Java Beans

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Java Beans

• RickardEnander_at_parallelconsulting.com

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Who is Rickard Enander?

• Uppsala Universitet 97
• Ph.D in something unimportant
• IBM 97-98
• methodology, OO, et.c.
• Parallel Consulting Group 98 -gt
• serious internet application
• Participated in courses like this one at KTH

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Whats demanded from you today?

• Some knowledge about Java
• Interest for components
• Patience with the demo daemons

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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ArchitecturesClient/Server, 3-tier and N-tier

• The evolution for architecture is
• Client/server (2-tier)
• 3-tier
• N-tier
• Why?
• A changing world...

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Component model mostly resides in the middle tier

• Component and component model?
• Many different definitions
• Different goals
• JavaBeans vs. CORBA
• Different with respect to OO-fundamentals
• CORBA vs. COM/DCOM
• Binaries or not
• COM vs JavaBeans

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Different definitions of the term component
A component is a piece of software small enough
to create and maintain, big enough to deploy and
support and with standard interfaces for
interoperability (Jed Harris)
A non-trivial, nearly independent, and
replaceable part of a system that fulfills a
clear function in the context of a well-defined
architecture. A component conforms to and
provides the physical realization of a set of
interfaces. RUP
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Component models today is very religious

• Statefull vs. stateless components
• should a component have state or not
• Microsoft vs. Java
• depends on who is talking
• think for yourselves
• dont talk before you know what your customer
  prays to

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Modern component models often have a clear
programming model

• Programming model
• how we shall/should do
• different roles developing / distributing
• Without any strong programming model
• CORBA, COM/DCOM
• With a strong programming model
• JavaBeans but most EJB

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Components in the daily work

• Component based development
• Build apps from ready and tested things
• Small amount of new code lines
• Make connections graphically
• Pros
• Spread of investment
• Third party
• Better focus in own development

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Components in the daily work...

• Properties of a good component
• Is a well defined operation or entity
• Is general enough to be reused
• Configurable
• Is able to present its own interface
• Possible to assemble larger components
• Self containing ? possible to distribute

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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Goals of the JavaBeans model

• A Bean in JavaBeans is a reusable software
  component that can be manipulated visually in a
  builder tool.
• Should be attractive to reuse
• Object oriented development
• Tools at assembly time should not influence run
  time performance

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Java Beans is really a two class pattern
myClass
myClassBeanInfo
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JavaBeans is supported by some Java language
features

• Name convention
• Introspection / Refelction
• Event model
• Inner classes
• NLS - National Language Support
• Serializing

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The naming convention is simple must be followed

• Name convention
• Introspection / reflection
• Event model
• Inner classes
• NLS - National Language Support
• Serializing

• Setters and getters
• Events

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Introspection means that a tool can figure out
a JavaBean

• Name convention
• Introspection / reflection
• Event model
• Inner classes
• NLS - National Language Support
• Serializing

• Is built on the naming convention
• Is supported by Java Reflection

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The event model makes JBs talk without previous
knowledge

• Name convention
• Introspection / reflection
• Event model
• Inner classes
• NLS - National Language Support
• Serializing

• Built on observer pattern
• The most complicated part
• Inner classes is a language construction used to
  support Events

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NLS is not JavaBean specific but needed for the
web

• Name convention
• Introspection / reflection
• Event model
• Inner classes
• NLS - National Language Support
• Serializing

• Many language problem
• Not easy but easier than with other techniques
• We will not handle this explicitly

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Serializing makes JavaBeans possible to persist
and transport

• Name convention
• Introspection / reflection
• Event model
• Inner classes
• NLS - National Language Support
• Serializing

• Was defined to support JB
• Also used in other situations
• Elegant
• Slow

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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Ways of working

• Configure and combine Beans to an application or
  to a more complicated beans with a graphical tool
• Save the assembled Bean
• identity
• initial state
• behaviour
• Distribute the Bean

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Three roles can be identified

• Developer of JavaBeans
• needs deep knowledge
• Toolmaker (and tool)
• needs deeper knowledge
• End user (bean assembler)
• needs to be able to draw straight lines -)

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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The interface of a Bean

• The interface is defined by
• Properities
• Events
• Methods

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The interface of a Bean ...

• How do you get the interface?

Introspection
BeanInfo
The BeanInfo-class is fetched
Discover by reflection
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The interface of a Bean ...

• BeanInfo
• getAdditionalBeanInfo()
• getBeanDescriptor()
• getDefaultEventIndex()
• getDefaultPropertyIndex()
• getEventSetDescriptors()
• getIcon(int)
• getMethodDescriptors()
• getPropertyDescriptors()

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Creation of BeanInfo

• The Introspector class creates BeanInfo
• BeanInfo class from developer
• Reflection
• Combination
• Reflection Class java.lang.Class
• Can get iformation about everything
• Can create instances

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Rules for a JavaBean

• A normal Java class
• Can be instanciated
• Has a constructor without arguments
• Implements java.io.Serializable
• Methods follows the naming conventions
• Eventually a BeanInfo-class

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Naming conventions

• Properties defined by name
• lttypegt x
• lttypegt getX() //Read
• void setX(lttypegt value) //Write
• isX() //If Boolean
• Example property Temperature
• int temperature
• int getTemperature()
• void setTemperature( int value )

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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Different types of properties

• Normal
• Indexed
• Bound
• Constrained

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Different types of properties Indexed

• A number of values indexed with an int
• lttypegt getX( int index )
• void setX( int index, lttypegt value )
• Implemented e.g. with an array

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Different types of properties Bound

• When the property value is changed all registered
  listeners are notified
• java.beans.PropertyChangeEvent
• Listeners registered with
• bean.addPropertyChangeListener(listener)

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Different types of properties Constrained

• When a property value is going to be changed the
  change must be OKed
• exception thrown for veto
• silent accept
• Property methods thows exception
• lttypegt getX()
• void setX( lttypgt value ) thows java.beans.Property
  VetoException

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Change of properties in tool

• Tool editors
• Simple editorer, for simple data types
• Inherits from
• PropertyEditor - interface
• PropertyEditorSupport - class
• Coupling between property an special editor is
  done in BeanInfo class
• Complex editor, Cutomizer

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Change of properties in tool Customizer

• Class supporting the Interface java.bean.Customize
  r
• More complex customization
• Implemented by graphical tool used for
  customization
• Coupling between property an special editor is
  done in BeanInfo class

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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Events

• Delegation model

Source
Listener
Eventobject
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Event object

• Contains data about the event
• Source
• Other specific data
• Named to xxxEvent
• Example
• KeyEvent
• MouseEvent

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Event source

• If the source can generate the event xxx the
  following methods are present
• addxxxListener
• removexxxListener
• manages list of listeners
• Notifies the listener when xxx happens

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Event Listener

• Support the xxxListener interface
• must contain at least one methos
• may contain many methods
• Listeners can use adaptors
• An adaptor is an abstarct class supporting a
  listener interface
• the logical listener has a inner class inheriting
  from the adaptor class

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Example of the use of an inner calss and an
adaptor
import java.awt.event. public class MyPanel
extends Canvas public MyPanel()
addMouseListener( new MyMouseListener() )
class MyMouseListener extends MouseAdapter
public void mouseClicked(MouseEvent e)
// Take care of mouse click here // or call
other method
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Event Class Diagram
xxxSource
yyySource
xxxAdapter
yyyAdapter
xxxAdapter- Child
yyyAdapter- Child
MyObectMultipleListener
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Example of classes for MouseEvent

• MouseEvent
• MouseListener
• MouseAdapter
• Source must implement
• addMouseEventListener(MouseListener m)
• removeMouseEventListener(MouseListener m)

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To think about when using Events

• You can define your own events
• extends AWTEvent
• tools will help you
• Events is executed from the source thread
• the listener should be synchronizerad
• How to handle exceptions
• the source can not handle then

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Agenda

• Architecture, component models
• Goals with the JavaBeans model
• Ways of working
• The interface of a Bean
• Properties
• Event model
• Something more about Java facilities

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Serializing

• To make an object to a bit stream
• Is automatically done
• class A implements Serializable
• If the standard behaviour is not enough, override
  the methods
• writeObject
• readObject

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Core Reflection, very short

• Uses the class Class for inspection
• Class xClass Class.forName(X)
• Mehod xMethods xClass.getMethods()
• Can also create instances
• X xInstance xClass.newInstance()

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Internationalization (NLS)

• Pattern for NLS
• Uses the classes
• Locale
• ResourceBoundle
• also on file
• Locally in all objects
• X.asString()
• X.format( Locale ) or X.format()