Notes on JavaBeans

JavaBeans Concepts Using the NetBeans GUI Builder Writing a Simple Bean Properties Simple Properties Bound Properties Constrained Properties Indexed Properties Manipulating Events Bean Persistence Long Term Persistence Introspection Bean Customization Using the BeanContext API Overview of the BeanContext API Bean Context #1: Containment Only Bean Context #2: Containment and Services AWT Containers and the BeanContextProxy Interface

The JavaBeans™ architecture is based on a component model which enables developers to create software units called components. Components are self-contained, reusable software units that can be visually assembled into composite components, applets, applications, and servlets using visual application builder tools. JavaBean components are known as beans. A set of APIs describes a component model for a particular language. The JavaBeans API specificationdescribes the core detailed elaboration for the JavaBeans component architecture. Beans are dynamic in that they can be changed or customized. Through the design mode of a builder tool you can use the Properties window of the bean to customize the bean and then save (persist) your beans using visual manipulation. You can select a bean from the toolbox, drop it into a form, modify its appearance and behavior, define its interaction with other beans, and combine it and other beans into an applet, application, or a new bean. The following list briefly describes key bean concepts. •

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Builder tools discover a bean's features (that is, its properties, methods, and events) by a process known as introspection. Beans support introspection in two ways: o By adhering to specific rules, known as design patterns, when naming bean features. The Introspector class examines beans for these design patterns to discover bean features. The Introspector class relies on the core reflection API. The trail The Reflection API is an excellent place to learn about reflection. o By explicitly providing property, method, and event information with a related bean information class. A bean information class implements the BeanInfo interface. A BeanInfo class explicitly lists those bean features that are to be exposed to application builder tools. Properties are the appearance and behavior characteristics of a bean that can be changed at design time. Builder tools introspect on a bean to discover its properties and expose those properties for manipulation. Beans expose properties so they can be customized at design time. Customization is supported in two ways: by using property editors, or by using more sophisticated bean customizers. Beans use events to communicate with other beans. A bean that is to receive events (a listener bean) registers with the bean that fires the event (a source bean). Builder tools can examine a bean and determine which events that bean can fire (send) and which it can handle (receive). Persistence enables beans to save and restore their state. After changing a bean's properties, you can save the state of the bean and restore that bean at a later time

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with the property changes intact. The JavaBeans architecture uses Java Object Serialization to support persistence. A bean's methods are no different from Java methods, and can be called from other beans or a scripting environment. By default all public methods are exported.

Beans vary in functionality and purpose. You have probably met some of the following beans in your programming practice: • • • •

GUI (graphical user interface) Non-visual beans, such as a spelling checker Animation applet Spreadsheet application

Lesson: Using the NetBeans GUI Builder his lesson explains how to use the NetBeans IDE GUI Builder to work with beans. In preparation for working with the GUI Builder, you should be first familiar with the key NetBeans concepts which are explained in the NetBeans IDE Java Quick Start Tutorial. This lesson guides you through the process of creating a bean pattern in the NetBeans projects, introduces the user interface of the GUI Builder, and explains how to add your bean object to the palette.

Creating a New Project In the NetBeans IDE, you always work in a project where you store sources and files. To create a new project, perform the following steps: 1. Select New Project from the File menu. You can also click the New Project button in the IDE toolbar. 2. In the Categories pane, select the General node. In the Projects pane, choose the Java Application type. Click the Next button. 3. Enter MyBean in the Project Name field and specify the project location. Do not create a Main class here, because later you will create a new Java class in this project. 4. Click the Finish button. This figure represents the expanded MyBean node in the Projects list.

Creating a New Form After creating a new project, the next step is to create a form within which the JavaBeans components and other required GUI components, will be placed. To create a new form, perform the following sequence of actions: 1. In the Projects list, expand the MyBean node, right-click on the node and choose New|JFrame Form from the pop-up menu. 2. Enter MyForm as the Class Name. 3. Click the Finish button. The IDE creates the MyForm form and the MyForm class within the MyBean application and opens the MyForm form in the GUI Builder. This figure represents the Projects list, where the MyForm class is located.

The GUI Builder Interface When the JFrame form is added to your application, the IDE opens the newly-created form in an Editor tab with a toolbar containing the following buttons: • • • • •

– Selection Mode enables you to select one or more objects in the Design Area. – Connection Mode enables you to set a connection between objects by specifying an event. – Preview Design enables you to preview the form layout. – Align commands enable you to align selected objects. – Change Resizability enables you to set up vertical and horizontal resizing.

When the MyForm form opens in the GUI Builder's Design view, three additional windows appear, enabling you to navigate, organize, and edit GUI forms. These windows include the following: •

Design Area. The primary window for creating and editing Java GUI forms. Source and Design toggle buttons enable you to switch between view a class's source code and a graphical view of the GUI components. Click on an object to select it in the Design Area. For a multiple selection, hold down the Ctrl key while clicking on objects.

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Inspector. Representation of a tree hierarchy of all the components in your application. The Inspector highlights the component in the tree that is currently being edited. Palette. A customizable list of available components containing groups for Swing, AWT, Borders, and Beans components. This window enables you to create, remove, and rearrange the categories displayed in the palette using the customizer. Properties Window. A display of the properties of the component currently selected in the GUI Builder, Inspector window, Projects window, or Files window.

If you click the Source button, the IDE displays the application's Java source code in the editor. Sections of code that are automatically generated by the GUI Builder are indicated by blue areas. These blue areas are protected from editing in the Source view. You can only edit code appearing in the white areas of the editor when in Source view. When you make your changes in the Design View, the IDE updates the file's sources.

Creating a Bean To create your own bean object and add it to the palette for the bean group, execute the following procedure: 1. Select the node in the MyBean project. 2. Choose New|Java Class from the pop-up menu. 3. Specify the name for the new class, for example, MyBean, then press the Finish button. 4. Open the MyBean.java file. 5. In the editor window, select inside the class. Right-click and choose Insert Code. Then select Add Property. 6. In the Name field of the Add Property dialog box, type YourName and press OK.

7. Now you can analyze the automatically generated code. Notice that set and get methods were included: public class MyBean { /** Creates a new instance of MyBean */ public MyBean() { }

/** * Holds value of property yourName. */ private String yourName; /** * Getter for property yourName. * @return Value of property yourName. */ public String getYourName() { return this.yourName; } /** * Setter for property yourName. * @param yourName New value of property yourName. */ public void setYourName(String yourName) { this.yourName = yourName; } }

8. Right-click the MyBean node in the MyBean project tree and choose Tools |Add to Palette from the pop-up menu. 9. Select the Beans group in the Palette tree to add your bean. Now you can switch to the Palette window by choosing Palette from the Windows menu and make sure that the MyBean component was added to the Beans group. So far you have created a bean, set the YourName property, and added this bean as a component to the palette.

Adding Components to the Form Now you can use the Free Design of the GUI Builder and add the MyBean component and other standard Swing components to MyForm. 1. Select the MyForm node in the project tree. 2. Drag the JLabel Swing component from the Palette window to the Design Area. Double-click the component and change the text property to "Enter your name:". 3. Drag the JTextField component from the Palette window to the Design Area. Double-click the component and empty the text field. 4. Drag the JButton component from the Palette window to the Design Area. Double-click the component and enter "OK" as the text property. 5. Add another button and enter "Cancel" as its text property. 6. Align components by using the appropriate align commands. 7. Before you drag the MyBean component from the Pallete you must compile your project because the MyBean component is non-visual and cannot be operated as a visual component. When you Drag and Drop the MyBean component it will not

appear in the Design Area. However, you can view it in the Inspector window by expanding the Other Components node, as shown in the following figure. 8.

To summarize, in the previous steps you created a project, developed a JFrame form, added a bean object and included it in your project as a non-visual component. Later in this trail you will learn how to change properties for the bean component and handle events by using the NetBeans GUI Builder.

In this section you will learn more about beans by performing the following actions: • • • • •

Creating a simple bean Compiling the bean Generating a Java Archive (JAR) file Loading the bean into the GUI Builder of the NetBeans IDE Inspecting the bean's properties and events

Your bean will be named SimpleBean. Here are the steps to create it: 1. Write the SimpleBean code. Put it in a file named SimpleBean.java, in the directory of your choice. Here's the code: 2. import java.awt.Color; 3. import java.beans.XMLDecoder; 4. import javax.swing.JLabel; 5. import java.io.Serializable; 6. 7. public class SimpleBean extends JLabel 8. implements Serializable { 9. public SimpleBean() { 10. setText( "Hello world!" ); 11. setOpaque( true ); 12. setBackground( Color.RED ); 13. setForeground( Color.YELLOW ); 14. setVerticalAlignment( CENTER ); 15. setHorizontalAlignment( CENTER ); 16. } 17. }

extends the javax.swing.JLabel graphic component and inherits its properties, which makes the SimpleBean a visual component. SimpleBean also implements the java.io.Serializable interface. Your bean may implement either the Serializable or the Externalizable interface. SimpleBean

18. Create a manifest, the JAR file, and the class file SimpleBean.class. Use the Apache Ant tool to create these files. Apache Ant is a Java-based build tool that enables you to generate XML-based configurations files as follows: 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.



30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

43. 44. 45. 46. 47. 48. 49. 50.

It is recommended to save an XML script in the build.xml file, because Ant recognizes this file name automatically. 51. Load the JAR file. Use the NetBeans IDE GUI Builder to load the jar file as follows: 1. Start NetBeans. 2. From the File menu select "New Project" to create a new application for your bean. You can use "Open Project" to add your bean to an existing application. 3. Create a new application using the New Project Wizard. 4. Select a newly created project in the List of Projects, expand the Source Packages node, and select the Default Package element. 5. Click the right mouse button and select New|JFrameForm from the pop-up menu. 6. Select the newly created Form node in the Project Tree. A blank form opens in the GUI Builder view of an Editor tab. 7. Open the Palette Manager for Swing/AWT components by selecting Palette Manager in the Tools menu. 8. In the Palette Manager window select the beans components in the Palette tree and press the "Add from JAR" button. 9. Specify a location for your SimpleBean JAR file and follow the Add from JAR Wizard instructions. 10. Select the Palette and Properties options from the Windows menu. 11. Expand the beans group in the Palette window. The SimpleBean object appears. Drag the SimpleBean object to the GUI Builder panel. The following figure represents the SimpleBean object loaded in the GUI Builder panel:

52. Inspect Properties and Events. The SimpleBean properties will appear in the Properties window. For example, you can change a background property by selecting another color. To preview your form, use the Preview Design button of the GUI Builder toolbar. To inspect events associated with the SimpleBean object, switch to the Events tab of the Properties window. You will learn more about bean properties and events in the lessons that follow.

Lesson: Properties In the following sections you will learn how to implement bean properties. 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. The JavaBeans™ specification defines the following types of bean properties: •

Simple – A bean property with a single value whose changes are independent of changes in any other property.

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Indexed – A bean property that supports a range of values instead of a single value. Bound – A bean property for which a change to the property results in a notification being sent to some other bean. Constrained – 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.

Bean properties can also be classified as follows: •

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Writable – A bean property that can be changed o Standard o Expert o Preferred Read Only – A bean property that cannot be changed. Hidden – A bean property that can be changed. However, these properties are not disclosed with the BeanInfo class

BeanBuilder uses this schema to group and represent properties in the Properties window. Simple Properties To add simple properties to a bean, add appropriate getXXX and setXXX methods (or isXXX and setXXX methods for a boolean property). The names of these methods follow specific rules called design patterns. These design pattern-based method names allow builder tools such as the NetBeans GUI Builder, to provide the following features: • • • • • •

Discover a bean's properties Determine the properties' read/write attributes Determine the properties' types Locate the appropriate property editor for each property type Display the properties (usually in the Properties window) Alter the properties (at design time)

Adding a Title Property In previous lessons you learned how to create a simple property by using the NetBeans GUI Builder. The following procedure shows how to create a simple property in detail: 1. Right-click on the Bean Patterns node in the MyBean class hierarchy. 2. Select Add|Property from the pop-up menu. 3. Fill out the New Property Pattern form as shown in the following figure and click OK.

4. The following code is automatically generated:

5. public class MyBean { 6. 7. /** Creates a new instance of MyBean */ 8. public MyBean() { 9. } 10. 11. /** 12. * Holds value of property title. 13. */ 14. private String title; 15. 16. /** 17. * Getter for property title. 18. * @return Value of property title. 19. */ 20. public String getTitle() { 21. return this.title; 22. } 23. 24. /** 25. * Setter for property title. 26. * @param title New value of property title. 27. */ 28. public void setTitle(String title) { 29. this.title = title; 30. } 31. 32. }

33. Now make your bean visual by extending the JComponent class and implement the Serializable interface. Then, add the paintComponent method to represent your bean. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65.

import java.awt.Graphics; import java.io.Serializable; import javax.swing.JComponent; /** * Bean with a simple property "title". */ public class MyBean extends JComponent implements Serializable { private String title; public String getTitle() { return this.title; } public void setTitle( String title ) { this.title = title; } protected void paintComponent( Graphics g ) { g.setColor( getForeground() ); int height = g.getFontMetrics().getHeight(); if ( this.title != null ) g.drawString(this.title, 0, height ); }

}

Inspecting Properties Select the MyBean component in the Other Components node in the Inspector window. Now you can analyze the title property in the Properties window and change it. To change the title property press the "..." button and enter any string you wish. The following figure represents the title property set to the "The title" value.

The NetBeans GUI Builder enables you to restrict the changing of a property value. To restrict the changing of the title property, right-click the title property in the Bean Patterns node of the MyBean project. Select Properties from the pop-up menu and the Properties window appears. Choose one of the following property access types from the Mode combo box: • • •

Read/Write Read only Write only

The Read only property has only the get method only, while the Write only property has only the set method only. The Read/Write type property has both of these methods. Bound Properties Bound properties support the PropertyChangeListener (in the API reference documentation) class. Sometimes when a Bean property changes, another object might need to be notified of the change, and react to the change. Whenever a bound property changes, notification of the change is sent to interested listeners. The accessor methods for a bound property are defined in the same way as those for simple properties. However, you also need to provide the event listener registration methods forPropertyChangeListener classes and fire a PropertyChangeEvent (in the

API reference documentation) event to the PropertyChangeListener objects by calling their propertyChange methods The convenience PropertyChangeSupport (in the API reference documentation) class enables your bean to implement these methods. Your bean can inherit changes from the PropertyChangeSupportclass, or use it as an inner class. In order to listen for property changes, an object must be able to add and remove itself from the listener list on the bean containing the bound property. It must also be able to respond to the event notification method that signals a property change. The PropertyChangeEvent class encapsulates property change information, and is sent from the property change event source to each object in the property change listener list with the propertyChange method.

Implementing Bound Property Support Within a Bean To implement a bound property in your application, follow these steps: 1. Import the java.beans package. This gives you access to the PropertyChangeSupport class. 2. Instantiate a PropertyChangeSupport object. This object maintains the property change listener list and fires property change events. You can also make your class a PropertyChangeSupport subclass. 3. Implement methods to maintain the property change listener list. Since a PropertyChangeSupport subclass implements these methods, you merely wrap calls to the property-change support object's methods. 4. Modify a property's set method to fire a property change event when the property is changed.

Creating a Bound Property To create the title property as a bound property for the MyBean component in the NetBeans GUI Builder, perform the following sequence of operations: 1. Right-click the Bean Patterns node in the MyBean class hierarchy. 2. Select Add|Property from the pop-up menu. 3. Fill the New Property Pattern form as shown on the following figure and click OK.

4. Note that the title property and the multicast event source pattern PropertyChangeListener were added to the Bean Patterns structure. You can also modify existing code generated in the previous lesson to convert the title and lines properties to the bound type as follows (where newly added code is shown in bold): import import import import import

java.awt.Graphics; java.beans.PropertyChangeListener; java.beans.PropertyChangeSupport; java.io.Serializable; javax.swing.JComponent;

/** * Bean with bound properties. */ public class MyBean extends JComponent implements Serializable { private String title; private String[] lines = new String[10]; private final PropertyChangeSupport pcs = new PropertyChangeSupport( this ); public String getTitle() { return this.title;

} public void setTitle( String title ) { String old = this.title; this.title = title; this.pcs.firePropertyChange( "title", old, title ); } public String[] getLines() { return this.lines.clone(); } public String getLines( int index ) { return this.lines[index]; } public void setLines( String[] lines ) { String[] old = this.lines; this.lines = lines; this.pcs.firePropertyChange( "lines", old, lines ); } public void setLines( int index, String line ) { String old = this.lines[index]; this.lines[index] = line; this.pcs.fireIndexedPropertyChange( "lines", index, old, lines ); } public void addPropertyChangeListener( PropertyChangeListener listener ) { this.pcs.addPropertyChangeListener( listener ); } public void removePropertyChangeListener( PropertyChangeListener listener ) { this.pcs.removePropertyChangeListener( listener ); } protected void paintComponent( Graphics g ) { g.setColor( getForeground() ); int height = g.getFontMetrics().getHeight(); paintString( g, this.title, height ); if ( this.lines != null ) { int step = height; for ( String line : this.lines )

}

paintString( g, line, height += step );

} private void paintString( Graphics g, String str, int height ) { if ( str != null ) g.drawString( str, 0, height ); }

}

Constrained Properties A bean property is constrained if the bean supports the VetoableChangeListener(in the API reference documentation) and PropertyChangeEvent(in the API reference documentation) classes, and if the set method for this property throws a PropertyVetoException(in the API reference documentation). Constrained properties are more complicated than bound properties because they also support property change listeners which happen to be vetoers. The following operations in the setXXX method for the constrained property must be implemented in this order: 1. Save the old value in case the change is vetoed. 2. Notify listeners of the new proposed value, allowing them to veto the change. 3. If no listener vetoes the change (no exception is thrown), set the property to the new value. The accessor methods for a constrained property are defined in the same way as those for simple properties, with the addition that the setXXX method throws a PropertyVetoException exception. The syntax is as follows: public void setPropertyName(PropertyType pt) throws PropertyVetoException {code}

Handling Vetoes If a registered listener vetoes a proposed property change by throwing a PropertyVetoException exception, the source bean with the constrained property is responsible for the following actions: • •

Catching exceptions. Reverting to the old value for the property.

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Issuing a new VetoableChangeListener.vetoableChange call to all listeners to report the reversion.

The VetoableChangeListener class throws a PropertyVetoException and handles the PropertyChangeEvent event fired by the bean with the constrained property. The VetoableChangeSupport provides the following operations: • • • •

Keeping track of VetoableChangeListener objects. Issuing the vetoableChange method on all registered listeners. Catching any vetoes (exceptions) thrown by listeners. Informing all listeners of a veto by calling vetoableChange again, but with the old property value as the proposed "new" value.

Creating a Constrained Property To create a constrained property, set the appropriate option in the New Property Pattern form as shown on the following figure.

Note that the Multicast Source Event Pattern - vetoableChangeListener was added to the Bean Patterns hierarchy. You can also modify the existing code generated in the previous lesson to make the title and lines properties constrained as follows (where newly added code is shown in bold):

import import import import import import import import

java.io.Serializable; java.beans.PropertyChangeListener; java.beans.PropertyChangeSupport; java.beans.PropertyVetoException; java.beans.VetoableChangeListener; java.beans.VetoableChangeSupport; java.awt.Graphics; javax.swing.JComponent;

/** * Bean with constrained properties. */ public class MyBean extends JComponent implements Serializable { private String title; private String[] lines = new String[10]; private final PropertyChangeSupport pcs = new PropertyChangeSupport( this ); private final VetoableChangeSupport vcs = new VetoableChangeSupport( this ); public String getTitle() { return this.title; }

/** * This method was modified to throw the PropertyVetoException * if some vetoable listeners reject the new title value */ public void setTitle( String title ) throws PropertyVetoException { String old = this.title; this.vcs.fireVetoableChange( "title", old, title ); this.title = title; this.pcs.firePropertyChange( "title", old, title ); } public String[] getLines() { return this.lines.clone(); } public String getLines( int index ) { return this.lines[index]; }

/** * This method throws the PropertyVetoException * if some vetoable listeners reject the new lines value */ public void setLines( String[] lines ) throws PropertyVetoException

{

}

String[] old = this.lines; this.vcs.fireVetoableChange( "lines", old, lines ); this.lines = lines; this.pcs.firePropertyChange( "lines", old, lines );

public void setLines( int index, String line ) throws PropertyVetoException { String old = this.lines[index]; this.vcs.fireVetoableChange( "lines", old, line ); this.lines[index] = line; this.pcs.fireIndexedPropertyChange( "lines", index, old, line ); } public void addPropertyChangeListener( PropertyChangeListener listener ) { this.pcs.addPropertyChangeListener( listener ); } public void removePropertyChangeListener( PropertyChangeListener listener ) { this.pcs.removePropertyChangeListener( listener ); } /** * Registration of the VetoableChangeListener */ public void addVetoableChangeListener( VetoableChangeListener listener ) { this.vcs.addVetoableChangeListener( listener ); } public void removeVetoableChangeListener( VetoableChangeListener listener ) { this.vcs.removeVetoableChangeListener( listener ); } protected void paintComponent( Graphics g ) { g.setColor( getForeground() ); int height = g.getFontMetrics().getHeight(); paintString( g, this.title, height );

}

if ( this.lines != null ) { int step = height; for ( String line : this.lines ) paintString( g, line, height += step ); }

private void paintString( Graphics g, String str, int height ) { if ( str != null ) g.drawString( str, 0, height ); }

}

Indexed Properties An indexed property is an array of properties or objects that supports a range of values and enables the accessor to specify an element of a property to read or write. Indexed properties are specified by the following methods: //Methods to access individual values public PropertyElement getPropertyName(int index) public void setPropertyName(int index, PropertyElement element)

and //Methods to access the entire indexed property array public PropertyElement[] getPropertyName() public void setPropertyName(PropertyElement element[]) Note that the distinction between the get and set methods for indexed properties is subtle. The get method either has an argument that is the array index of the property, returns an array. The set method either has two arguments, namely an integer array

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index and the property element object that is being set, or has the entire array as an argument.

Creating an Indexed Property To create an indexed property for your MyBean component, right-click the Bean Patterns node and select Add|Indexed Property from the pop-up menu. Set up Non-Index Options as shown in the following figure.

The code in the Source window will be changed automatically as follows: import java.awt.Graphics; import java.io.Serializable; import javax.swing.JComponent; /** * Bean with simple property 'title'. */ public class MyBean extends JComponent implements Serializable { private String title; public String getTitle() { return this.title; } public void setTitle( String title ) { this.title = title; }

protected void paintComponent( Graphics g ) { g.setColor( getForeground() ); int height = g.getFontMetrics().getHeight(); if ( this.title != null ) g.drawString(this.title, 0, height );

}

/** * Holds value of property lines. */ private String[] lines; /** * Indexed getter for property lines. * @param index Index of the property. * @return Value of the property at index. */ public String getLines(int index) { return this.lines[index]; } /** * Getter for property lines. * @return Value of property lines. */ public String[] getLines() { return this.lines; } /** * Indexed setter for property lines. * @param index Index of the property. * @param lines New value of the property at index. */ public void setLines(int index, String lines) { this.lines[index] = lines; } /** * Setter for property lines. * @param lines New value of property lines. */ public void setLines(String[] lines) { this.lines = lines; }

}

Add the following code to the MyBean.java component to present the user with a list of choices. You can provide and change these choices at design time. (Newly added code is shown in bold.) import java.awt.Graphics; import java.io.Serializable; import javax.swing.JComponent; /**

* Bean with a simple property "title" * and an indexed property "lines". */ public class MyBean extends JComponent implements Serializable { private String title; private String[] lines = new String[10]; public String getTitle() { return this.title; } public void setTitle( String title ) { this.title = title; } public String[] getLines() { return this.lines.clone(); } public String getLines( int index ) { return this.lines[index]; } public void setLines( String[] lines ) { this.lines = lines; } public void setLines( int index, String line ) { this.lines[index] = line; } protected void paintComponent( Graphics g ) { g.setColor( getForeground() ); int height = g.getFontMetrics().getHeight(); paintString( g, this.title, height );

}

if ( this.lines != null ) { int step = height; for ( String line : this.lines ) paintString( g, line, height += step ); }

private void paintString( Graphics g, String str, int height ) {

if ( str != null ) g.drawString( str, 0, height ); }

}

The following figure represents the lines property in the Properties window.

Notice that this property has a null value. To set up an alternative value, press the "..." button. The form shown in the following figure enables you to add ten items for the lines property list. First remove the default null items. Then add custom items to the list by entering each item value into the Item field and pressing the Add button each time.

Lesson: Manipulating Events Event passing is the means by which components communicate with each other. Components broadcast events, and the underlying framework delivers the events to the components that are to be notified. The notified components usually perform some action based on the event that took place. The event model was designed to accommodate the JavaBeans™ architecture. To understand how events and event handling work in the JavaBeans component model, you must understand the concepts of events, listeners, and sources. To refresh your knowledge in these areas, read the Writing Event Listeners lesson of the Swing tutorial. The event model that is used by the JavaBeans architecture is a delegation model. This model is composed of three main parts: sources, events, and listeners. The source of an event is the object that originates or fires the event. The source must define the events it will fire, as well as the methods for registering listeners of those events. A listener is an object that indicates that it is to be notified of events of a

particular type. Listeners register for events using the methods defined by the sources of those events. From the Properties lesson you discovered two event listeners. The PropertyChangeListener(in the API reference documentation) interface provides a notification whenever a bound property value is changed and the VetoableChangeListener(in the API reference documentation) creates a notification whenever a bean changes a constrained property value.

Simple Event Example This example represents an application that performs an action when a button is clicked. Button components are defined as sources of an event type called ActionEvent(in the API reference documentation). Listeners of events of this type must register for these events using the addActionListener method. Therefore, the addActionListener method is used to register the ButtonHandler object as a listener of the ActionEvent event that is fired by the button. In addition, according to the requirements of the ActionListener class, you must define an actionPerformed method, which is the method that is called when the button is clicked. import import import import import import import

java.awt.event.ActionEvent; java.awt.event.ActionListener; javax.swing.JTextArea; java.awt.BorderLayout; javax.swing.JButton; javax.swing.JFrame; javax.swing.WindowConstants;

public class ButtonHandler implements ActionListener { /** * Component that will contain messages about * events generated. */ private JTextArea output; /** * Creates an ActionListener that will put messages in * JTextArea everytime event received. */ public ButtonHandler( JTextArea output ) { this.output = output; } /** * When receives action event notification, appends * message to the JTextArea passed into the constructor. */ public void actionPerformed( ActionEvent event )

{ }

this.output.append( "Action occurred: " + event + '\n' );

}

class ActionTester { public static void main(String args[]) { JFrame frame = new JFrame( "Button Handler" ); JTextArea area = new JTextArea( 6, 80 ); JButton button = new JButton( "Fire Event" ); button.addActionListener( new ButtonHandler( area ) ); frame.add( button, BorderLayout.NORTH ); frame.add( area, BorderLayout.CENTER ); frame.pack(); frame.setDefaultCloseOperation( WindowConstants.DISPOSE_ON_CLOS E ); frame.setLocationRelativeTo( null ); frame.setVisible( true ); } }

Using Introspection to Discover the Events A Bean Fires The JavaBeans API provides event-oriented design patterns to give introspecting tools the ability to discover what events a bean can fire. For a bean to be the source of an event, it must implement methods that add and remove listener objects for that type of event. The design patterns for these methods are the following: public void add( a) public void remove( a)

These methods let a source bean know where to fire events. The source bean then fires events at those listener beans using the methods for those particular interfaces. For example, if a source bean registers ActionListener objects, it will fire events at those objects by calling the actionPerformed method on those listeners. package java.awt.event; import java.util.EventListener;

public interface ActionListener extends EventListener { }

public void actionPerformed(ActionEvent e);

Using the NetBeans GUI Builder to Set Events In the lesson "Using the NetBeans GUI Builder," you learned how to create a MyBean component, add the yourName property, and design a simple form. Now you will set an event by which a value entered in the JTextField component is stored in the yourName property. Use the GUI Builder as follows to set such an event: 1. Left click the MyForm node.

2. Switch to the Connection Mode by clicking the appropriate button on the GUI Builder toolbar. 3. In the Design Area or Inspector window select the OK button (jButton1). Notice that the button is highlighted in red when it is selected. 4. In the Inspector window select the myBean1 component. 5. In the Connection wizard's Select Source Event page, select the action| actionPerformed[jButton1ActionPerformed1] event by expanding the event type directory nodes, as represented in the following figure.

6. Click the Next button. 7. In the Specify Target Operation page, specify the yourName property in the MyBean component, and click the Next button. 8. In the Enter Parameters page, specify the target property by selecting the Property radio button. 9. Press the ellipsis (...) button to display the Select Property dialog box. 10. In the Select Property dialog box select the jTextField component from the Component combobox and choose the text property from the list that is presented, as shown on the following figure.

11. Click the Finish button. The Source Editor window is now displayed. Since the GUI Builder automatically generates the code to connect the form's components, the following code will be added to the MyForm class: private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) { myBean1.setYourName(jTextField1.getText());

}

Lesson: Bean Persistence A bean has the property of persistence when its properties, fields, and state information are saved to and retrieved from storage. Component models provide a mechanism for persistence that enables the state of components to be stored in a non-volatile place for later retrieval. The mechanism that makes persistence possible is called serialization. Object serialization means converting an object into a data stream and writing it to storage. Any applet, application, or tool that uses that bean can then "reconstitute" it by deserialization. The object is then restored to its original state. For example, a Java application can serialize a Frame window on a Microsoft Windows machine, the serialized file can be sent with e-mail to a Solaris machine, and then a Java application can restore the Frame window to the exact state which existed on the Microsoft Windows machine.

Any applet, application, or tool that uses that bean can then "reconstitute" it by deserialization. All beans must persist. To persist, your beans must support serialization by implementing either the java.io.Serializable(in the API reference documentation) interface, or the java.io.Externalizable(in the API reference documentation) interface. These interfaces offer you the choices of automatic serialization and customized serialization. If any class in a class's inheritance hierarchy implements Serializable or Externalizable, then that class is serializable.

Classes That Are Serializable Any class is serializable as long as that class or a parent class implements the java.io.Serializable interface. Examples of serializable classes include Component, String, Date, Vector, and Hashtable. Thus, any subclass of the Component class, including Applet, can be serialized. Notable classes not supporting serialization include Image, Thread, Socket, and InputStream. Attempting to serialize objects of these types will result in an NotSerializableException. The Java Object Serialization API automatically serializes most fields of a Serializable object to the storage stream. This includes primitive types, arrays,and strings. The API does not serialize or deserialize fields that are marked transient or static.

Controlling Serialization You can control the level of serialization that your beans undergo. Three ways to control serilization are: • • •

Automatic serialization, implemented by the Serializable interface. The Java serialization software serializes the entire object, except transient and static fields. Customized serialization. Selectively exclude fields you do not want serialized by marking with the transient (or static) modifier. Customized file format, implemented by the Externalizable interface and its two methods. Beans are written in a specific file format.

Default Serialization: The Serializable Interface The Serializable interface provides automatic serialization by using the Java Object Serialization tools. Serializable declares no methods; it acts as a marker, telling the Object Serialization tools that your bean class is serializable. Marking your class Serializable means you are telling the Java Virtual Machine (JVM) that you have made sure your class will work with default serialization. Here are some important points about working with the Serializable interface:

•

• •

Classes that implement Serializable must have an access to a no-argument constructor of supertype. This constructor will be called when an object is "reconstituted" from a .ser file. You don't need to implement Serializable in your class if it is already implemented in a superclass. All fields except static and transient fields are serialized. Use the transient modifier to specify fields you do not want serialized, and to specify classes that are not serializable.

Selective Serialization Using the transient Keyword To exclude fields from serialization in a Serializable object mark the fields with the transient modifier. transient int status;

Default serialization will not serialize transient and static fields.

Selective Serialization: writeObject and readObject If your serializable class contains either of the following two methods (the signatures must be exact), then the default serialization will not take place. private void writeObject(java.io.ObjectOutputStream out) throws IOException; private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException;

You can control how more complex objects are serialized, by writing your own implementations of the writeObject and readObject methods. Implement writeObject when you need to exercise greater control over what gets serialized when you need to serialize objects that default serialization cannot handle, or when you need to add data to the serialization stream that is not an object data member. Implement readObject to reconstruct the data stream you wrote with writeObject.

The Externalizable Interface Use the Externalizable interface when you need complete control over your bean's serialization (for example, when writing and reading a specific file format). To use the Externalizable interface you need to implement two methods: readExternal and writeExternal. Classes that implement Externalizable must have a no-argument constructor. Lesson: Long Term Persistence Long-term persistence is a model that enables beans to be saved in XML format.

Information on the XML format and on how to implement long-term persistence for nonbeans can be found in XML Schema and Using XMLEncoder .

Encoder and Decoder The XMLEncoder class is assigned to write output files for textual representation of Serializable objects. The following code fragment is an example of writing a Java bean and its properties in XML format: XMLEncoder encoder = new XMLEncoder( new BufferedOutputStream( new FileOutputStream( "Beanarchive.xml" ) ) ); encoder.writeObject( object ); encoder.close();

The XMLDecoder class reads an XML document that was created with XMLEncoder: XMLDecoder decoder = new XMLDecoder( new BufferedInputStream( new FileInputStream( "Beanarchive.xml" ) ) ); Object object = decoder.readObject(); decoder.close();

What's in XML? An XML bean archive has its own specific syntax, which includes the following tags to represent each bean element: • • • • • •

an XML preamble to describe a version of XML and type of encoding a tag to embody all object elements of the bean an tag to represent a set of method calls needed to reconstruct an object from its serialized form Ok

or statements Cancel

•

tags to define appropriate primitive types:

o o o o o o o o •

5555

• •

a tag to represent an instance of Class.

• • •

an tag to define an array

java.swing.JFrame



The following code represents an XML archive that will be generated for the SimpleBean component: true

Lesson: Introspection Introspection is the automatic process of analyzing a bean's design patterns to reveal the bean's properties, events, and methods. This process controls the publishing and discovery of bean operations and properties. This lesson explains the purpose of introspection, introduces the Introspection API, and gives an example of introspection code.

Purpose of Introspection A growing number of Java object repository sites exist on the Internet in answer to the demand for centralized deployment of applets, classes, and source code in general. Any developer who has spent time hunting through these sites for licensable Java code to incorporate into a program has undoubtedly struggled with issues of how to quickly and cleanly integrate code from one particular source into an application. The way in which introspection is implemented provides great advantages, including: 1. Portability - Everything is done in the Java platform, so you can write components once, reuse them everywhere. There are no extra specification files that need to be maintained independently from your component code. There are no platform-specific issues to contend with. Your component is not tied to one component model or one proprietary platform. You get all the advantages of the evolving Java APIs, while maintaining the portability of your components. 2. Reuse - By following the JavaBeans design conventions, implementing the appropriate interfaces, and extending the appropriate classes, you provide your component with reuse potential that possibly exceeds your expectations.

Introspection API The JavaBeans API architecture supplies a set of classes and interfaces to provide introspection. The BeanInfo (in the API reference documentation) interface of the java.beans package defines a set of methods that allow bean implementors to provide explicit information about their beans. By specifying BeanInfo for a bean component, a developer can hide methods, specify an icon for the toolbox, provide descriptive names for properties, define which properties are bound properties, and much more. The getBeanInfo(beanName) (in the API reference documentation) of the Introspector (in the API reference documentation) class can be used by builder tools and other automated environments to provide detailed information about a bean. The getBeanInfo method relies on the naming conventions for the bean's properties, events, and methods. A call to getBeanInfo results in the introspection process analyzing the bean’s classes and superclasses. The Introspector class provides descriptor classes with information about properties, events, and methods of a bean. Methods of this class locate any descriptor information that has been explicitly supplied by the developer through BeanInfo classes. Then the Introspector class applies the naming conventions to determine what properties the bean has, the events to which it can listen, and those which it can send. The following figure represents a hierarchy of the FeatureDescriptor classes:

Each class represented in this group describes a particular attribute of the bean. For example, the isBound method of the PropertyDescriptor class indicates whether a PropertyChangeEvent event is fired when the value of this property changes.

Editing Bean Info with the NetBeans BeanInfo Editor To open the BeanInfo dialog box, expand the appropriate class hierarchy to the bean Patterns node. Right-click the bean Patterns node and choose BeanInfo Editor from the pop-up menu. All elements of the selected class that match bean-naming conventions will be displayed at the left in the BeanInfo Editor dialog box as shown in the following figure:

Select one of the following nodes to view and edit its properties at the right of the dialog box: • • • • •

BeanInfo Bean Properties Methods Event Sources

Special symbols (green and red) appear next to the subnode to indicate whether an element will be included or excluded from the BeanInfo class. If the Get From Introspection option is not selected, the node's subnodes are available for inclusion in the BeanInfo class. To include all subnodes, right-click a node and choose Include All. You can also include each element individually by selecting its subnode and setting the Include in BeanInfo property. If the Get From Introspection option is selected, the setting the properties of subnodes has no effect in the generated BeanInfo code. The following attributes are available for the nodes for each bean, property, event sources, and method: • • • • • • • • • • •

Name - A name of the selected element as it appears in code. Preferred - An attribute to specify where this property appears in the Inspector window under the Properties node. Expert - An attribute to specify where this property appears in the Inspector window under the Other Properties node. Hidden - An attribute to mark an element for tool use only. Display Name Code - A display name of the property. Short Description Code - A short description of the property. Include in BeanInfo - An attribute to include the selected element in the BeanInfo class. Bound - An attribute to make the bean property bound. Constrained - An attribute to make the bean property constrained. Mode - An attribute to set the property's mode and generate getter and setter methods. Property Editor Class - An attribute to specify a custom class to act as a property editor for the property.

For Event Source nodes, the following Expert properties are available: • •

Unicast (read-only) In Default Event Set

Introspection Sample The following example represents code to perform introspection:

import import import import

java.beans.BeanInfo; java.beans.Introspector; java.beans.IntrospectionException; java.beans.PropertyDescriptor;

public class SimpleBean { private final String name = "SimpleBean"; private int size; public String getName() { return this.name; } public int getSize() { return this.size; } public void setSize( int size ) { this.size = size; } public static void main( String[] args ) throws IntrospectionException { BeanInfo info = Introspector.getBeanInfo( SimpleBean.class ); for ( PropertyDescriptor pd : info.getPropertyDescriptors() ) System.out.println( pd.getName() ); } }

This example creates a non-visual bean and displays the following properties derived from the BeanInfo object: • • •

class name size

Note that a class property was not defined in the SimpleBean class. This property was inherited from the Object class. To get properties defined only in the SimpleBean class, use the following form of the getBeanInfo method: Introspector.getBeanInfo( SimpleBean.class, Object.class );

Lesson: Bean Customization

Customization provides a means for modifying the appearance and behavior of a bean within an application builder so it meets your specific needs. There are several levels of customization available for a bean developer to allow other developers to get maximum benefit from a bean’s potential functionality. The following links are useful for learning about property editors and customizers: • • • • •

PropertyEditor(in the API reference documentation) interface PropertyEditorSupport(in the API reference documentation) class PropertyEditorManager(in the API reference documentation) class Customizer(in the API reference documentation) interface BeanInfo(in the API reference documentation) interface

A bean's appearance and behavior can be customized at design time within beanscompliant builder tools. There are two ways to customize a bean: •

•

By using a property editor. Each bean property has its own property editor. The NetBeans GUI Builder usually displays a bean's property editors in the Properties window. The property editor that is associated with a particular property type edits that property type. By using customizers. Customizers give you complete GUI control over bean customization. Customizers are used where property editors are not practical or applicable. Unlike a property editor, which is associated with a property, a customizer is associated with a bean.

Property Editors A property editor is a tool for customizing a particular property type. Property editors are activated in the Properties window. This window determines a property's type, searches for a relevant property editor, and displays the property's current value in a relevant way. Property editors must implement the PropertyEditor interface, which provides methods to specify how a property should be displayed in a property sheet. The following figure represents the Properties window containing myBean1 properties:

You begin the process of editing these properties by clicking the property entry. Clicking most of these entries will bring up separate panels. For example, to set up the foreground or background use selection boxes with choices of colors, or press the "..." button to work with a standard ColorEditor window. Clicking on the toolTipText property opens a StringEditor window. The support class PropertyEditorSupport provides a default implementation of the PropertyEditor interface. By subclassing your property editor from PropertyEditorSupport, you can simply override the methods you need. To display the current property value "sample" within the Properties window, you need to override isPaintable to return true. You then must override paintValue to paint the current property value in a rectangle in the property sheet. Here's how ColorEditor implements paintValue: public void paintValue(java.awt.Graphics gfx, java.awt.Rectangle box) { Color oldColor = gfx.getColor(); gfx.setColor(Color.black); gfx.drawRect(box.x, box.y, box.width-3, box.height-3); gfx.setColor(color); gfx.fillRect(box.x+1, box.y+1, box.width-4, box.height-4); gfx.setColor(oldColor); }

To support the custom property editor, override two more methods. Override supportsCustomEditor to return true, and then override getCustomEditor to return a custom editor instance. ColorEditor.getCustomEditor returns this.

In addition, the PropertyEditorSupport class maintains a PropertyChangeListener list, and fires property change event notifications to those listeners when a bound property is changed.

How Property Editors are Associated with Properties Property editors are discovered and associated with a given property in the following ways: •

Explicit association by way of a BeanInfo object. The editor of the title's property is set with the following line of code:

• •

pd.setPropertyEditorClass(TitleEditor.class);

Explicit registration by way of the method. This method takes two arguments: the bean class type, and the editor class to be associated with that type. Name search. If a class has no explicitly associated property editor, then the PropertyEditorManager searchs for that class's property editor in the following ways: o Appending "Editor" to the fully qualified class name. For example, for the my.package.ComplexNumber class, the property editor manager would search for the my.package.ComplexNumberEditor class. o Appending "Editor" to the class name and searching a class path. java.beans.PropertyEditorManager.registerEditor

•

Customizers You have learned that builder tools provide support for you to create your own property editors. What other needs should visual builders meet for complex, industrial-strength beans? Often it is undesirable to have all the properties of a bean revealed on a single (sometimes huge) property sheet. What if one single root choice about the type of the bean rendered half the properties irrelevant? The JavaBeans specification provides for user-defined customizers, through which you can define a higher level of customization for bean properties than is available with property editors. When you use a bean Customizer, you have complete control over how to configure or edit a bean. A Customizer is an application that specifically targets a bean's customization. Sometimes properties are insufficient for representing a bean's configurable attributes. Customizers are used where sophisticated instructions would be needed to change a bean, and where property editors are too primitive to achieve bean customization. All customizers must: •

Extend java.awt.Component or one of its subclasses.

•

• •

Implement the java.beans.Customizer interface This means implementing methods to register PropertyChangeListener objects, and firing property change events at those listeners when a change to the target bean has occurred. Implement a default constructor. Associate the customizer with its target class via BeanInfo.getBeanDescriptor.

Lesson: Using the BeanContext API As stated in the specification, the purpose of the Extensible Runtime Containment and Services Protocol is "to introduce the concept of a relationship between a Component and its environment, or Container, wherein a newly instantiated Component is provided with a reference to its Container or Embedding Context. The Container, or Embedding Context not only establishes the hierarchy or logical structure, but it also acts as a service provider that Components may interrogate in order to determine, and subsequently employ, the services provided by their Context."

Overview of the BeanContext API This section introduces extensible mechanisms and represents inheritance diagram of the BeanContext API.

Bean Context #1: Containment Only This section teaches how to use the BeanContextSupport class to provide the basic BeanContext functionality.

Bean Context #2: Containment and Services This section teaches how to use service capability defined by the BeanContextServices interface.

AWT Containers and the BeanContextProxy Interface This section describes how an AWT Container can act as a BeanContext.

Additional Resources • •

The Extensible Runtime Containment and Services Protocol Specification The java.beans.beancontext API documentation

Overview of the BeanContext API

The Extensible Runtime Containment and Services Protocol supports extensible mechanisms that: • • •

•

Introduce an abstraction for the environment, or context, in which a JavaBean logically functions during its life-cycle, that is a hierarchy of JavaBeans Enable the dynamic addition of arbitrary services to a JavaBean's environment Provide a single service discovery mechanism through which JavaBeans may interrogate their environment in order both to ascertain the availability of particular services and to subsequently employ those services. Provide better support for JavaBeans that are also Applets.

In English, this means that there now exists a standard mechanism through which Java developers can logically group a set of related JavaBeans into a "context" that the beans can become aware of and/or interact with. This context, or "containing environment", is known as the BeanContext. There are two distinct types of BeanContext included in this protocol: one which supports membership only (interface java.beans.beancontext.BeanContext) and one which supports membership and offers services (interface java.beans.beancontext.BeanContextServices) to its JavaBeans nested within. To orient yourself with the classes and interfaces of java.beans.beancontext, take a minute to look over the following hierarchy diagram. You will notice that the majority of the package is defined as interfaces, which allow for multiple inheritance.

Inheritance Diagram of the BeanContext API The classes and interfaces relevant to the BeanContext API are listed in the following diagrams. As you study the diagrams, take note of the BeanContext and BeanContextServices interfaces, and that each has its own concrete implementation that you can subclass or instantiate directly (classes java.beans.beancontext.BeanContextSupport and java.beans.beancontext.BeanContextServicesSupport respectively). Also take note of the location of the java.beans.beancontext.BeanContextChild interface. This is the interface that allows nested JavaBeans to become aware of their enclosing BeanContext.

Bean Context #1: Containment Only The "containment" portion of the Extensible Runtime Containment and Services Protocol is defined by the BeanContext interface. In its most basic form, a BeanContext is used to logically group a set of related java beans, bean contexts, or arbitrary objects. JavaBeans nested into a BeanContext are known as "child" beans. Once nested, a child bean can query its BeanContext for various membership information, as illustrated in the following examples. Here are some possible BeanContext containment scenarios:

The sample code presented in this chapter uses instances of the BeanContextSupport helper class to provide the basic BeanContext functionality. A BeanContextSupport object is simply a concrete implementation of the BeanContext interface. With a BeanContextSupport instance, it is possible to: • • • • •

• • •

Add an object, bean, or BeanContext: boolean add(Object o) Remove an object, bean, or BeanContext: boolean remove(Object o) Add a BeanContextMembershipListener: void addBeanContextMembershipListener(BeanContextMembershipListener bcml) Remove a BeanContextMembershipListener: void removeBeanContextMembershipListener(BeanContextMembershipListener bcml) Get all JavaBean or BeanContext instances currently nested in this BeanContext as an array or as an Iterator: Object[] toArray(), Object[] toArray(Object[] a), and Iterator iterator()

Determine whether or not a specified object is currently a child of the BeanContext: boolean contains(Object o) Get the number of children currently nested in this BeanContext: int size() Determine whether or not the BeanContext currently has zero children: boolean isEmpty()

•

Instantiate a new JavaBean instance as a child of the target BeanContext: Object instantiateChild(String beanName)

The following test programs, which are run from the command line, illustrate the use of these methods. The comments in the source code explain the purpose of each. File: Example1.java import java.beans.beancontext.*; public class Example1 { private static BeanContextSupport context = new BeanContextSupport(); // The BeanContext

private static BeanContextChildSupport bean = new BeanContextChildSupport(); // The JavaBean public static void main(String[] args) { report(); // Add the bean to the context System.out.println("Adding bean to context..."); context.add(bean); report();

}

private static void report() { // Print out a report of the context's membership state. System.out.println("=========================================== =="); // Is the context empty? System.out.println("Is the context empty? " + context.isEmpty()); // Has the context been set for the child bean? boolean result = (bean.getBeanContext()!=null); System.out.println("Does the bean have a context yet? " +

result);

// Number of children in the context System.out.println("Number of children in the context: " + context.size()); // Is the specific bean a member of the context? System.out.println("Is the bean a member of the context? " + context.contains(bean)); // Equality test if (bean.getBeanContext() != null) { boolean isEqual = (bean.getBeanContext()==context); // true means both references point to the same object System.out.println("Contexts are the same? " + isEqual); } System.out.println("=========================================== =="); } }

Output:

============================================= Is the context empty? true Does the bean have a context yet? false Number of children in the context: 0 Is the bean a member of the context? false ============================================= Adding bean to context... ============================================= Is the context empty? false Does the bean have a context yet? true

Number of children in the context: 1 Is the bean a member of the context? true Contexts are the same? true ============================================= File: Example2.java import java.beans.beancontext.*; public class Example2 { public static void main(String[] args) { // A BeanContext BeanContextSupport context = new BeanContextSupport(); // Many JavaBeans BeanContextChildSupport[] beans = new BeanContextChildSupport[100]; System.out.println("Number of children in the context: " + context.size()); // Create the beans and add them to the context for (int i = 0; i < beans.length; i++) { beans[i] = new BeanContextSupport(); context.add(beans[i]); } System.out.println("Number of children in the context: " + context.size()); // Context now has 100 beans in it, get references to them all Object[] children = context.toArray(); System.out.println("Number of objects retrieved from the context: " + children.length); } }

Output: Number of children in the context: 0 Number of children in the context: 100 Number of objects retrieved from the context: 100 File: Example3.java import java.beans.beancontext.*; import java.io.*; public class Example3 { public static void main(String[] args) { BeanContextSupport context = new BeanContextSupport(); System.out.println("Number of children nested into the context: " + context.size()); BeanContextChildSupport child = null; try { child = (BeanContextChildSupport)context.instantiateChild("java.beans.beanconte xt.BeanContextChildSupport"); }

catch(IOException e){ System.out.println("IOException occurred: " + e.getMessage()); } catch(ClassNotFoundException e){ System.out.println("Class not found: " + e.getMessage()); } System.out.println("Number of children nested into the context: " + context.size()); } }

Output:

Number of children nested into the context: 0 Number of children nested into the context: 1

BeanContextMembershipEvent Notification The BeanContext API uses the standard Java event model to register listeners and deliver events. For an overview of this standard event model, refer to Writing Event Listeners. For details about handling specific events, see Writing Event Listeners. In a basic BeanContext, the event classes and interfaces involved are: •

•

java.beans.beancontext.BeanContextMembershipEvent:

Encapsulates the list of children added to, or removed from, the membership of a particular BeanContext. An instance of this event is fired whenever a successful add(), remove(), retainAll(), removeAll(), or clear() is invoked on a given BeanContext instance. java.beans.BeanContextMembershipListener: Objects wishing to receive BeanContextMembershipEvents implement this interface. It defines methods void childrenAdded(BeanContextMembershipEvent bcme) and void childrenRemoved(BeanContextMembershipEvent bcme), which are called when a child is added to or removed from a given BeanContext instance.

BeanContextMembershipEvent Notification: Sample Code File: MembershipTest.java import java.beans.beancontext.*; public class MembershipTest { public static void main(String[] args) { BeanContextSupport context = new BeanContextSupport(); // the context MyMembershipListener listener = new MyMembershipListener(); BeanContextChildSupport bean = new BeanContextChildSupport(); // a JavaBean context.addBeanContextMembershipListener(listener); // now listening! context.add(bean); context.remove(bean);

}

}

class MyMembershipListener implements BeanContextMembershipListener { public void childrenAdded(BeanContextMembershipEvent bcme) { System.out.println("Another bean has been added to the context."); } public void childrenRemoved(BeanContextMembershipEvent bcme) { System.out.println("A bean has been removed from the context."); } }

Output:

Another bean has been added to the context. A bean has been removed from the context.

The same example, implemented using an anonymous inner class import java.beans.beancontext.*; public class MembershipTest { public static void main(String[] args) { BeanContextSupport context = new BeanContextSupport(); context.addBeanContextMembershipListener(new BeanContextMembershipListener() { public void childrenAdded(BeanContextMembershipEvent bcme) { System.out.println("Another bean has been added to the context."); } public void childrenRemoved(BeanContextMembershipEvent

bcme) { context.");

}

System.out.println("A bean has been removed from the

} }); BeanContextChildSupport bean = new BeanContextChildSupport(); context.add(bean); context.remove(bean);

}

Output:

Another bean has been added to the context. A bean has been removed from the context.

« Previous • Trail • Next »

Bean Context #2: Containment and Services As mentioned in the introduction, the BeanContext API also provides a standard mechanism through which JavaBeans can discover and utilize the services offered by

their enclosing BeanContext. Service capability is defined by the BeanContextServices interface. Because this interface is a BeanContext extension, it inherits all BeanContext membership capabilities. The discovery and requesting of services can be summarized in the following steps: 1. A JavaBean that implements the java.beans.beancontext.BeanContextServicesListener interface joins the bean context (the context itself is a BeanContextServices implementation), and

registers its intent to be notified of new services via the context's addBeanContextServicesListener(BeanContextServicesListener bcsl)

method. 2. A java.beans.beancontext.BeanContextServiceProvider registers a new service with the context via the context's addService() method. The context notifies all currently registered listeners that this new service has been added. 3. After being notified of the newly available service, the listening JavaBean requests an instance of the service from the context. 4. The context tells the service provider to deliver the service to the requesting JavaBean. BeanContextServices: Service Related Methods Using a java.beans.beancontext.BeanContextServicesSupport object as the bean context, it is possible to: • • • •

• • • • •

Add a service to this BeanContext: boolean addService(java.lang.Class

serviceClass, BeanContextServiceProvider serviceProvider) Add a service to this BeanContext: boolean addService(Class serviceClass, BeanContextServiceProvider bcsp, boolean fireEvent) Revoke a service: void revokeService(java.lang.Class serviceClass, BeanContextServiceProvider serviceProvider, boolean revokeCurrentServicesNow) Release a BeanContextChild's (or any arbitrary object associated with a BeanContextChild) reference to the specified service: void releaseService(BeanContextChild child, java.lang.Object requestor, java.lang.Object service) Add a BeanContextServicesListener void addBeanContextServicesListener(BeanContextServicesListener bcsl) Remove a BeanContextServicesListener: void removeBeanContextServicesListener(BeanContextServicesListener bcsl) Get the currently available services for this context: Iterator getCurrentServiceClasses()

Determine whether or not a given service is currently available from this context: boolean hasService(java.lang.Class serviceClass) Get a service from the context: Object getService(BeanContextChild child, java.lang.Object requestor, java.lang.Class serviceClass, java.lang.Object serviceSelector, BeanContextServiceRevokedListener bcsrl)

•

Get the list of service dependent service parameters (Service Selectors) for the specified service: Iterator getCurrentServiceSelectors(java.lang.Class serviceClass)

Service Event Notification JavaBeans nested into a BeanContextServices implement BeanContextServicesListener to listen for new services being added, and/or BeanContextServiceRevokedListener to listen for services being revoked. There are two event types that may be intercepted by such listeners: • •

BeanContextServiceAvailableEvent: received by the BeanContextServicesListener in order to identify the service being registered. BeanContextServiceRevokedEvent: received by the BeanContextServiceRevokedListener in order to identify the service being

revoked. The Service Provider JavaBeans can query their enclosing bean context for a list of available services, or ask for a specific service by name. The service itself, however, is actually delivered by a BeanContextServiceProvider. The provider can be any object that implements the java.beans.beancontext.BeanContextServiceProvider interface. Services become available in a context via the bean context's addService() registration method. BeanContextServiceProvider offers the following three methods, which will be automatically called when a bean requests (or releases) a service from its context: • • •

Object getService(BeanContextServices bcs, java.lang.Object requestor, java.lang.Class serviceClass, java.lang.Object serviceSelector) Iterator getCurrentServiceSelectors(BeanContextServices bcs, java.lang.Class serviceClass) public void releaseService(BeanContextServices bcs, java.lang.Object requestor, java.lang.Object service) Release a

service from any object that currently has a reference to it The Service The service itself is best described by this paragraph from the specification: A service, represented by a Class object, is typically a reference to either an interface, or to an implementation that is not publicly instantiable. This Class defines an interface protocol or contract between a BeanContextServiceProvider, the factory of the service, and an arbitrary object associated with a BeanContextChild that is currently nested within the BeanContext the service is registered with.

The following section presents a sample application that uses a word counting service to count the number of words in a given text file.

A Word Counting Service Example The classes defined in this sample application are: •

•

• •

DocumentBean.java:

A JavaBean that encapsulates a File object. Create an instance of this bean by passing it a String indicating the name of the text file to represent. This bean extends BeanContextChildSupport, which allows it to listen for addition/revocation of services in its context. When the bean detects that a WordCount service has been added to the context, it requests the service to count the number of words it contains. WordCountServiceProvider.java: A class that acts as the factory for delivering the WordCount service. This class implements the BeanContextServiceProvider interface. WordCount.java: This interface defines the service itself. DocumentTester.java: The main test program.

File: DocumentBean.java import java.beans.beancontext.*; import java.io.*; import java.util.*; public final class DocumentBean extends BeanContextChildSupport { private File document; private BeanContextServices context; public DocumentBean(String fileName) { document = new File(fileName); } public void serviceAvailable(BeanContextServiceAvailableEvent bcsae) { System.out.println("[Detected a service being added to the context]"); // Get a reference to the context BeanContextServices context = bcsae.getSourceAsBeanContextServices(); System.out.println("Is the context offering a WordCount service? " + context.hasService(WordCount.class)); // Use the service, if it's available if (context.hasService(WordCount.class)) { System.out.println("Attempting to use the service..."); try {

this,

WordCount service = (WordCount)context.getService(this,

WordCount.class, document, this); System.out.println("Got the service!"); service.countWords(); } catch(Exception e) { } } } public void serviceRevoked(BeanContextServiceRevokedEvent bcsre) { System.out.println("[Detected a service being revoked from the context]"); } }

File: WordCountServiceProvider.java import java.beans.beancontext.*; import java.util.*; import java.io.*; public final class WordCountServiceProvider implements BeanContextServiceProvider { public Object getService(BeanContextServices bcs, Object requestor, Class serviceClass, Object serviceSelector) { // For this demo, we know that the cast from serviceSelector // to File will always work. final File document = (File)serviceSelector; return new WordCount() { public void countWords() { try { // Create a Reader to the DocumentBean's File BufferedReader br = new BufferedReader(new FileReader(document)); String line = null; int wordCount = 0; while ((line = br.readLine()) != null) { StringTokenizer st = new StringTokenizer(line); while (st.hasMoreTokens()) { System.out.println("Word " + (++wordCount) + " is: " + st.nextToken()); } } System.out.println("Total number of words in the document: " + wordCount); System.out.println("[WordCount service brought to you by WordCountServiceProvider]");

br.close(); } catch(Exception e) { } };

}

} public void releaseService(BeanContextServices bcs, Object requestor, Object service) { // do nothing } public Iterator getCurrentServiceSelectors(BeanContextServices bcs, Class serviceClass) { return null; // do nothing } }

File: WordCount.java public interface WordCount { public abstract void countWords(); }

File: DocumentTester.java import java.beans.beancontext.*; import java.util.*; public class DocumentTester { public static void main(String[] args) { BeanContextServicesSupport context = new BeanContextServicesSupport(); // a bean context DocumentBean doc1 = new DocumentBean("Test.txt"); context.add(doc1); context.addBeanContextServicesListener(doc1); // listen for new services WordCountServiceProvider provider = new WordCountServiceProvider(); context.addService(WordCount.class, provider); // add the service to the context } }

File: Test.txt This

text will

by the WordCount

be analyzed

service.

Output: [Detected a service being added to the context] Is the context offering a WordCount service? true Attempting to use the service... Got the service! Word 1 is: This Word 2 is: text Word 3 is: will Word 4 is: be Word 5 is: analyzed Word 6 is: by Word 7 is: the Word 8 is: WordCount Word 9 is: service. Total number of words in the document: 9 [WordCount service brought to you by WordCountServiceProvider]

AWT Containers and the BeanContextProxy Interface Sometimes, it is desirable for an AWT Container to act as a BeanContext. However, AWT Containers cannot implement the BeanContext interface directly, because of a method name collision between Component and Collection. If some AWT Component needs to act as a BeanContext, it must internally create a BeanContext instance and delegate work to it. Third parties, such as visual builder tools, can discover this BeanContext instance if the Component implements the BeanContextProxy interface.

The BeanContextProxy Interface public BeanContextChild getBeanContextProxy()

subinterface) associated with this object.

- Gets the BeanContextChild (or