Java beans

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Java beans

• Part 1 introspecting beans

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What are Java beans?

• Java beans are simply Java classes that exposes
  properties and events by a naming convention
• The Java beans mechanism allows introspection and
  manipulation of such classes using reflection
• Get a list of properties
• Get or set a specific property
• Get a list of events
• Add and remove listeners

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Why Java beans?

• Reflection is too general to handle software
  components well. A naming convention standard
  mechanism encourages using comp.
• The most commonly needed form of reflection is
  exposing properties and events
• Properties determines the state of the object
• Configuration by external tools
• Displaying, serializing
• Events allows activating it

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Java beans properties

• A property is determined by methods names
• A property named foo of type Goo can have
• Getter for reading Goo getFoo()
• Setter for writing void setFoo(Goo goo)
• So a class having such getter/setter method,
  defines its properties
• The Java beans mechanism uses reflection to find
  these methods to describe properties

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Class and properties example

• class MyBean
• public Color getForeground()
• public void setForeground(Color c)
• public int getSize()
• public void setFont(Font f)

• The class MyBean has the following properties
• Read/Write foreground Color
• Read-only size int
• Write-only font Font

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Other properties

• Beside the simple properties, a bean can have
• Indexed properties like simple but with an index
• Getter Goo getFoo(int i)
• Setter void setFoo(int i, Goo foo)
• All getter Goo getFoo
• All setter void setFoo(Goo foo)
• Bounded properties when changed, a
  PropertyChangedEvent is fired
• Constrained properties can be vetoed by listener

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Introspecting a bean class

• Java beans classes package java.beans
• The class Introspector allows getting a BeanInfo
  on a certain bean class
• The BeanInfo allows getting
• Property descriptors (PropertyDescriptor)
• Event descriptors (EventDescriptor)
• Method descriptors (MethodDescriptor)
• For the general use

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Property descriptors

• The PropertyDescriptor allow getting
• The propertys name, type and other info
• The getter method (if any)
• The setter method (if any)
• From this point, straight reflection can be used
  to effect the bean object itself
• The descriptor can be obtained either
• From a class introspection
• Directly by constructor (if property name is
  known)

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Other property descriptors

• The general property descriptor also describes
  bounded and constrained properties
• Has checks isBound() and isConstrained()
• A sub-class IndexedPropertyDescriptor allows
  accessing the methods for indexed property access

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Property editors (1)

• One main problem with opaque beans is that the
  input/output is many times in text format.
• To help, the beans mechanism includes a way to
  translate strings to objects and vice verse.
• The PropertyEditor interface declares methods for
  translating to/from text
• A static PropertyEditorManager handles getting
  and registering editors

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Property editors (2)

• An editor for a given property can be acquired by
  calling the managers static method
  findEditor(Class type)
• Notice that the type is the type of the property
• Returns a property editor for the given property
• The editor has the translation methods
• setAsText(text), String getAsText()
• Object getValue(), setValue(Object)

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Possible usage of Java beans

• Visual editing
• Display a table of properties names and values
• When values change, it effects the displayed bean
• Storage
• Get properties and values and write them to a
  file
• Load the file, create a new object and set
  properties
• ? The new XML serializer for beans!
• Display
• Use a generic table to display content of any
  object

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Problems with Java beans

• The Java beans mechanism is more structured and
  organized then raw reflection, but it still has
  its drawbacks
• It is based on a naming convention(And therefore
  the good will of other programmers)
• It requires a lot of type checks
• It still uses reflection!
• ? Better then reflection, but still a last
  resort