Chapter 9: JSP and JavaBeans

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Chapter 9 JSP and JavaBeans
Reference Beginning JSP
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Objectives

• What are JavaBeans?
• JavaBean Usage within JSPs
• Access JavaBeans with Standard Actions
• Access JavaBeans with EL
• Access JavaBeans from scriptlets
• Front Controller JSPs
• JSP Input Validation
• Error Message JavaBean

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Review JSP Action and JavaBeans

• Reference Pages 55 - 58
• JavaBeans (more details in Chapter 9)
• Reusable software components that has properties,
  methods and events
• Can be used to generate dynamic content for the
  portal pages
• To create a JavaBean attached to the current
  request
• ltjspuseBean idnewsfeed" class"com.wrox.begjsp.
  ch2.NewsFeed
• scope"request"gtlt/jspuseBeangt
• To set the topic property to news
• ltjspsetProperty name"newsfeed"
  property"topic" value"news"/gt
• To get the value property
• ltjspgetProperty name"newsfeed"
  property"value"/gt

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What Are JavaBeans?

• Javas software component model specification
• Written in Java platform neutral
• JavaBean Characteristics
• Must have a default (zero-argument) constructor
• Used when restoring JavaBean from a file
• Must be serializable
• Implements java.io.Serializable
• Can save JavaBean to a file
• Note Many non-visual JavaBeans do not conform to
  this requirement.
• Private instance variables
• Public features (methods, properties, events)

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JavaBean Property Types

• Simple
• Use standard getter/setter signatures
• E.g., public String getName()
• Boolean
• Use isltPropertyNamegt for getter method
• E.g., public boolean isValid()
• Better use getValid() to allow access using EL
• Indexed
• Represents an array of values
• Use java.util.ArrayList for dynamic arrays
• E.g., public ArrayList getValues()

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Coding a JavaBean

• Reference Page 276-284
• NetBeans 4.1
• Use wizard to create getter/setter methods
• E.g., Bid.java
• Add instance variable and read/write access
• private String item
• public String getItem() return this.item
  
• public void setItem(String item) this.item
  item
• JavaBean method
• Can be wrapped as an EL function
• public double calculateTotal ()
• return price quantity

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JavaBean as Data Transfer Objects

• Reference Chapter 9, Example 1
• JavaBeans carry data between pages
• Between business logic and presentation layer.
• index.jsp (controller JSP)
• Creates Bid JavaBean instance named bidinfo
• ltjspuseBean id"bidinfo" class"com.wrox.begjsp.c
  h09.Bid" scope"request"gt
• Sets property values
• ltjspsetProperty name"bidinfo" property""/gt
• showbid.jsp (presentation JSP)
• Access bidinfo property values
• Using EL bidinfo.item
• Using Scriptlet lt bidinfo.getPrice() gt

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Front Controller JSPs

• Reference Page 284-288, Example 1
• Front controller (e.g., index.jsp) switches
  incoming request to different presentation JSPs
• Incoming request from enterbid.jsp
• ltinput type"hidden" name"action" value"bid"/gt
• redirects request to appropriate JSP page
• ltcwhen test"empty param.action"gt
• ltjspforward page"enterbid.jsp"/gt
• lt/cwhengt
• Perform common tasks such as input validation
• ltcif test"(param.price lt 0) (param.price
  gt 999)"gt

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JSP Input Validation

• Reference Page 288-291, Example 1
• Create BidError JavaBean with msg property
• index.jsp creates BidError instance
• ltcif test"(param.price lt 0) (param.price
  gt 999)"gt
• ltjspuseBean id"biderror" class"com.wrox.begjs
  p.ch09.BidError" scope"request"gt
• ltjspsetProperty name"biderror"
  property"msg" value"Sorry, your bid is not in
  range. Please enter again."/gt
• enterbid.jsp displays input error
• ltcif test"!(empty biderror)"gt
• lttrgtlttd class"errorText" colspan"2"gt
• biderror.msg lt/tdgt
• lt/trgt
• lt/cifgt

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Input Validation Error Example

• Reference Chapter 9, Example 1
• Enter a bid higher than 999 (e.g, 2000)

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How to Remember Bid Price Entered

• index.jsp set bidinfo properties before forward
  enterbid.jsp
• enterbid.jsp add value attribute to ltinputgt tag
• ltinput name"price" type"text"
  value"bidinfo.price" /gt

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Testing ltjspuseBeangt Behavior

• In index.jsp, change scope of bidinfo from
  request to session
• ltjspuseBean id"bidinfo" class"com.wrox.begjsp.c
  h09.Bid" scope"session"gt
• Findings
• Only the first bid price is remembered in bidinfo
• ltjspuseBeangts body is executed only if a new
  bean instance is created.
• Result ltjspsetPropertygt is executed the first
  time bidinfo is created.
• Solution
• Add ltcremove var "bidinfo"/gt before
  ltjspuseBeangt

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Next Steps

• Try It Out sections of Textbook (Chapter 9)
• Example 1
• Exercise 1
• You can skip Example 2 and Exercise 2
• Do Assignment 3