Web Tier Design Patterns
Objectives
! " Learn design patterns relevant to web application
programming
! " Model View Controller
! " Front Controller
! " View Helper
! " Composite View
! " Business Delegate
! " Dispatcher View
! " Service To Worker
! " Understand how enterprise applications can benefit from
these patterns
The Problem
! " Application construction
! " Applications built by many groups of developers
! " Groups have different skills
! " Groups have different objectives
! " Groups have different timelines
! " This translates into various pieces of the application being
developed independently
! " Application use
! " Applications often also need to support multiple clients
! " Different clients can interact with application in different ways
! " Share business data in different ways
! " Interactions follow different flows
! " This increases the complexity and risk of an application
The Solution: Model-View-Controller
! " Model
! " Represents business data and business rules
! " Typically a Java Bean or a POJO (plain old java object)
! " Controller
! " Maps user interactions to calls on the model, which often results
in a change to the business data
! " View
! " Renders the data encapsulated by the model and guarantees
coordination with any changes to the business data
! " Each view must remain consistent with the backend data it
represents
Benefits of MVC for the Web Tier
! " Separation of concern
! " Maintain modularized approach
! " Increased flexibility
! " For example, can move web-resident model to the EJB tier
! " Model components can be viewed in multiple ways
! " Support for new clients is simple
MVC Diagram
MODEL!
-Encapsulates business data!
and business rules!
-Handles calls from the
CONTROLLER!
-Notifies VIEW of changes"
VIEW!
-Displays the MODEL data!
-Sends user request to
CONTROLLER!
-Notified by MODEL of state
change"
User request"
Select View"
CONTROLLER!
-Provides business logic!
-Translates user requests into
calls to MODEL!
-Selects VIEW for response"
The Model 1 Architecture
! " A variation of MVC for the web
! " Useful for smaller, simpler applications
! " Not as loosely coupled as larger-scale applications require
! " Not as complex to build / maintain
! " Model-1 architecture focuses on two components
! " JSP
! " Each JSP is responsible for handling requests
! " Also responsible for providing responses
! " JavaBeans
! " Model-1 solutions typically have large amount of JSP
scriptlet code
Model 1 Diagram
request!
(view)!
jsp!
response!
client!
JavaBean!
(controller)!
database!
(model)!
The Model 2 Architecture
! " Also a variation of MVC
! " Looser coupling
! " Higher cohesion
! " Increased modularity = increased flexibility
! " Better for larger-scale applications
! " Model-2 architecture adds servlet to Model-1 architecture
! " Servlet acts as a controller
! " Servlet focuses on low level tasks
! " Model-2 architecture changes role of JSP
! " JSP focuses on presentation only
! " Lessens scriplet code
! " Much stricter separation of model from view from controller
Model 2 Diagram
(view)!
request!
response!
jsp!
Servlet"
(controller)!
client!
database!
JavaBean!
(model)!
Front Controller
! " The “front door” to your application
! " Generally a pre-processor, but can be used for post-
processing too
! " Often used to enforce
! " Validation rules
! " Authentication constraints
! " Dispatching of requests
! " Session management
Front Controller Class Diagram
Client!
request!
Front
Controller!
<<servlet>>!
ServletFC!
<<jsp>>!
JSPFC!
Front Controller Sequence Diagram
Client!
Controller!
1. Send Request!
Dispatcher!
View!
1.1 Delegate Request!
1.1.1 Forward Request!
2. Send Request!
2.1 Forward Request!
Front Controller Servlet Strategy
! " This is the most common approach
! " All requests route through the servlet
! " Front Controller processing is more logically separated
from the view
Front Controller JSP Strategy
! " Generally, this is undesirable due to the mismatch of low-
level processing and view processing
! " Tends to lead to complicated JSP code
Modularized Front Controller
! " For complex logic, a front controller can be broken into
smaller components
! " For example, an authentication component, validation
component, and dispatching component
! " This continues the pursuit of architectural modularity
View Helper
! " Separates presentation logic from presentation
components (views)
! " Represent the model from a web tier perspective
! " Typically implemented as a JavaBean component
! " The JSP and View Helper can be implemented
independently, allowing for
! " Separation of developer roles
! " Flexibility in the web tier
! " Multiple views sharing the same helper
View Helper Class Diagram
Client!
1..*!
View!
0..*!
Helper!
View Helper Sequence Diagram
Client!
1. Request!
View!
Helper!
1.1 Get Data!
1.1.1 Get Data!
Business"
Service!
Implementing View Helper as a Tag
Library
! " Requires more work upfront to create the tag library
! " May be more reusable than a simple JavaBean
component
! " Custom tags also add manageability concerns
! " Deploying tags, tag libraries, and descriptors
Business Delegate
! " Used to provide a façade over a complex business tier
! " Normally, presentation components interact with business
services directly, which may cause problems
! " If the business tier API changes, it affects the presentation tier
! " Heavy interaction with the business tier may result in too many
network calls
! " A Business Delegate can perform caching and throttling
Business Delegate Class Diagram
Client!
Business Delegate!
Business Service!
Business Delegate Sequence Diagram
Helper!
1. Create!
Business
Delegate!
1.1 Invoke!
Business
Service!
Business Delegate as a Proxy
! " The Business Delegate exposes the same interface as
the business tier
! " Calls to the Business Delegate can be forwarded to the
business tier
! " Data can also be cached and calls short-circuited
! " This can reduce network calls and increase performance
Business Delegate as an Adapter
! " Excellent for communication with disparate business
components
! " For example, communication with a web service
component could be accomplished
! " The delegate would parse the XML request
! " The data in the request could be sent to an existing, non-XML
aware, business tier
! " A response could work the same way
Composite View
! " Modern web pages are built from smaller “sub-pages”
! " All Composite View does is define a pattern for creating
modularized views that can be combined
! " Complicated views can be managed much simpler via
separate view components
! " This does add some runtime overhead in exchange for
the added flexibility
Composite View Class Diagram
View Component!
Sub View!
Composite View!
Dispatcher View
! " Combines Front Controller and View Helper
! " May also involve Composite View
Dispatcher View Class Diagram
Client!
<<servlet>>!
Front Controller!
1..*!
<<jsp>>!
View!
1..*!
View Helper!
Dispatcher View Sequence Diagram
Client!
1. Request!
Controller!
View!
Helper!
1.1 Dispatch!
1.1.1 Get Data!
To business tier!
Service To Worker
! " Structurally identical to Dispatcher View
! " Difference is in the behavior and location of the
intelligence
! " Now, the Front Controller interacts with View Helper
components before selecting a view
Service To Worker Class Diagram
Client!
<<servlet>>!
Front Controller!
1..*!
<<jsp>>!
View!
1..*!
1..*!
View Helper!
Service To Worker Sequence
Diagram
Client!
1. Request!
Controller!
View!
Helper!
1.1 Get Data!
To business tier!
1.1.1 Dispatch!
Summary
! " There are several relevant design patterns for the web
tier
! " Model View Controller
! " Front Controller
! " View Helper
! " Composite View
! " Business Delegate
! " Dispatcher View
! " Service To Worker
! " These patterns can benefit enterprise applications by
promoting modularity, flexibility, and reuse
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