Download Slides

Introduction to Java Beans
From 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
Introduction to JavaBeans
2
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 bean, entire applications
Introduction to JavaBeans
3
Requirements for JavaBeans
classes
• 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 not be public
– Data should be accesses via get and set methods
• Should be thread safe
– Use synchronized carefully
Introduction to JavaBeans
4
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
Introduction to JavaBeans
5
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
• Like a property with a new value
Introduction to JavaBeans
6
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
Introduction to JavaBeans
7
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
Introduction to JavaBeans
8
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
Introduction to JavaBeans
9
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()
• Properties of type boolean has special naming
conventions
– setXx, isXx
Introduction to JavaBeans
10
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.
Introduction to JavaBeans
11
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
Introduction to JavaBeans
12
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
Introduction to JavaBeans
13
Examples
• AccountBound.java
• ScreenLogListener.java
• AccountBoundApp.java
Introduction to JavaBeans
14
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)
Introduction to JavaBeans
15
Sequence diagram: Firing a
property change event
:Account
:PropertyChangeSupport
:PropertyChangeListener
:PropertyChangeListener
setBalance
firePropertyChange
propertyChange
propertyChange
Introduction to JavaBeans
16
Sequence diagram: Adding a
property change listener to a bean
:PropertyChangeListener
:Account
:PropertyChangeSupport
addPropertyChangeListener(this)
addPropertyChangeListener(...)
Introduction to JavaBeans
17
Classes and interfaces
• Package java.beans
– Class PropertyChangeEvent
• extends EventObject
– Interface PropetyChangeListener
• Extends EventListener
– Class PropertyChangeSupport
• Helps you to make bound properties
Introduction to JavaBeans
18
PropertyChangeListener
• Interface PropertyChangeListener extends
EventListener
– EventListener is a marker interface
• No methods to implement
• PropertyChangeListener methods
– void propertyChange(PropertyChangeEvent evt)
Introduction to JavaBeans
19
PropertyChangeEvent
• After 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()
Introduction to JavaBeans
20
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)
Introduction to JavaBeans
21
Event loops
• Problem
– 2 beans send propertyChangeEvents to each other
• Example: User interface objects events a model object, 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
Introduction to JavaBeans
22
Example bean: JButton
• Javax.swing.JButton is a bean
– Does not 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.
Introduction to JavaBeans
23
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
Introduction to JavaBeans
24
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.
– Example: AccountAdminFrame.java
Introduction to JavaBeans
25
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)
Introduction to JavaBeans
26
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
Introduction to JavaBeans
27
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:
beanbank/introspection/IntrospectionApp.java
Introduction to JavaBeans
28
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="... " />
Introduction to JavaBeans
29
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
Introduction to JavaBeans
30
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.]
Introduction to JavaBeans
31
References
• Niemeyer &
Knudsen Learning
Java, 3rd edition,
O’Reilly 2005
– 22. JavaBeans, page
751-786
Introduction to JavaBeans
32