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INTRODUCTION TO JAVA BEANS
Software components are self-contained software units developed according to the motto “Developed them once, run and reused them everywhere”. Or in other words, reusability is the main concern behind the component model. A software component is a reusable object that can be plugged into any target software application. You can develop software components using various programming languages, such as C, C++, Java, and Visual Basic. A “Bean” is a reusable software component model based on sun’s java bean specification that can be manipulated visually in a builder tool. The term software component model describe how to create and use reusable software components to build an application Builder tool is nothing but an application development tool which lets you both to create new beans or use existing beans to create an application. To enrich the software systems by adopting component technology JAVA came up with the concept called Java Beans. Java provides the facility of creating some user defined components by means of Bean programming. We create simple components using java beans. We can directly embed these beans into the software.
Advantages of Java Beans: The java beans posses the property of “Write once and run anywhere”. Beans can work in different local platforms. Beans have the capability of capturing the events sent by other objects and vice versa enabling object communication. The properties, events and methods of the bean can be controlled by the application developer.(ex. Add new properties) Beans can be configured with the help of auxiliary software during design time.(no hassle at runtime) The configuration setting can be made persistent.(reused) Configuration setting of a bean can be saved in persistent storage and restored later.
What can we do/create by using JavaBean: There is no restriction on the capability of a Bean. It may perform a simple function, such as checking the spelling of a document, or a complex function, such as forecasting the performance of a stock portfolio. A Bean may be visible to an end user. One example of this is a button on a graphical user interface. Software to generate a pie chart from a set of data points is an example of a Bean that can execute locally. Bean that provides real-time price information from a stock or commodities exchange.
Definition of a builder tool:
Builder tools allow a developer to work with JavaBeans in a convenient way. By examining a JavaBean by a process known as Introspection, a builder tool exposes the discovered features of the JavaBean for visual manipulation. A builder tool maintains a list of all JavaBeans available. It allows you to compose the Bean into applets, application, servlets and composite components (e.g. a JFrame), customize its behavior and appearance by modifying its properties and connect other components to the event of the Bean or vice versa.
Some Examples of Application Builder tools:
TOOL VENDOR DESCRIPTION
Java Workshop2.0 Sun MicroSystems., Inc., Complete IDE that support applet, application and bean development
Visual age for java IBM Bean Oriented visual development toolset.
Jbuilder Borland Inc. Suit of bean oriented java development tool
Beans Development Kit SunMicroSystems., Inc., Supports only Beans development
JavaBeans basic rules A JavaBean should: be public implement the Serializable interface have a no-arg constructor be derived from javax.swing.JComponent or java.awt.Component if it is visual
The classes and interfaces defined in the java.beans package enable you to create JavaBeans. The Java Bean components can exist in one of the following three phases of development:
- Construction phase
- Build phase
- Execution phase It supports the standard component architecture features of Properties Events Methods Persistence. In addition Java Beans provides support for Introspection (Allows Automatic Analysis of a java beans) Customization(To make it easy to configure a java beans component)
Elements of a JavaBean:
Properties Similar to instance variables. A bean property is a named attribute of a bean that can affect its behavior or appearance. Examples of bean properties include color, label, font, font size, and display size. Methods Same as normal Java methods. Every property should have accessor (get) and mutator (set) method. All Public methods can be identified by the introspection mechanism. There is no specific naming standard for these methods. Events Similar to Swing/AWT event handling.
Beans Development Kit Is a development environment to create, configure, and test JavaBeans. The features of BDK environment are:
- Provides a GUI to create, configure, and test JavaBeans.
- Enables you to modify JavaBean properties and link multiple JavaBeans in an application using BDK.
- Provides a set of sample JavaBeans.
- Enables you to associate pre-defined events with sample JavaBeans.
Identifying BDK Components
- Execute the run.bat file of BDK to start the BDK development environment.
- The components of BDK development environment are:
- ToolBox
- BeanBox
- Properties
- Method Tracer
ToolBox window : Lists the sample JavaBeans of BDK. The following figure shows the ToolBox window:
BeanBox window : Is a workspace for creating the layout of JavaBean application.
- The following figure shows the BeanBox window:
Properties window : Displays all the exposed properties of a JavaBean. You can modify JavaBean properties in the properties window. The following figure shows the Properties window:
Method Tracer window : Displays the debugging messages and method calls for a JavaBean application. The following figure shows the Method Tracer window:
Steps to Develop a User-Defined JavaBean:
- Create a directory for the new bean
- Create the java bean source file(s)
- Compile the source file(s)
- Create a manifest file
- Generate a JAR file
- Start BDK
- Load Jar file
- Test. 1. Create a directory for the new bean Create a directory/folder like C:\Beans 2. Create bean source file - MyBean.java import java.awt.*; public class MyBean extends Canvas { public MyBean() { setSize(70,50); setBackground(Color.green); } } 3. Compile the source file(s) C:\Beans >Javac MyBean.java
7. Load Jar file Go to Beanbox->File->Load jar. Here we have to select our created jar file when we click on ok, our bean(userdefined) MyBean appear in the ToolBox. 8. Test our created user defined bean Select the MyBean from the ToolBox when we select that bean one + simple appear then drag that Bean in to the Beanbox. If you want to apply events for that bean, now we apply the events for that Bean.
Introspection: Introspection can be defined as the technique of obtaining information about bean properties, events and methods. Basically introspection means analysis of bean capabilities. Introspection is the automatic process by which a builder tool finds out which properties, methods, and events a bean supports. Introspection describes how methods, properties, and events are discovered in the beans that you write. This process controls the publishing and discovery of bean operations and properties Without introspection, the JavaBeans technology could not operate.
BDK Introspection: Allows automatic analysis of a java beans component Enables a builder tool to analyze how a bean works. (Or) A mechanism that allows classes to publish the operations and properties they support and a mechanism to support the discovery of such mechanism. Introspection can be defined as the technique of obtaining information about bean properties, events and methods. Basically introspection means analysis of bean capabilities.
There are two ways in which the developer of a Bean can indicate which of its properties, events, and methods should be exposed by an builder tool. With the first method, simple naming conventions are used. These allow the introspection mechanisms to infer information about a Bean. In the second way, an additional class is provided that explicitly supplies this information.
Design patterns for JavaBean Properties:-
A property is a subset of a Bean’s state. A bean property is a named attribute of a bean that can affect its behavior or appearance. Examples of bean properties include color, label, font, font size, and display size. Properties are the private data members of the JavaBean classes. Properties are used to accept input from an end user in order to customize a JavaBean. Properties can retrieve and specify the values of various attributes, which determine the behavior of a JavaBean.
Types of JavaBeans Properties
- Simple properties
- Boolean properties
- Indexed properties
Simple Properties: Simple properties refer to the private variables of a JavaBean that can have only a single value. Simple properties are retrieved and specified using the get and set methods respectively. A read/write property has both of these methods to access its values. The get method used to read the value of the property .The set method that sets the value of the property.
The setXXX() and getXXX() methods are the heart of the java beans properties mechanism. This is also called getters and setters. These accessor methods are used to set the property.
The syntax of get method is: public return_type get() public T getN(); public void setN(T arg) N is the name of the property and T is its type
Ex: public double getDepth() { return depth; } Read only property has only a get method. The syntax of set method is: public void set(data_type value) Ex: public void setDepth(double d) { Depth=d; } Write only property has only a set method.
Boolean Properties :
A Boolean property is a property which is used to represent the values True or False.
Have either of the two values, TRUE or FALSE. It can identified by the following methods: Syntax: Let N be the name of the property and T be the type of the value then public boolean isN(); public void setN(boolean parameter); public Boolean getN();
- public boolean is()
- public boolean get()
First or second pattern can be used to retrieve the value of a Boolean.
- public void set(boolean value) For getting the values isN() and getN() methods are used and for setting the Boolean values setN() method is used.
Bound Properties : A bean that has a bound property generates an event when the property is changed. Bound Properties are the properties of a JavaBean that inform its listeners about changes in its values. Bound Properties are implemented using the PropertyChangeSupport class and its methods. Bound Properties are always registered with an external event listener. The event is of type PropertyChangeEvent and is sent to objects that previously egistered an interest in receiving such notifications bean with bound property - Event source Bean implementing listener -- event target
In order to provide this notification service a JavaBean needs to have the following two methods:
public void addPropertyChangeListener(PropertyChangeListener p) { changes.addPropertyChangeListener(p); } public void removePropertyChangeListener(PropertyChangeListener p) { changes.removePropertyChangeListener(p); }
PropertyChangeListener is an interface declared in the java.beans package. Observers which want to be notified of property changes have to implement this interface, which consists of only one method: public interface PropertyChangeListener extends EventListener { public void propertyChange(PropertyChangeEvent e ); }
Constrained Properties:
It generates an event when an attempt is made to change it value Constrained Properties are implemented using the PropertyChangeEvent class. The event is sent to objects that previously registered an interest in receiving an such notification Those other objects have the ability to veto the proposed change This allows a bean to operate differently according to the runtime environment
A bean property for which a change to the property results in validation by another bean. The other bean may reject the change if it is not appropriate. Constrained Properties are the properties that are protected from being changed by other JavaBeans. Constrained Properties are registered with an external event listener that has the ability to either accept or reject the change in the value of a constrained property. Constrained Properties can be retrieved using the get method. The prototype of the get method is: Syntax: public string get() Can be specified using the set method. The prototype of the set method is: Syntax :public string set(String str)throws PropertyVetoException
Design Patterns for Events:
Handling Events in JavaBeans: Enables Beans to communicate and connect together. Beans generate events and these events can be sent to other objects. Event means any activity that interrupts the current ongoing activity. Example: mouse clicks, pressing key…
User-defined JavaBeans interact with the help of user-defined events, which are also called custom events. You can use the Java event delegation model to handle these custom events. The components of the event delegation model are:
- Event Source : Generates the event and informs all the event listeners that are registered with it.
- Event Listener : Receives the notification, when an event source generates an event.
- Event Object : Represents the various types of events that can be generated by the event sources. Creating Custom Events: The classes and interfaces that you need to define to create the custom JavaBean events are:
- An event class to define a custom JavaBean event.
- An event listener interface for the custom JavaBean event.
- An event handler to process the custom JavaBean event.
- A target Java application that implements the custom event. Creating the Event Class: The event class that defines the custom event extends the EventObject class of the java.util package. For example, public class NumberEvent extends EventObject { public int number1,number2; public NumberEvent(Object o,int number1,int number2) { super(o); this.number1=number1; this.number2=number2; } } Beans can generate events and send them toother objects.
Creating Event Listeners
- When the event source triggers an event, it sends a notification to the event listener interface.
- The event listener interface implements the java.util.EventListener interface.
- Syntax:
- public void addTListener(TListener eventListener);
- public void addTListener(TListener eventListener)throws TooManyListeners;
- public void removeTListener(TListener eventListener);
- The target application that uses the custom event implements the custom listener. For example, public interface NumberEnteredListener extends EventListener { public void arithmeticPerformed(NumberEvent mec); }
Creating Event Handler Custom event handlers should define the following methods:
- addXXListener(): Registers listeners of a JavaBean event.
- fireXX(): Notifies the listeners of the occurrence of a JavaBean event.
- removeXXListener(): Removes a listener from the list of registered listeners of a JavaBean.
The Java Beans API:
The Java Beans functionality is provided by a set of classes and interfaces in the java.beans package. Table 25- 2 lists the interfaces in java.beans and provides a brief description of their functionality. Table 25-3 lists the classes in java.beans.
Set of classes and interfaces in the java.beans package
Package java.beans
Contains classes related to developing beans -- components based on the JavaBeansTM^ architecture
Interface Summary
AppletInitializer This interface is designed to work in collusion with java.beans.Beans.instantiate.
BeanInfo
A bean implementor who wishes to provide explicit information about their bean may provide a BeanInfo class that implements this BeanInfo interface and provides explicit information about the methods, properties, events, etc, of their bean.
Customizer A customizer class provides a complete custom GUI for customizing a target Java Bean.
DesignMode
This interface is intended to be implemented by, or delegated from, instances of java.beans.beancontext.BeanContext, in order to propagate to its nested hierarchy of java.beans.beancontext.BeanContextChild instances, the current "designTime" property.
ExceptionListener An ExceptionListener is notified of internal exceptions.
PropertyChangeListener A "PropertyChange" event gets fired whenever a bean changes a "bound" property.
PropertyEditor A PropertyEditor class provides support for GUIs that want to allow users to edit a property value of a given type.
VetoableChangeListener A VetoableChange event gets fired whenever a bean changes a "constrained" property.
Visibility Under some circumstances a bean may be run on servers where a GUI is not available.
Class Summary
BeanDescriptor A BeanDescriptor provides global information about a "bean", including its Java class, its displayName, etc.
Beans This class provides some general purpose beans control methods.
DefaultPersistenceDelegate
The DefaultPersistenceDelegate is a concrete implementation of the abstract PersistenceDelegate class and is the delegate used by default for classes about which no information is available.
Encoder An state of a collection of JavaBeans in terms of their public APIs.^ Encoder^ is a class which can be used to create files or streams that encode the
EventHandler
The EventHandler class provides support for dynamically generating event listeners whose methods execute a simple statement involving an incoming event object and a target object.
EventSetDescriptor An EventSetDescriptor describes a group of events that a given Java bean fires.
Expression
An Expression object represents a primitive expression in which a single method is applied to a target and a set of arguments to return a result - as in "a.getFoo()".
FeatureDescriptor The FeatureDescriptor class is the common baseclass for PropertyDescriptor, EventSetDescriptor, and MethodDescriptor, etc.
IndexedPropertyChangeEvent
An "IndexedPropertyChange" event gets delivered whenever a component that conforms to the JavaBeans specification (a "bean") changes a bound indexed property.
IndexedPropertyDescriptor An IndexedPropertyDescriptor describes a property that acts like an array and has an indexed read and/or indexed write method to access specific elements of the array.
Introspector The Introspector class provides a standard way for tools to learn about the properties, events, and methods supported by a target Java Bean.
MethodDescriptor A MethodDescriptor describes a particular method that a Java Bean supports for external access from other components.
ParameterDescriptor
The ParameterDescriptor class allows bean implementors to provide additional information on each of their parameters, beyond the low level type information provided by the java.lang.reflect.Method class.
PersistenceDelegate The PersistenceDelegate class takes the responsibility for expressing the state of an instance of a given class in terms of the methods in the class's public API.
PropertyChangeEvent A "PropertyChange" event gets delivered whenever a bean changes a "bound" or "constrained" property.
PropertyChangeListenerProxy A class which extends the named PropertyChangeListener^ EventListenerProxy.^ specifically for adding a
PropertyChangeSupport This is a utility class that can be used by beans that support bound properties.
PropertyDescriptor A PropertyDescriptor describes one property that a Java Bean exports via a pair of accessor methods.
PropertyEditorManager The PropertyEditorManager can be used to locate a property editor for any given type name.
PropertyEditorSupport This is a support class to help build property editors.
SimpleBeanInfo This is a support class to make it easier for people to provide BeanInfo classes.
Statement A applied to a target and a set of arguments - as in^ Statement^ object represents a primitive statement in which a single method is "a.setFoo(b)".
VetoableChangeListenerProxy A class which extends the^ EventListenerProxy^ specifically for associating a VetoableChangeListener with a "constrained" property.
VetoableChangeSupport This is a utility class that can be used by beans that support constrained properties.
XMLDecoder The XMLEncoder^ XMLDecoder and is used just like the^ class is used to read XML documents created using the ObjectInputStream.
XMLEncoder
The XMLEncoder class is a complementary alternative to the ObjectOutputStream and can used to generate a textual representation of a JavaBean in the same way that the ObjectOutputStream can be used to create binary representation of Serializable objects.
The BeanBox Menus
This section explains each item in the BeanBox File, Edit, and View menus. File Menu Save Saves the Beans in the BeanBox, including each Bean’s size, position, and internal state. The saved file can be loaded via File|Load. SerializeComponent... Saves the Beans in the BeanBox to a serialized ( .ser ) file. This file must be put in a .jar file to be useable. MakeApplet... Generates an applet from the BeanBox contents. Load... Loads Saved files into the BeanBox. Will not load .ser files. LoadJar... Loads a Jar file’s contents into the ToolBox. Print Prints the BeanBox contents. Clear Removes the BeanBox contents. Exit Quits the BeanBox without offering to save. Edit Menu Cut Removes the Bean selected in the BeanBox. The cut Bean is serialized, and can then be pasted. Copy Copies the Bean selected in the BeanBox. The copied Bean is serialized, and can then be pasted. Paste Drops the last cut or copied Bean into the BeanBox. Report... Generates an introspection report on the selected Bean. Events Lists the event−firing methods, grouped by interface. Bind property... Lists all the bound property methods of the selected Bean. View Menu Disable Design Mode Removes the ToolBox and the Properties sheet from the screen. Eliminates all 14 beanBox design and test behavior (selected Bean, etc.), and makes the BeanBox behave like an application. Hide Invisible Beans Hides invisible Beans.
