Visual programming and Beans

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and Beans

What is a Bean?

After the dust settles, then, a component is really just a block of code, typically embodied in a class. The key issue is the ability for the application builder tool to discover the properties and events for that component. To create a VB component, the programmer had to write a fairly complicated piece of code following certain conventions to expose the properties and events. Delphi was a second-generation visual programming tool and the language was actively designed around visual programming so it is much easier to create a visual component. However, Java has brought the creation of visual components to its most advanced state with Java Beans, because a Bean is just a class. You don’t have to write any extra code or use special language extensions in order to make something a Bean. The only thing you need to do, in fact, is slightly modify the way that you name your methods. It is the method name that tells the application builder tool whether this is a property, an event, or just an ordinary method.

  1. For a property named xxx, you typically create two methods: getXxx( ) and setXxx( ). Note that the first letter after get or set is automatically lowercased to produce the property name. The type produced by the “get” method is the same as the type of the argument to the “set” method. The name of the property and the type for the “get” and “set” are not related.
  2. For a boolean property, you can use the “get” and “set” approach above, but you can also use “is” instead of “get.”
  3. Ordinary methods of the Bean don’t conform to the above naming convention, but they’re public.
  4. For events, you use the “listener” approach. It’s exactly the same as you’ve been seeing: addFooBarListener(FooBarListener) and removeFooBarListener(FooBarListener) to handle a FooBarEvent. Most of the time the built-in events and listeners will satisfy your needs, but you can also create your own events and listener interfaces.
//: Frog.java
// A trivial Java Bean
package frogbean;
import java.awt.*;
import java.awt.event.*;
 
class Spots {}
 
public class Frog {
  private int jumps;
  private Color color;
  private Spots spots;
  private boolean jmpr;
  public int getJumps() { return jumps; }
  public void setJumps(int newJumps) { 
    jumps = newJumps;
  }
  public Color getColor() { return color; }
  public void setColor(Color newColor) { 
    color = newColor; 
  }
  public Spots getSpots() { return spots; }
  public void setSpots(Spots newSpots) {
    spots = newSpots; 
  }
  public boolean isJumper() { return jmpr; }
  public void setJumper(boolean j) { jmpr = j; }
  public void addActionListener(
      ActionListener l) {
    //...
  }
  public void removeActionListener(
      ActionListener l) {
    // ...
  }
  public void addKeyListener(KeyListener l) {
    // ...
  }
  public void removeKeyListener(KeyListener l) {
    // ...
  }
  // An "ordinary" public method:
  public void croak() {
    System.out.println("Ribbet!");
  }
} ///:~ 

First, you can see that it’s just a class. Usually, all your fields will be private, and accessible only through methods. Following the naming convention, the properties are jumps, color, spots, and jumper (notice the change in case of the first letter in the property name). Although the name of the internal identifier is the same as the name of the property in the first three cases, in jumper you can see that the property name does not force you to use any particular name for internal variables (or, indeed, to even have any internal variable for that property).

The events this Bean handles are ActionEvent and KeyEvent, based on the naming of the “add” and “remove” methods for the associated listener. Finally, you can see that the ordinary method croak( ) is still part of the Bean simply because it’s a public method, not because it conforms to any naming scheme.

Extracting BeanInfo

with the Introspector

One of the most critical parts of the Bean scheme occurs when you drag a Bean off a palette and plop it down on a form. The application builder tool must be able to create the Bean (which it can do if there’s a default constructor) and then, without access to the Bean’s source code, extract all the necessary information to create the property sheet and event handlers.

//: BeanDumper.java
// A method to introspect a Bean
import java.beans.*;
import java.lang.reflect.*;
 
public class BeanDumper {
  public static void dump(Class bean){
    BeanInfo bi = null;
    try {
      bi = Introspector.getBeanInfo(
        bean, java.lang.Object.class);
    } catch(IntrospectionException ex) {
      System.out.println("Couldn't introspect " +
        bean.getName());
      System.exit(1);
    }
    PropertyDescriptor[] properties = 
      bi.getPropertyDescriptors();
    for(int i = 0; i < properties.length; i++) {
      Class p = properties[i].getPropertyType();
      System.out.println(
        "Property type:\n  " + p.getName());
      System.out.println(
        "Property name:\n  " + 
        properties[i].getName());
      Method readMethod = 
        properties[i].getReadMethod();
      if(readMethod != null)
        System.out.println(
          "Read method:\n  " + 
          readMethod.toString());
      Method writeMethod = 
        properties[i].getWriteMethod();
      if(writeMethod != null)
        System.out.println(
          "Write method:\n  " +
          writeMethod.toString());
      System.out.println("====================");
    }
    System.out.println("Public methods:");
    MethodDescriptor[] methods =
      bi.getMethodDescriptors();
    for(int i = 0; i < methods.length; i++)
      System.out.println(
        methods[i].getMethod().toString());
    System.out.println("======================");
    System.out.println("Event support:");
    EventSetDescriptor[] events = 
      bi.getEventSetDescriptors();
    for(int i = 0; i < events.length; i++) {
      System.out.println("Listener type:\n  " +
        events[i].getListenerType().getName());
      Method[] lm = 
        events[i].getListenerMethods();
      for(int j = 0; j < lm.length; j++)
        System.out.println(
          "Listener method:\n  " +
          lm[j].getName());
      MethodDescriptor[] lmd = 
        events[i].getListenerMethodDescriptors();
      for(int j = 0; j < lmd.length; j++)
        System.out.println(
          "Method descriptor:\n  " +
          lmd[j].getMethod().toString());
      Method addListener = 
        events[i].getAddListenerMethod();
      System.out.println(
          "Add Listener Method:\n  " +
        addListener.toString());
      Method removeListener =
        events[i].getRemoveListenerMethod();
      System.out.println(
        "Remove Listener Method:\n  " +
        removeListener.toString());
      System.out.println("====================");
    }
  }
  // Dump the class of your choice:
  public static void main(String[] args) {
    if(args.length < 1) {
      System.err.println("usage: \n" +
        "BeanDumper fully.qualified.class");
      System.exit(0);
    }
    Class c = null;
    try {
      c = Class.forName(args[0]);
    } catch(ClassNotFoundException ex) {
      System.err.println(
        "Couldn't find " + args[0]);
      System.exit(0);
    }
    dump(c);
  }
} ///:~ 

BeanDumper.dump( ) is the method that does all the work. First it tries to create a BeanInfo object, and if successful calls the methods of BeanInfo that produce information about properties, methods, and events. In Introspector.getBeanInfo( ), you’ll see there is a second argument. This tells the Introspector where to stop in the inheritance hierarchy. Here, it stops before it parses all the methods from Object, since we’re not interested in seeing those.

class name: Frog
Property type:
  Color
Property name:
  color
Read method:
  public Color getColor()
Write method:
  public void setColor(Color)
====================
Property type:
  Spots
Property name:
  spots
Read method:
  public Spots getSpots()
Write method:
  public void setSpots(Spots)
====================
Property type:
  boolean
Property name:
  jumper
Read method:
  public boolean isJumper()
Write method:
  public void setJumper(boolean)
====================
Property type:
  int
Property name:
  jumps
Read method:
  public int getJumps()
Write method:
  public void setJumps(int)
====================
Public methods:
public void setJumps(int)
public void croak()
public void removeActionListener(ActionListener)
public void addActionListener(ActionListener)
public int getJumps()
public void setColor(Color)
public void setSpots(Spots)
public void setJumper(boolean)
public boolean isJumper()
public void addKeyListener(KeyListener)
public Color getColor()
public void removeKeyListener(KeyListener)
public Spots getSpots()
======================
Event support:
Listener type:
  KeyListener
Listener method:
  keyTyped
Listener method:
  keyPressed
Listener method:
  keyReleased
Method descriptor:
  public void keyTyped(KeyEvent)
Method descriptor:
  public void keyPressed(KeyEvent)
Method descriptor:
  public void keyReleased(KeyEvent)
Add Listener Method:
  public void addKeyListener(KeyListener)
Remove Listener Method:
  public void removeKeyListener(KeyListener)
====================
Listener type:
  ActionListener
Listener method:
  actionPerformed
Method descriptor:
  public void actionPerformed(ActionEvent)
Add Listener Method:
  public void addActionListener(ActionListener)
Remove Listener Method:
  public void removeActionListener(ActionListener)
====================

This reveals most of what the Introspector sees as it produces a BeanInfo object from your Bean. You can see that the type of the property and its name are independent. Notice the lowercasing of the property name. (The only time this doesn’t occur is when the property name begins with more than one capital letter in a row.) And remember that the method names you’re seeing here (such as the read and write methods) are actually produced from a Method object that can be used to invoke the associated method on the object.

The public method list includes the methods that are not associated with a property or event, such as croak( ), as well as those that are. These are all the methods that you can call programmatically for a Bean, and the application builder tool can choose to list all of these while you’re making method calls, to ease your task.

Finally, you can see that the events are fully parsed out into the listener, its methods, and the add- and remove-listener methods. Basically, once you have the BeanInfo, you can find out everything of importance for the Bean. You can also call the methods for that Bean, even though you don’t have any other information except the object (again, a feature of reflection).

A more sophisticated Bean

This next example is slightly more sophisticated, albeit frivolous. It’s a canvas that draws a little circle around the mouse whenever the mouse is moved. When you press the mouse, the word “Bang!” appears in the middle of the screen, and an action listener is fired.

//: BangBean.java
// A graphical Bean
package bangbean;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
 
public class BangBean extends Canvas
     implements Serializable {
  protected int xm, ym;
  protected int cSize = 20; // Circle size
  protected String text = "Bang!";
  protected int fontSize = 48;
  protected Color tColor = Color.red;
  protected ActionListener actionListener;
  public BangBean() {
    addMouseListener(new ML());
    addMouseMotionListener(new MML());
  }
  public int getCircleSize() { return cSize; }
  public void setCircleSize(int newSize) {
    cSize = newSize;
  }
  public String getBangText() { return text; }
  public void setBangText(String newText) {
    text = newText;
  }
  public int getFontSize() { return fontSize; }
  public void setFontSize(int newSize) {
    fontSize = newSize;
  }
  public Color getTextColor() { return tColor; }
  public void setTextColor(Color newColor) {
    tColor = newColor;
  }
  public void paint(Graphics g) {
    g.setColor(Color.black);
    g.drawOval(xm - cSize/2, ym - cSize/2, 
      cSize, cSize);
  }
  // This is a unicast listener, which is
  // the simplest form of listener management:
  public void addActionListener (
      ActionListener l) 
        throws TooManyListenersException {
    if(actionListener != null)
      throw new TooManyListenersException();
    actionListener = l;
  }
  public void removeActionListener(
      ActionListener l) {
    actionListener = null;
  }
  class ML extends MouseAdapter {
    public void mousePressed(MouseEvent e) {
      Graphics g = getGraphics();
      g.setColor(tColor);
      g.setFont(
        new Font(
          "TimesRoman", Font.BOLD, fontSize));
      int width = 
        g.getFontMetrics().stringWidth(text);
      g.drawString(text, 
        (getSize().width - width) /2,
        getSize().height/2);
      g.dispose();
      // Call the listener's method:
      if(actionListener != null)
        actionListener.actionPerformed(
          new ActionEvent(BangBean.this,
            ActionEvent.ACTION_PERFORMED, null));
    }
  }
  class MML extends MouseMotionAdapter {
    public void mouseMoved(MouseEvent e) {
      xm = e.getX();
      ym = e.getY();
      repaint();
    }
  }
  public Dimension getPreferredSize() {
    return new Dimension(200, 200);
  }
  // Testing the BangBean:
  public static void main(String[] args) {
    BangBean bb = new BangBean();
    try {
      bb.addActionListener(new BBL());
    } catch(TooManyListenersException e) {}
    Frame aFrame = new Frame("BangBean Test");
    aFrame.addWindowListener(
      new WindowAdapter() {
        public void windowClosing(WindowEvent e) {
          System.exit(0);
        }
      });
    aFrame.add(bb, BorderLayout.CENTER);
    aFrame.setSize(300,300);
    aFrame.setVisible(true);
  }
  // During testing, send action information
  // to the console:
  static class BBL implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      System.out.println("BangBean action");
    }
  }
} ///:~ 

Packaging a Bean

Manifest-Version: 1.0
 
Name: bangbean/BangBean.class
Java-Bean: True

The first line indicates the version of the manifest scheme, which until further notice from Sun is 1.0. The second line (empty lines are ignored) names the BangBean.class file, and the third says, “It’s a Bean.” Without the third line, the program builder tool will not recognize the class as a Bean.

The only tricky part is that you must make sure that you get the proper path in the “Name:” field. If you look back at BangBean.java, you’ll see it’s in package bangbean (and thus in a subdirectory called “bangbean” that’s off of the classpath), and the name in the manifest file must include this package information. In addition, you must place the manifest file in the directory above the root of your package path, which in this case means placing the file in the directory above the “bangbean” subdirectory. Then you must invoke jar from the same directory as the manifest file, as follows:

You might wonder “What about all the other classes that were generated when I compiled BangBean.java?” Well, they all ended up inside the bangbean subdirectory, and you’ll see that the last argument for the above jar command line is the bangbean subdirectory. When you give jar the name of a subdirectory, it packages that entire subdirectory into the jar file (including, in this case, the original BangBean.java source-code file – you might not choose to include the source with your own Beans). In addition, if you turn around and unpack the JAR file you’ve just created, you’ll discover that your manifest file isn’t inside, but that jar has created its own manifest file (based partly on yours) called MANIFEST.MF and placed it inside the subdirectory META-INF (for “meta-information”). If you open this manifest file you’ll also notice that digital signature information has been added by jar for each file, of the form:

Digest-Algorithms: SHA MD5 
SHA-Digest: pDpEAG9NaeCx8aFtqPI4udSX/O0=
MD5-Digest: O4NcS1hE3Smnzlp2hj6qeg==

In general, you don’t need to worry about any of this, and if you make changes you can just modify your original manifest file and re-invoke jar to create a new JAR file for your Bean. You can also add other Beans to the JAR file simply by adding their information to your manifest.

One thing to notice is that you’ll probably want to put each Bean in its own subdirectory, since when you create a JAR file you hand the jar utility the name of a subdirectory and it puts everything in that subdirectory into the JAR file. You can see that both Frog and BangBean are in their own subdirectories.

More complex Bean support

You can see how remarkably simple it is to make a Bean. But you aren’t limited to what you’ve seen here. The Java Bean design provides a simple point of entry but can also scale to more complex situations. These situations are beyond the scope of this book but they will be briefly introduced here. You can find more details at http://java.sun.com/beans.

  1. You can provide a custom property sheet for your particular Bean. The ordinary property sheet will be used for all other Beans, but yours is automatically invoked when your Bean is selected.
  2. You can create a custom editor for a particular property, so the ordinary property sheet is used, but when your special property is being edited, your editor will automatically be invoked.
  3. You can provide a custom BeanInfo class for your Bean that produces information that’s different from the default created by the Introspector.
  4. It’s also possible to turn “expert” mode on and off in all FeatureDescriptors to distinguish between basic features and more complicated ones.

More to Beans



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