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How does a builder tool examine a Bean and expose its features (properties, events, and methods) in a property sheet? By using the
The following documentation will help you learn about reflection, introspection, and theBeanInfo
class:
- Chapter 8 of the JavaBeans API Specification
- Core Reflection Documentation:
- The Reflection API trail
- JDK Reflection documentation
- Beans API Documentation:
BeanInfo
interfaceSimpleBeanInfo
classIntrospector
classFeatureDescriptor
classBeanDescriptor
classEventSetDescriptor
classPropertyDescriptor
classIndexedPropertyDescriptor
classMethodDescriptor
classParameterDescriptor
classjava.beans.Introspector
class. TheIntrospector
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 aBeanInfo
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:Each of these methods returns an array of descriptors for each feature.PropertyDescriptor[] getPropertyDescriptors(); MethodDescriptor[] getMethodDescriptors(); EventSetDescriptor[] getEventSetDescriptors();
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 ofPropertyDescriptor
, 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.
This section uses the
ExplicitButtonBeanInfo
demo class to illustrate creating aBeanInfo
class. Here are the general steps to make aBeanInfo
class:
- Name your
BeanInfo
class. You must append the string "BeanInfo" to the target class name. If the target class name isExplicitButton
, then its associated Bean information class must be namedExplicitButtonBeanInfo
- Subclass
SimpleBeanInfo
. This is a convenience class that implementsBeanInfo
methods to return null, or an equivalent no-op value.Usingpublic class ExplicitButtonBeanInfo extends SimpleBeanInfo {SimpleBeanInfo
saves you from implementing all theBeanInfo
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 thegetPropertyDescriptors()
method to return four properties:There are two important things to note here: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()); } }
- 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 theSimpleBeanInfo
default method, which returns null, low-level reflection will be used for that feature.
- Optionally associate an icon with the target Bean.
The BeanBox displays this icon next to the Bean name in the ToolBox. You can expect builder tools to do the same.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; }
- 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'sBeanInfo
class in the target Bean's package path. If noBeanInfo
is found, then the Bean information package search path (maintained by theIntrospector
) is searched. The default Bean information search path issun.beans.infos
. If noBeanInfo
class is found, then low-level reflection is used to discover a Bean's features.
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
, thenfoo
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:Then low-level reflection will be used to discover your Bean's public methods.public MethodDescriptor[] getMethodDescriptors() { return null; }
Before examining a Bean, the
Introspector
will attempt to find aBeanInfo
class associated with the Bean. By default, theIntrospector
takes the target Bean's fully qualified package name, and appends "BeanInfo" to form a new class name. For example, if the target Bean issunw.demo.buttons.ExplicitButton
, then theIntrospector
will attempt to locatesunw.demo.buttons.ExplicitButtonBeanInfo
.If that fails, then each package in the
BeanInfo
search path is searched. TheBeanInfo
search path is maintained byIntrospector.setBeanInfoSearchPath()
andIntrospector.getBeanInfoSearchPath()
.
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