Persistence

1
Persistence Bean Properties

• Properties in Beans
• Accessors Mutators
• Indexed Bound properties
• Constrained properties
• Examples

• Persistence in JavaBeans
• Java Object Serialization
• The java.io.Serializable Interface
• Versioning
• Validating Objects
• The java.io.Externalizable Interface

2
What is a Property?

• Named attributes / characteristics that define
  the behavior state of the object (bean)
• How is it different from a variable?

Bean
Bean
Properties
Bean actions tied to properties
3
Design Patterns (coding convention)

• Why?
• Accessors
• function to get value of a variable, no params.
• ltproperty typegt ltproperty namegt
• ltproperty typegt getltproperty namegt( )
• Example
• public class Temperature
• protected double currentTemperature 25
• public double getCurrentTemperature()
• return currentTemperature
• public boolean isltPropertyNamegt( )

4
Mutators

• Functions that change the value of a property
• Takes parameters
• Design pattern
• public void setltproperty namegt(ltproperty
  typegt value)
• Example
• public class Temperature
• protected double currentTemperature 25
• public double setCurrentTemperature(double
  cTemp)
• currentTemperature cTemp

5
Indexed Bound properties

• Design pattern indexed property
• public ltPropertyTypegt getltPropertyNamegt( )
• public void setltPropertyNamegt (
  ltPropertyTypegt value )
• public ltPropertyTypegt getltPropertyNamegt (int
  index)
• public void setltPropertyNamegt (int index,
  ltPropertyTypegt value)
• (Hint java.lang.ArrayIndexOutOfBoundsException
  should be thrown !)
• Bound properties support change notifications
• PropertyChangeListener PropertyChangeEvent

6
Bound properties
Notifying a listener about a change in property
addPropertyChangeListener removePropertyChangeList
ener
Bean 1
Property Owner
(producer)
Property 1 Changed
Property 2 Changed
Property n Changed

PropertyChangeEvent
PropertyChangeEvent
PropertyChangeEvent
propertyChange( )
PropertyChangeListener
Bean 2
(consumer)
7
java.beans.PropertyChangeEvent

• Constructor
• public PropertyChangeEvent (Object sourceBean,
  String propertyName, Object oldValue, Object
  newValue )
• Methods
• public Object getNewValue ( )
• public Object getOldValue ( )
• public String getPropertyName( )
• public Object getPropagationId ( )
• public Object setPropagationId ( Object newId )

8
java.beans.PropertyChangeSupport utility class
Listener
Bean
Properties
Listener
Property Support
Listener

• public PropertyChangeSupport ( Object sourceBean
  )
• public synchronized void addPropertyChangeListener
  (PropertyChangeListener newListener )
• public synchronized void addPropertyChangeListener
  (PropertyChangeListener existingListener )
• public void firePropertyChange ( String
  propertyName, Object oldValue, Object newValue )

9
Constrained Properties
Change rejected
setProperty( )
PropertyVetoException
vetoableChange( )
Change rejected
VetoableChangeListeners

• VetoableChangeListener
• Public abstract void vetoableChange(
  PropertyChangeEvent vetoablePropertyEvent )
• throws PropertyVetoException
• PropertyVetoException
• public PropertyVetoException (String
  reasonForVeto, PropertyChangeEvent eventVetoed )
• VetoableChangeSupport

10
Persistence!
Save
Restore

• Object Serialization java.io package
• java.io.FileInputStream java.io.FileOutputStream
  
• Java.io.ObjectInputStream java.io.ObjectOutputSt
  ream

Object Stream source and target may vary
11
An example

• //save a String and double to the stream
• String str Sample
• double d 3.14
• FileOutputStream fos new FileOutputStream
  (Beans.ser)
• ObjectOutputStream oos new ObjectOutputStream
  (fos)
• oos.writeObject (str)
• oos.writeObject (d)
• oos.flush()
• // restore the String and double
• FileInputStream fis new
  FileInputStream (Beans.ser)
• ObjectInputStream ois new
  ObjectInputStream (fis)
• String str (String) ois.readObject()
• double d s.readDouble()

12
java.io.Serializable interface

• No methods
• Just a marker, indicating class is serializable
• Object / Class tree traversed to serialize
• java.io.NotSerializableException thrown, if
  unserializable member found
• Static and Transient members not serialized
• import java.io. Saver.java
• import java.awt.
• public class Saver
• Saver()
• Button b new Button("Bean Button")
• b.setFont(new Font("System", Font.BOLD, 20))
• try
• FileOutputStream f new FileOutputStream
  ("Saver.ser")
• ObjectOutputStream o new ObjectOutputStream
  (f)
• o.writeObject(b)
• o.flush()
• catch (Exception e) System.out.println(e)

13
Reader.java
//ltapplet code"Reader" height100
width200gtlt/appletgt import java.applet. import
java.io. import java.awt. public class
Reader extends Applet public void init()
try FileInputStream f new
FileInputStream ("Saver.ser") ObjectInputStream
o new ObjectInputStream (f) Button b
(Button)o.readObject() add(b)
catch (Exception e) System.out.println(e)

14
Versioning

• inevitable tendency for an object to evolve over
  time and gain new functionality
• New version (vs.) old version conflict
• Compatible changes
• Do not render serialized data unusable by older
  class
• ? Adding new methods
• ? Changing the base class
• java.io.InvalidClassException thrown
• serialver ltclass namegt - generates a SUID
  (Stream Unique ID )
• E.g., gt serialver Reader
• Reader static final long serialVersionUID
  463333450398306235L

15
java.io.Externalizable interface

• How is it different from Serializable interface?
• writeExternal ( ) and readExternal ( )
• Class hierarchy not traversed during
  serialization
• Versioning responsibility of the object
• Instantiating Serialized Objects
• the instantiate( ) method in java.beans.Beans
  (static)
• 2 parameters class loader, beans fully
  qualified name
• If 1st parameter null -gt uses default class
  loader
• Instantiate() looks for a pickled bean, if
  not found uses new to create an instance
• Requires that the bean have constructor with no
  params.

16
References

• Developing JavaBeans Robert Englander
• ( OReilly publications )
• JavaBeans Developers Resource Prashant
  Sridharan ( Prentice Hall )
• Presenting JavaBeans Michael Morrison
• (MCP , Sams publishing )
• JavaBeans Developer's Guide Piroz Mohseni
• - ( IDG Books worldwide )
• http//java.sun.com/products/javabeans
• Questions ?!!