Download Introduction to Java Beans

Introduction to Java Beans
by Anders Børjesson
JavaBeans components
• JavaBeans are the components in the
Java environment
– COM / ActiveX are components in the
Microsoft environment
• Many classes in J2SE are JavaBeans
– Like the Java Swing classes
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Characteristics
• Visibility
– Some JavaBeans are visual
• Buttons, TextFields, etc.
– Some JavaBeans are “invisible”
• Model components (worker classes)
• Size (granularity)
– Some JavaBeans are small (in terms of features)
• Buttons, TextFields, etc.
– Some JavaBeans are big (in terms of features)
• SMTP beans, entire applications
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Requirements for JavaBeans
• Must have a no-arg (no parameters) constructor
– More constructors are allowed
– Remember: If you add a constructor with parameters
then the default no-arg constructor is gone.
• Must implements java.io.Serializable
• Data should no be public
– Data should be accesses via get and set methods
• Should be thread safe
– Use synchronized carefully
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Non-requirements for JavaBeans
• An applet must extend java.lang.Applet
• A servlet must extend javax.servlet.Servlet
• A thread must implement
java.lang.Runnable
• Beans does not need to extend or
implement anything special
– Except for java.io.Serializable
• Which is only a marker interface
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JavaBeans terms
• Properties
– To be read or set by other objects
• Get and set methods
• Methods
– To be called by other objects
• Ordinary Java methods
• Events
– Sent to other objects when something happens to the
bean object
• A property with a new value
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Access levels
• 3 access levels
– Read-only
• Make only a get method
– Write-only
• Make only a set method
– Quite rare
– Read / write
• Make get and set methods
• Most common
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Different kinds of properties
• Different kinds of properties
– Simple or indexed (array) properties
– Bound properties
• Event after a new value is set on the property
• To update other components (e.g. visual
components)
– Constrained properties
• Event before a new value is set on the property
• To check if the new value is legal according to the
current business rules
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Simple properties
• Represents a single value
• A simple property has
– 1 name
– 1 type
• Primitive data type
or
• Class based data type
• Example: size
–
–
–
–
Name: size
Type: int
Public void setSize(int size)
Public int getSize()
AccountSimple.java
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Example bean: JButton
• Some properties of javax.swing.JButton
– Text: setText(String), getText()
– Font: setFont(Font), getFont()
– Icon: setIcon(Icon), getIcon()
– Enabled: setEnabled(boolean), isEnabled()
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Exercise: Car bean
• You are supposed to program parts of a
system for car registration
– Program a Car bean
• Properties: Registration number, color, etc.
• Methods: toString, equals
• Constructors: No-arg constructor + maybe other
constructors
– Make a JUnit test to test the Car bean
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Indexed properties
• Represents an array of values
• Example: Property: address Type: Address
– Public void setAddress(int index, Address addr)
– Public Address getAddress(int index)
– Bulk operations:
• Public void setAddresses(Address[] addr)
• Public Address[] getAddresses()
– AccountIndexedOwner.java
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Programming JavaBeans
• JavaBeans can be programmed using
– Ordinary text editors
• TextPad, even Notepad
– Builder tools
• NetBeans, Eclipse, JBuilder, etc.
• Builder tools often have special features that
makes programming JavaBeans easier.
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Exercise: Owner bean
• Additional classes for the car registration
system
– Program a (car) owner bean
• Properties: Name, address, etc.
• Indexed property: Cars
– One owner may have several cars
– Make a JUnit test to test the Owner and Car
beans.
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Bound properties
• Sends an event to another object after the
property is assigned a new value.
– The other object is sometimes called:
Observer, listener, etc.
• The other object may react on the event
– Update its own state
• Content of an event
– propertyName, oldValue, newValue, source
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Examples
• AccountBound.java
• ScreenLogListener.java
• AccountBoundApp.java
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Observer pattern
Account
0..n
#pcs : PropertyChangeSupport
+addPropertyChangeListener(l : PropertyChangeListener)
+removePropertyChangeListener(l : PropertyChangeListener)
«Interface»
PropertyChangeListener
0..n
+propertyChange(evt : PropertyChangeEvent)
c
MyPropertyChangeListener
+propertyChange(evt : PropertyChangeEvent)
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Sequence diagram: Firing a
property change event
:Account
:PropertyChangeSupport
:PropertyChangeListener
:PropertyChangeListener
setBalance
firePropertyChange
propertyChange
propertyChange
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Sequence diagram: Adding a
property change listener to a bean
:PropertyChangeListener
:Account
:PropertyChangeSupport
addPropertyChangeListener(this)
addPropertyChangeListener(...)
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Classes and interfaces
• Package java.beans
– Class PropertyChangeEvent
• extends EventObject
– Interface PropetyChangeListener
• Extends EventListener
– Class PropertyChangeSupport
• Helps you to make bound properties
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PropertyChangeListener
• Interface PropertyChangeListener extends
EventListener
– EventListener is a marker interface
• No methods to implement
• PropertyChangeListener methods
– void propertyChange(PropertyChangeEvent evt)
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PropertyChangeEvent
• Af changing the value of a property
– firePropertyChange(propertyChangeName, oldValue,
newValue)
• Listener called
– propertyChange(PropertyChangeEvent ev)
• PropertyChangeEvent methods:
–
–
–
–
String getPropertyName()
Object getOldValue()
Object getNewValue()
Object getSource()
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PropertyChangeSupport
• Helps you to make bound properties
• Constructor
– PropertyChangeSupport(Object sourceBean)
• Some methods
– addPropertyChangeListener(PropertyChangeListener listener)
– removePropertyChangeListener(PropertyChangeListener listener)
– addPropertyChangeListener(String propertyName,
PropertyChangeListener listener)
– removePropertyChangeListener(String propertyName,
PropertyChangeListener listener)
– firePropertyChange(String propertyName, Object oldValue,
Object newValue)
– firePropertyChange(String propertyName, int oldValue,
int newValue)
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Exercises: Car bean with bound
properties
• Program a car bean with bound properties
– You may either change your “old” car bean or make a
new car bean.
• Program a PropertyChangeListener
– LogListener implements PropertyChangeListener
– Every time a property changes the listener writes to
the screen the data in the propertyChangeEvent
• Source, property name, old value, new value.
• Make a JUnit test to test the bound properties.
• Extra (if you have time): Make a more general
log listener that writes to a general stream.
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Event loops
• Problem
– 2 beans sends propertyChangeEvents to each other
• Example: User interface objects events a model objec, and
vice versa
– The 2 beans might event each other forever.
• Solution
– Check if the new value is really a new value
• i.e. newValue != oldValue
• PropertyChangeSupport.firePropertyChange() does the
checking
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Example bean: JButton
• Javax.swing.JButton is a bean
– Doesn’t use ProperChangeEvent, etc.
– addActionListener(ActionListener l)
• Listens for button presses
– addChangeListener(ChangeListener l)
• Listens for changes in the button’s state
– Size, font, text, etc.
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Constrained properties
• A bean object asks one (or more) observers if an
update of a property is legal
– Observer may throw PropertyVetoException
– Example: account.setBalance(-100)
– A property is often bound and constrained
• Examples
–
–
–
–
–
AccountConstrained.java
MinimumBalanceChecker.java
MaximumWithdrawChecker.java
AccountConstrainedTest.java
AccountFrame.java + AccountGuiApp.java
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Exercise: Car with constrained
properties
• Program a car bean with bound properties
– You may either change your “old” car bean or make a new car bean.
• Program a few VetoableChangeListeners
– ColorChecker
• Checks if a color is illegal
– Pink, orange, etc. are considered illegal colors of a car
– RegistrationNumberChecker
• Checks if the registration number is legal
– A proper registration number must have the structure AB12345, i.e. 2 letter
followed by 5 digits.
• Make a JUnit test to test the Car bean with the new listeners.
• Extra (if you have time)
– Program a configurable ColorChecker as a bean +
VetoableChangeListener
• Illegal colors is an indexed property
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Dynamic vs. static characteristics
on properties
• Static characteristic
– Follows the object for its life time.
• Dynamic characteristic
– Changes over time during the objects life time
– Characteristics may be (de)aggregated using
constrained properties.
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Low coupling
• The event mechanism in JavaBeans
provides low coupling
– The bean object doesn’t know much about the
listeners
• Except that listeners must implement
PropertyChangeListener or PropertyVetoListener
– The listeners doesn’t know much about the
bean
• It’s just an Object
– Which we sometimes have to typecast to the Bean class
(which is very bad)
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Using JavaBeans in builder tools
• JavaBeans can be used in a builder tool
like
– NetBeans
– Eclipse
– Borland JBuilder
– etc.
• The bean must be “installed” in the builder
– The builder usually helps you to during
installation
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Packing JavaBeans in JAR files
• JAR = Java Archive
– The standard package format in Java
– JAR = ZIP file with a manifest
– A JAR file may contain
• Class files
• Icons, etc.
• Manifest file describing the contents of the JAR file
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Introspection
• The builder tool unzip’s the JAR file to get the
class files.
• The builder tool “learns” about the beans’
properties, methods, and events using the
introspection API
– Package java.beans
•
•
•
•
•
Class Introspector
Interface BeanInfo
Interface PropertyDescriptor
Interface MethodDescriptor
Interface EventSetDescriptor
– Example: IntrospectionApp.java
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Using JavaBeans
• JavaBeans can be used as ordinary classes
in
– Java applications, applets, servlets
– JSP (JavaServer Pages)
• special tags to manipulated JavaBeans
– <jsp:useBean id="cart" class="shop.cart.ShoppingCart"
scope="session" />
– <jsp:getProperty name="cart" property="numberOfItems" />
– <jsp:setProperty name="cart" property="... " value="... " />
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JavaBeans vs. Enterprise
JavaBeans
• JavaBeans
– Are relatively simple
– Part of Java Standard Edition (J2SE)
– Can be used in an ordinary Java application / applet
• Enterprise JavaBeans
–
–
–
–
More advanced
Part of Java Enterprise Edition (J2EE)
Middle tier: Between (web)application and database.
Can only be used in an enterprise Java application
• Requires an J2EE server to run the application
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Job types using JavaBeans
and other types of components
• Designing components
• Programming components
• Documenting components
– Technical writer
•
•
•
•
Design using components
Programming using components
Marketing / selling components
Testing components
– Correctness, performance, security [Trojan horses,
etc.]
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References
• Niemeyer &
Knudsen Learning
Java, 3rd edition,
O’Reilly 2005
– 22. JavaBeans, page
751-786
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