The BeanInfo Interface

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Trail: JavaBeans(TM)

Lesson: The BeanInfo Interface

The following documentation will help you learn about reflection, introspection, and the BeanInfo class:

JavaBeans API Specification Chapter 8

Core Reflection Documentation
The Reflection API trail
JDK Reflection documentation

Beans API Documentation
BeanInfo interface
SimpleBeanInfo class
Introspector class
FeatureDescriptor class
BeanDescriptor class
EventSetDescriptor class
PropertyDescriptor class
IndexedPropertyDescriptor class
MethodDescriptor class
ParameterDescriptor class

How does a builder tool examine a Bean and expose its features (properties, events, and methods) in a property sheet? By using the java.beans.Introspector class. The Introspector class uses the JDK core reflection API to discover a Bean's methods, and then applies the JavaBeans design patterns to discover the Beans features. This discovery process is named introspection.
Alternatively, you can explicitly expose a Bean's features in a separate, associated class that implements the BeanInfo interface. By associating a BeanInfo class with your Bean, you can:

Expose only those features you want to expose.
Rely on BeanInfo to expose some Bean features while relying on low-level reflection to expose others.
Associate an icon with the target Bean.
Specify a customizer class.
Segregate features into normal and expert categories.
Provide a more descriptive display name, or additional information about a Bean feature.

BeanInfo defines methods that return descriptors for each property, method, or event that you want exposed. Here's the prototypes for these methods:
PropertyDescriptor[] getPropertyDescriptors();
MethodDescriptor[]   getMethodDescriptors();
EventSetDescriptor[] getEventSetDescriptors();
Each of these methods returns an array of descriptors for each feature.

Feature Descriptors

BeanInfo classes contain descriptors that precisely describe the target Bean's features. The BDK implements the following descriptor classes:

FeatureDescriptor is the base class for the other descriptor classes. It declares the aspects common to all descriptor types.
BeanDescriptor describes the target Bean's class type and name, and describes the target Bean's customizer class if it exists.
PropertyDescriptor describes the target Bean's properties.
IndexedPropertyDescriptor is a subclass of PropertyDescriptor, and describes the target Bean's indexed properties.
EventSetDescriptor describes the events the target Bean fires.
MethodDescriptor describes the target Bean's methods.
ParameterDescriptor describes method parameters.

The BeanInfo interface declares methods that return arrays of the above descriptors.

Creating a BeanInfo Class

This section uses the ExplicitButtonBeanInfo demo class to illustrate creating a BeanInfo class. Here are the general steps to make a BeanInfo class:

Name your BeanInfo class. You must append the string "BeanInfo" to the target class name. If the target class name is ExplicitButton, then its associated Bean information class must be named ExplicitButtonBeanInfo
Subclass SimpleBeanInfo. This is a convenience class that implements BeanInfo methods to return null, or an equivalent no-op value.
```
public class ExplicitButtonBeanInfo extends SimpleBeanInfo {
```
Using SimpleBeanInfo saves you from implementing all the BeanInfo methods; you only have to override those methods you need.
Override the appropriate methods to return the properties, methods, or events that you want exposed. ExplicitButtonBeanInfo overrides the getPropertyDescriptors() method to return four properties:
```
public PropertyDescriptor[] getPropertyDescriptors() {
    try {  
        PropertyDescriptor background =
            new PropertyDescriptor("background", beanClass);
        PropertyDescriptor foreground =
            new PropertyDescriptor("foreground", beanClass);
        PropertyDescriptor font =
            new PropertyDescriptor("font", beanClass);
        PropertyDescriptor label =
            new PropertyDescriptor("label", beanClass);
 
        background.setBound(true);
        foreground.setBound(true);
        font.setBound(true);
        label.setBound(true);
         
        PropertyDescriptor rv[] =
            {background, foreground, font, label};
        return rv;
    } catch (IntrospectionException e) {
         throw new Error(e.toString());
    }
}        
```
There are two important things to note here:
- If you leave a descriptor out, that property, event or method will not be exposed. In other words, you can selectively expose properties, events, or methods by leaving out those you don't want exposed.
- If a feature's getter (for example, getMethodDescriptor()) method returns null, low-level reflection is then used for that feature. This means, for example, that you can explicitly specify properties, and let low-level reflection discover the methods. If you don't override the SimpleBeanInfo default method, which returns null, low-level reflection will be used for that feature.

Optionally associate an icon with the target Bean.

public java.awt.Image getIcon(int iconKind) {
    if (iconKind == BeanInfo.ICON_MONO_16x16 ||
        iconKind == BeanInfo.ICON_COLOR_16x16 )
    {
        java.awt.Image img = loadImage("ExplicitButtonIcon16.gif");
        return img;
    }
    if (iconKind == BeanInfo.ICON_MONO_32x32 ||
        iconKind == BeanInfo.ICON_COLOR_32x32 )
    {
        java.awt.Image img = loadImage("ExplicitButtonIcon32.gif");
        return img;
    }
    return null;
}

The BeanBox displays this icon next to the Bean name in the ToolBox. You can expect builder tools to do the same.

Specify the target Bean class, and, if the Bean has a customizer, specify it also.

public BeanDescriptor getBeanDescriptor() {
    return new BeanDescriptor(beanClass, customizerClass);
}
...
private final static Class beanClass = ExplicitButton.class;
private final static Class customizerClass = OurButtonCustomizer.class;

Keep the BeanInfo class in the same directory as its target class. The BeanBox first searches for a target Bean's BeanInfo class in the target Bean's package path. If no BeanInfo is found, then the Bean information package search path (maintained by the Introspector) is searched. The default Bean information search path is sun.beans.infos. If no BeanInfo class is found, then low-level reflection is used to discover a Bean's features.

Using BeanInfo to Control What Features are Exposed

If you rely on low-level reflection to discover your Bean's features, all those properties, methods, and events that conform to the appropriate design patterns will be exposed in a builder tool. This includes any features in all base classes. If the BeanBox finds an associated BeanInfo class, then that information is used instead, and no more base classes are examined using reflection. In other words, BeanInfo information overrides low-level reflection information, and prevents base class examination.

By using a BeanInfo class, you can expose subsets of a particular Bean feature. For example, by not returning a method descriptor for a particular method, that method will not be exposed in a builder tool.

When you use a BeanInfo class:

Base class features will not be exposed. You can retrieve base class features by using the BeanInfo.getAdditionalBeanInfo method.
Properties, events, or methods that have no descriptor will not be exposed. For a particular feature, only those items returned in the descriptor array will be exposed. For example, if you return descriptors for all your Bean methods except foo, then foo will not be exposed.
Low-level reflection will be used for features with getter methods returning null. For example if your BeanInfo class contains this method implementation:
```
public MethodDescriptor[] getMethodDescriptors() {
    return null;
}
```
Then low-level reflection will be used to discover your Bean's public methods.

Locating BeanInfo Classes

Before examining a Bean, the Introspector will attempt to find a BeanInfo class associated with the Bean. By default, the Introspector takes the target Bean's fully qualified package name, and appends "BeanInfo" to form a new class name. For example, if the target Bean is sunw.demo.buttons.ExplicitButton, then the Introspector will attempt to locate sunw.demo.buttons.ExplicitButtonBeanInfo.

If that fails, then each package in the BeanInfo search path is searched. The BeanInfo search path is maintained by Introspector.setBeanInfoSearchPath() and Introspector.getBeanInfoSearchPath().

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