain: Ordered collection of independent filters
 Filter: Component that performs a specific pre and/or postprocessing
task.
 Target: The resource requested by the client.
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18. Intercepting Filter: Sequence Diagram
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19. Java Filters: Example
 Motivation: HTML forms that include a file upload use a different
encoding type than that of basic forms. As a result, form data that
accompanies the upload is not available via simple getParameter()
invocations. The following filter preprocesses requests to translate
the encoding type into a single consistent format that makes all
form data available as request attributes:
public class MultipartEncodeFilter extends javax.servlet.Filter {
public void doFilter(javax.servlet.ServletRequest request,
javax.servlet.ServletResponse response,
javax.servlet.FilterChain filterChain)
throws java.io.IOException, javax.servlet.ServletException {
String contentType = request.getContentType();
if (contentType.startsWith("multipart/form-data") ) {
For each pair (attributeName, value) obtained from the request
do request.setAttribute(attributeName, value)
}
filterChain.doFilter(request, response); }
}
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20. View Helper
 Problem: You want to separate a view from its processing logic.
 JSP and other template-based views allow embedding scriptlet code
 Embedded scriptlet code cannot be reused easily
 Embedded scriptlet code often acts as control code or performs view
preparation activities, such as content retrieval.
 Mingling control logic, data access logic and formatting logic leads to
problems with modularity, reuse, maintenance, and role separation.
 Forces:
 You want to use template-based views, such as JSP.
 You want to avoid embedding program logic in the view.
 You want to separate programming logic from the view to facilitate division of
labor between software developers and web page designers
 Solution: Use Views to encapsulate formatting code and Helpers to
encapsulate view-processing logic.
 A View delegates its processing responsibilities to its helper classes,
implemented as POJOs, custom tags, or tag files.
 Helpers serve as adapters between the view and the model, and perform
processing related to formatting logic, such as generating an HTML table.
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21. View Helper: Structure
 Helper: Encapsulates processing logic for generating and
formatting a View. A helper typically adapts a PresentationModel
for a view or provides access to the raw data of the
PresentationModel
 PresentationModel: Holds the data retrieved from the business
service, used to generate the View. It can be either a Business
Delegate or a DTO
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22. View Helper: Sequence Diagram
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23. View Helper: Example with JavaBean and JSTL
<%@ taglib uri=http://java.sun.com/jsp/jstl/core
prefix="c" %>
<jsp:useBean id="welcomeHelper" scope="request"
class="WelcomeHelper" />
<HTML><BODY bgcolor="FFFFFF">
<c:if test = "${welcomeHelper.nameExists == true}">
<center><H3> Welcome <b>
<c:out value='${welcomeHelper.name}'/></b><br><br></H3>
</center>
</c:if>
<H4><center>We are happy for your visit!</center></H4>
</BODY>
</HTML>
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24. Composite View
 Problem: You want to build a view from modular, atomic
component parts that are combined to create a composite whole,
while managing the content and the layout independently.
 Forces
 You want common subviews, such as headers, footers and
tables reused in multiple views, which may appear in different
locations within each page layout.
 You have content in subviews that frequently changes or might
be subject to certain access controls, such as limiting access to
users in certain roles.
 You want to avoid directly embedding and duplicating subviews
in multiple views which makes layout changes difficult to
manage and maintain.
 Solution: Use Composite Views that are composed of multiple
atomic subviews. Each subview of the overall template can be
included dynamically in the whole, and the layout of the page can
be managed independently of the content.
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25. Composite View: Structure
 Template: Represents the page layout
 ViewManager: Uses a Template to enforce a layout into which it places
the appropriate content.
 A simple ViewManager might use the standard JSP include tag (<jsp:include>)
to include SimpleView segments into a Template.
 A more sophisticated ViewManager might use POJOs or custom tag helpers to
provide content and layout management in a more comprehensive and
robust manner.
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26. Composite View: Sequence Diagram
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27. Transform View
 Problem: You want a view that processes domain data
element by element and transforms it into HTML.
 Forces: The data returned by the business layer is either in
XML or in something automatically transformable to it
 Solution: Use a Transform View that transforms directly from
domain-oriented XML into (X)HTML by using XSLT (eXtensible
Stylesheet LanguageTransformations). Normally XSLT does
this by transforming each XML element into an (X)HTML
element.
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28. Dispatcher View
 Problem: You want a view to handle a request and generate a
response, with little or no business processing performed
before rendering the view.
 Forces:
 You have static views
 You have views generated from an existing presentation model
 You have views which are independent of any business service
response
 You have limited business processing
 Solution: Use Dispatcher View with views as the initial access
point for a request. Business processing, if necessary in
limited form, is managed by the views.
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29. Dispatcher View: Structure
 BusinessHelper: Helps the View initiate business processing to handle a
request
 BusinessService: Encapsulates the business logic and business state. A
remote business service is accessed via a Business Delegate
 PresentationModel: Holds the data retrieved from the business service
(DTO).
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30. Dispatcher View: Sequence Diagram
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31. Service To Worker
 Problem: You want to perform core request handling and
invoke business logic before control is passed to the view
 Forces:
 You want specific business logic executed to service a request in
order to retrieve content that will be used to generate a
dynamic response.
 You have view selections which may depend on responses from
business service invocations.
 You may have to use a framework or library in the application
 Solution: Use Service to Worker to centralize control and
request handling to retrieve a presentation model before
turning control over to the view. The view generates a
dynamic response based on the presentation model.
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32. Service To Worker: Structure
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33. Service to Worker: Sequence Diagram
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34. References
 Books
 ALUR, Deepak et al. Core J2EE Patterns: Best Practices and
Design Strategies, 2on Edition, Prentice Hall PTR, 2003.
 FOWLER, Martin Patterns of Enterprise Application
Architecture, Addison Wesley, 2002.
 Webs
 www.corej2eepatterns.com
 java.sun.com/blueprints/corej2eepatterns/
 java.sun.com/blueprints/guidelines/designing_enterprise_appli
cations_2e/web-tier/web-tier5.html
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