Module 3
Web Component
Model
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Objectives
► Describe the role of web components in a Java EE
application
► Define the HTTP request-response model
► Compare Java servlets and JSP components
► Describe the basic session management strategies
► Manage thread safety issues in web components
► Describe the purpose of web-tier design patterns
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Role of Web Components in a Java EE
Application
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► Web-centric Java EE application architecture:
► EJB component-centric Java EE application architecture:
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HTTP Request-Response Model
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The GET and POST Requests
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Form Data
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HTML snippet:
<FORM ACTION=’form_test’ METHOD=’POST’>
<INPUT NAME=’input1’ SIZE=’20’/>
<INPUT TYPE=’SUBMIT’ VALUE=’OK’/>
</FORM>
Browser form:
Browser request:
POST /bank/form_test HTTP/1.1
... request headers...
input1=this+is+a+test
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Content Type and the Response Header
► The server response includes a Content-Type header that
contains MIME type values including but not limited to:
─ text/html
─ text/xml
─ image/jpeg
► Examples of additional response headers include:
─ Content-Encoding
─ Content-Length
─ Cache-Control
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Comparison of Servlets and JSP™
Components
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Life Cycle of a Web Component
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The service Method
The web container calls the service method once for each
incoming request. The service method then typically
completes the following operations:
► Validates any form data
► Updates the application’s data model
► Collects data from the model to be rendered
► Renders the data in HTML or passes the request and the
data to another component responsible for rendering them
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Servlet and JSP Component Examples
The examples on the following slides illustrate the similarities
and differences between servlets and JSP components.
► Servlet example
► JSP component example
► Servlet and JSP component collaboration
► Run-time behavior of servlets and JSP components
► Web context root and alias mapping
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Servlet Example
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The following code generates the same output as the
preceding servlet example:
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JSP Component Example
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Servlet and JSP Component Collaboration
Most modern web applications use servlets and JSP
components in collaboration. Their capabilities are similar,
but they are expressed differently.:
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Run-Time Behavior of Servlets and JSP
Components
► Because JSP components are translated into servlets, JSP
components and servlets share run-time behaviors:
• Life cycle and container management
• API and container services
• Client session access
► Both can be entered on multiple threads concurrently and
must be implemented accordingly.
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Web Component Thread Model
int x;
public void doGet(){
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Implications for the Developer
► Because the web container enters the service method on
multiple concurrent threads to support multiple simultaneous
requests, the developer must ensure that web components are
thread-safe:
► Use instance variables cautiously
► Use class variables very cautiously
► Provide access to external resources cautiously
► Use synchronization constructs to denote critical sections:
synchronized (this) {
// This section is only entered by 1 thread at a time
}
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Web Context Root and Alias Mapping
Servlets and JSP components are packaged into a web
application.
► Static content, such as HTML or images, is included.
► A web application URI has the following form:
http://server:port/context_root/resource
► The context root maps to a web application
► In the deployment descriptor a url pattern is specified that
maps from a JSP or Servlet resource to a url:
http://www.mybank.com/bank/main
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Session Management
The HTTP protocol is stateless. Conversational state might be
stored on either the browser or the server:
The Java EE model provides a simple mechanism for storing
conversational state on the server.
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Problems With Web-Tier Development
The complexities of web-based user interface development
include:
► Addressing HTTP’s stateless request-response sequence
► Avoiding the creation of large numbers of servlets and JSP
components that handle different types of requests
► Using JSP components and servlets effectively
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Model 1 and Model 2 Architectures
Two design strategies are involved in using JSP components
in the web tier:
► Model 1 architecture:
• JSP components handle request processing through
<jsp:useBean> classes, the JSTL, and custom tags.
• JSP components render data that is retrieved from the
business logic tier.
► Model 2 architecture:
• Servlets handle request processing, interact with the
business logic, and collect data for display.
• JSP components render the data for display.
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Traditional MVC Architecture
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Model 2 Architecture as MVC
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MVC in the Java EE Platform
The MVC paradigm divides application logic into three roles:
► Controller
► View
► Model
• When the application includes EJB components
• When there are no EJB components
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Using Web-Tier Design Patterns
► The web-tier developer should work with specific design
patterns that realize the broad, architectural paradigms of
MVC and Model 2 architecture.
► Of the many available patterns, consider these three
specific patterns, which can be used in combination:
• Service-to-Worker
• Dispatcher View
• Business Delegate
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Service-to-Worker and Dispatcher View
Patterns
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Web-Tier Design Framework Construction
The Service-to-Worker and Dispatcher View patterns provide
generic functionality that can be extracted to form a design
framework.
► Each request is handled in the same generic way, with
variations for the specific needs of the application.
► The controller element is the most generic, allowing reuse
in different applications.
► The controller’s functionality can be expressed
declaratively, rather than in code.
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Some Available Web-Tier Frameworks
Commonly used frameworks include:
► Struts
► JavaServer™ Faces technology
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Business Delegate Pattern
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