Introduction to Java Beans

From Anders Børjesson

Introduction to JavaBeans 2

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 bean, entire applications

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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

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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• Like 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()

• Properties of type boolean has special naming conventions

– setXx, isXx

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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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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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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

+addPropertyChangeListener(l : PropertyChangeListener)+removePropertyChangeListener(l : PropertyChangeListener)

#pcs : PropertyChangeSupport

«Interface»PropertyChangeListener

+propertyChange(evt : PropertyChangeEvent)0..n

0..n

MyPropertyChangeListener

+propertyChange(evt : PropertyChangeEvent)

c

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Sequence diagram: Firing a property change event

setBalance

firePropertyChange

:Account :PropertyChangeSupport :PropertyChangeListener

propertyChange

:PropertyChangeListener

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• 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()

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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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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

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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.

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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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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

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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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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

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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 JavaBeansand 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