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JavaBeans and JSP. JavaBeans. The Java component technology originally intended for the creation and management of “pluggable” GUI components; Java’s answer to Visual Basic’s VBX/OCXs

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Presentation Transcript
javabeans
JavaBeans
  • The Java component technology
    • originally intended for the creation and management of “pluggable” GUI components; Java’s answer to Visual Basic’s VBX/OCXs
    • becoming more popular for encapsulating business logic for server-side applications (especially Java Server Pages
    • many Java GUI Development tools have been modified to allow application development of server-side applications (Visual Café, Jbuilder, VisualAge for Java) along with the development tools being delivered with application servers (SilverStream, BEA Weblogic)
  • A JavaBean is nothing more than a class that maintains some state data (called properties) and follows a certain set of coding conventions. Along with certain Java runtime support (reflection and introspection) JavaBeans can be easily added to and maintained by most of the Java GUI Development Tools .
  • A bean encapsulates data about one entity
api requires that
API requires that:
  • Must implement java.io.Serializable or java.io.Externalizable
    • A Java bean must be in a package
    • Beans must be able to support their own persistence
      • this allows the bean to be saved and restored consistently
  • provide a no-arguments constructor
    • provides a single way for the Bean to be instantiated
    • insures consistent bean creation and initialization
  • private properties must have corresponding get/set methods that follow the appropriate naming patterns
    • each piece of state data to be exposed is called a property
      • a property is eitherread-only (has a set),write-only (has a get)orread-write (has both)
      • a property is case-sensitive and starts with a lower-case letter
    • made public via accessor and mutators (gets and sets)
      • accessor method names must start with “get” & have no arguments
        • for property int color the accessor would be getColor() (note cap C)
      • mutator method names must start with “set” and return void
        • for property fuelCapacity the mutator would be setFuelCapacity()
jsp usebean tag
jsp:useBean Tag
  • jsp:useBean does the following:
    • If the object is found within the specified scope it is retrieved and assigned to the object
    • if not found it is instantiated
    • if newly instantiated it executes the code specified in the body ( one or more jsp:setProperty tags or a scriptlet)
    • if newly instantiated it is saved into the scope via setAttribute( ) method
  • jsp:useBean also makes the bean visible to the JSP; there may be other objects in the context that were put there by other JSPs or servlets; jsp:useBean can make them visible to the current JSP
jsp usebean tag more
jsp:useBean Tag (more)
  • Attributes :
      • id
      • scope
      • class
      • beanName
      • type
    • <jsp:useBean id = today class = “java.util.Date”>
      • instantiates a bean called today of class java.util.Date( )
    • <jsp:useBean id = “count” class = “java.lang.Integer” type = “java.lang.Number”>
      • essentially does : Number = count ; count = new Integer( )
    • <jsp:useBean id = “count” class = “<%= request.getParameter(“beanName”)%> type = “Number” />
      • essentiallt does: Number count; count = java.beans.Beans.instantiate(request.getParameter(“beanName”));
jsp usebean tag more1
jsp:useBean Tag (more)

Scope of a Java Bean- Four Scopes Available

  • Page
    • available only within the JSP page and is destroyed when the page has finished generating its output for the request
  • Request
    • destroyed when the response is sent
  • Session
    • destroyed when the session is destroyed
  • Application
    • destroyed when the web application is destroyed.
jsp usebean tag more2
jsp:useBean Tag (more)
  • In general the syntax is:

<jsp:useBean id = “name” {scope = “page | request | session | application”}

{

}

/>

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