Java Server Pages

1
Java Server Pages

• Continued

2
JavaBeans

• Reusable Components for Java

3
What is a JavaBean?

• A Java class meeting specific requirements
• Must have a zero-argument constructor
• E.g. public MyBean()
• Must have no public fields
• All fields should be private (encapsulation)
• All data should be accessed via access methods
• Allow you to define properties
• These are virtual exposed fields
• Similar in nature to ActiveX controls

4
JavaBean Properties

• Read-only properties
• int getWidth() returns the width property
• Read/write properties
• void setHeight(int iHeight)
• int getHeight()
• Boolean properties
• boolean isSold()
• void setSold(boolean bSold)

5
More About JavaBeans

• JavaBeans can receive events, or cause events in
  other beans (or classes)
• JavaBeans usually implement the Serializable
  interface to allow them to be stored
• JavaBeans provide other methods (non-access
  methods) for their interfaces
• Properties and methods of JavaBeans can be
  determined at runtime, using a technique known as
  introspection
• Define these properties and methods in a bean
  info class
• I.e. A class that implements BeanInfo

6
Whats the Point?

• JavaBeans provide a framework for reusable
  components
• The zero-argument constructor and bean info
  classes make using JavaBeans easier for IDEs
• The properties can be easily connected
  graphically to components such as text fields,
  radio buttons, etc.
• They allow IDEs (like VisualAge, J, VisualCafe)
  to provide reusable components similar to those
  used by VisualBasic

7
JavaBean Example

• public class PythagorasBean
• private double dWidth 0.0
• private double dHeight 0.0
• public void setWidth(double dWidth)
• this.dWidth dWidth
• public double getWidth() return dWidth
• public void setHeight(double dHeight)
• this.dHeight dHeight
• public double getHeight() return dHeight
• public double getHypoteneuse()
• return Math.sqrt((dWidthdWidth)(dHeightdHeigh
  t))

8
When to use JavaBeans

• Client Applications or Applets
• GUI and non-GUI beans can be combined in a rapid
  application development app
• Server tier
• Used to represent server tier data model
• Can be conveniently accessed from JSPs

9
JavaBeans and JSPs

• Using JavaBeans from Java Server Pages

10
Creating JavaBean Instances

• ltjspuseBean idbean1 classcs334.MyBean /gt
• Creates a bean instance called bean1 of type
  cs334.MyBean
• Similar to the following JSP code
• lt cs334.MyBean bean1 new cs334.MyBean() gt

11
Instances with Different Types

• ltjspuseBean idbean1 classcs334.MyBean
  typeObject /gt
• Creates a bean instance called bean1 of type
  cs334.MyBean and stores it in a variable of
  type Object
• Similar to the following JSP code
• lt Object bean1 new cs334.MyBean() gt

12
JavaBean Scope

• Page
• The bean will be bound to a local variable and
  placed in the PageContext object
• Request
• The bean will be bound to a local variable and
  placed in the ServletRequest object
• These beans will not last after the request is
  completed
• The difference between these 2 scopes is very
  small
• Beans such as this do not allow you to share data
  between servlets and JSPs

13
JavaBean Scope

• Application
• The bean will be bound to a local variable and
  placed in the ServletContext object
• ServletContext is shared by all servlets and JSPs
• Session
• The bean will be bound to a local variable and
  also stored in the HttpSession object
• These beans will last between requests
• Again, the differences here are mostly cosmetic

14
JavaBean Scope

• Page
• ltjspuseBean idb1 classMyBean scopepage
  /gt
• Request
• ltjspuseBean idb1 classMyBean
  scoperequest /gt
• Application
• ltjspuseBean idb1 classMyBean
  scopeapplication /gt
• Session
• ltjspuseBean idb1 classMyBean
  scopesession /gt
• Sound familiar? (they are scoped variables)

15
Reading Bean Properties

• ltjspgetProperty namebean1 propertyprop /gt
• Inserts the value of bean1s prop property into
  the web page
• Basically the same operation as
• lt bean1.getProp() gt

16
Setting Bean Properties

• ltjspsetProperty namebean1 propertyprop
  valueabc /gt
• Sets the value of bean1s prop property to
  abc
• Basically the same operation as
• lt bean1.setProp(abc) gt

17
Setting Bean Properties Using JSP Parameters

• ltjspsetProperty namebean1 propertyprop
  paramabc /gt
• Sets the value of bean1s prop property to the
  value of the JSP parameter named abc
• If the parameter does not exist, the value of the
  bean property does not change
• Similar to
• lt bean1.setProp(request.getParameter(abc)) gt
• Except that it checks if the parameter is null
  before setting the value

18
Setting Bean Properties Using JSP Parameters

• ltjspsetProperty namebean1 property /gt
• Sets the value of all bean1s properties to JSP
  parameters with the same name
• If the parameters do not exist, the value of the
  bean properties do not change

19
An Example JSP

• lthtmlgt
• ltbodygt
• ltjspuseBean idpyth classPythagorasBean
  /gt
• lth2gtThe hypoteneuse of the triangle islt/h2gt
• ltjspsetProperty namepyth propertywidth
  value4 /gt
• ltjspsetProperty namepyth propertyheight
  value5 /gt
• ltjspgetProperty namepyth propertyhypoteneu
  se /gt
• lt/bodygt
• lt/htmlgt

20
Demonstration

• simple_bean.jsp

21
JavaBean Number Converter

• For additional practice, I have created a
  JavaBean which converts integer numbers into
  their English equivalent (and vice versa)
• This bean is used as an example which allows us
  to remove most code from the JSP, and put it
  elsewhere

22
Taglibs

• Customized JSP Tags

23
Taglibs

• Offer no technological advantage over servlets,
  but do provide convenience
• Custom JSP tags associated with tag handler
  classes
• The tag handler classes do the work that the
  custom tags represent
• Present a convenient method for code
  modularization in JSPs

24
When would you use taglibs?

• The same reasons they created HTML and JSP tags
• Whenever you need to perform the same operation
  in several places
• Examples
• A tag that generates a view of the users
  shopping cart, including total price
• A tag that filters text (the body of the tag) for
  swearing, etc.

25
How do you do it?

• Define a class that generates HTML
• This class can access
• JSP/Servlet-related information
• E.g. Parameters, Session
• The text in the body of the tag
• Create a tag library descriptor file
• XML file containing a description of the JSP tags
  you create
• Use the tag library from within a JSP file

26
A Simple TagHandler Class

• public class RandomNumberTag extends TagSupport
• public int doStartTag()
• try
• JspWriter out pageContext.getOut()
• out.print(Math.random())
• catch (IOException e)
• e.printStackTrace()
• return SKIP_BODY // we dont use the tag body

27
Tag Handler Descriptor File

• lttaglibgt
• lttlib-versiongt1.0lt/tlib-versiongt
• ltjsp-versiongt1.1lt/jsp-versiongt
• ltshort-namegtcs334lt/short-namegt
• ltdescriptiongtA sample tag librarylt/descriptiongt
• lttaggt
• ltnamegtrandomlt/namegt
• lttag-classgtRandomNumberTaglt/tag-classgt
• ltbody-contentgtEMPTYlt/body-contentgt
• ltdescriptiongtPrints a random
  numberlt/descriptiongt
• lt/taggt
• lt/taglibgt

28
The JSP File

• lt_at_ taglib urics334_taglib.tld prefixcs334
  gt
• lthtmlgt
• lth2gtA random numberlt/h2gt
• ltcs334random /gt
• lth2gtAnother random numberlt/h2gt
• ltcs334random /gt
• lt/htmlgt

29
Demonstration

• simple_taglib.jsp

30
Tag Attributes

• If you want to define an attribute called max,
  just define a method like
• public void setMax(String value)
• Define this method in the tag handler class

31
Tag Attributes

• The JSP file will look something like
• ltcs334random max10gt
• The taglib definition file will look like
• lttaggt
• ltnamegtrandomlt/namegt
• ltattributegt
• ltnamegtmaxlt/namegt
• ltrequiredgtfalselt/requiredgt
• lt/attributegt

32
Demonstration

• attrib_taglib.jsp

33
Tag Body Text

• The HTML text inside the tag can be retrieved
  using the method
• String strBody getBodyContent().getString()
• Obviously, this is done in the tag handler class
• You can now manipulate this body text, and print
  it using the normal out.print() and out.println()
  calls

34
Example

• A custom tag that converts everything into
  uppercase
• You can see how this knowledge could be used to
  develop any filtering tag

35
Uppercase Tag Handler

• public class UppercaseTag extends BodyTagSupport
  
• public int doAfterBody()
• try
• BodyContent body getBodyContent()
• JspWriter out body.getEnclosingWriter()
• String strBody body.getString()
• out.print(strBody.toUpperCase())
• catch (IOException e)
• e.printStackTrace()
• return SKIP_BODY // dont print the tag body
  again

36
Uppercase THD

• lttaglibgt
• lttlib-versiongt1.0lt/tlib-versiongt
• ltjsp-versiongt1.1lt/jsp-versiongt
• ltshort-namegtcs334lt/short-namegt
• ltdescriptiongtA sample tag librarylt/descriptiongt
• lttaggt
• ltnamegtuppercaselt/namegt
• lttag-classgtUppercaseTaglt/tag-classgt
• ltbody-contentgtJSPlt/body-contentgt
• ltdescriptiongtConverts to upper
  caselt/descriptiongt
• lt/taggt
• lt/taglibgt

37
The JSP File

• lt_at_ taglib urics334_taglib.tld prefixcs334
  gt
• lthtmlgt
• lt/bodygt
• lth2gtHere is the testlt/h2gt
• ltcs334uppercasegt
• This text is both uppercAse and lowerCASE.
• lt/cs334uppercasegt
• lt/bodygt
• lt/htmlgt

38
Demonstration

• uppercase_taglib.jsp

39
JavaBean Example

• A Bean for Converting Numbers to/from English
  Written Notation

40
Taglib Examples

• An Internationalization Tag Library