Lorenz: Visitor Beans: An AspectOriented Pattern

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Lorenz Visitor BeansAn Aspect-Oriented Pattern

• Aspect-oriented pattern describes a solution to
  a tangling problem in a particular context.

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AOP control of tangling

• Demeter/Java controls the following tangling
  problems
• structure/behavior,
• navigation/behavior,
• behavior/behavior,
• object construction/structure,
• synchronization/behavior,
• remote invocation/behavior.

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Java Bean a Java class

• Provides methods and events
• Requires listens to events from other beans
• pcs.firePropertyChange("n", old, new)

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Java Beans

• Software Component Model for Java
• Introspection what methods and events are
  available. Builder tool can inspect a bean.
• Support for customization
• Events for communication between beans

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Java Beans

• Three most important features
• set of properties it exposes
• set of methods it allows other beans to call
• set of events it fires
• event listeners can be associated with event
  source
• when the event source detects something, it will
  call an appropriate method on the event listener
  object

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Java Beans

• Design-time versus run-time. A bean must be
  capable of running in a
• design environment design information,
  customization (code can be large wizard style
  customization)
• run-time environment

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Clear separationdesign-time and run-time aspects

• Design-time interfaces can be supported in a
  separate class from the run-time interfaces
• It is possible to deploy a bean at run-time
  without needing to download all its design time
  code

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Links to remote state

• Java bean runs in same address space as their
  container
• Network access mechanisms available
• Java RMI
• Java IDL CORBA
• JDBC Java data base API

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Alternate type views

• First release each bean a single Java object
• Future releases beans that are implemented as a
  set of cooperating objects.
• Type view represents a view of a bean as a given
  type
• In future releases different type views of a
  bean may be implemented by different Java objects.

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Events
Source
Listener
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Goal

• Enable discovery of the events that a particular
  class may generate
• Enable the discovery of the events a class may
  observe
• Common event registration mechanism for dynamic
  manipulation of relationship between sources and
  listeners

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Design patternsused with a different meaning

• The standard design pattern for EventListener
  registration (multicast event source)
• public void add(
  listener)
• public void remove (
  listener)
• should be synchronized methods

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Java Beans preferred format

• public PropertyType get()
  // simple getter method
• public void set(PropertyType var)
  // simple setter method
• public PropertyType get(int index)
  // indexed getter method
• public void set(int index,
  PropertyType var) // setter method
• public PropertyType get()
  // indexed getter method
• public void set(PropertyType
  var) // indexed setter method

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Defining events and listeners

• public class ContainerEventObject extends
  EventObject
• public interface ContainerListener extends
  EventListener
• void handleContainerEvent(Conta
  inerEventObject ceo)

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Listeners for Container bean

• private Vector containerListeners new Vector()
• public synchronized void addContainerListener(Cont
  ainerListener l)
• containerListeners.addElement(l)
• public synchronized void removeContainerListener(C
  ontainerListener l)
• containerListeners.removeElement(l)

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Broadcasting event

• public void broadcastContainerEvent()
• Vector l
• ContainerEventObject ceo new
  ContainerEventObject(this)
• synchronized(this)
• l (Vector)containerListeners.clone()
• for (int i 0 i
• ContainerListener cl (ContainerListener)
  l.elementAt(i)
• cl.handleContainerEvent(ceo)

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Visitor Bean

• Accept methods use reflection to define a
  traversal visitor!
• Visit methods Instead of sending a visitor to
  visit an element, the element is fired to the
  visitor as part of an event.

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In class Visitor

• public void visit_dispatch(Object o)
• Class formal new Class1
• formal0 o.getClass()
• try
• Method m getClass().getMethod(visit,
  formal)
• Object actual new Object1
• actual0 o
• m.invoke(this, actual)
• catch (Exception e)

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Visit events

• Visitors report their actions via events
• event objects may contain visited objects
• visitors may register as event listeners to other
  visitors
• Events are typed registration is typesafe

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VisitEvent

• Package visitor.beans.event
• import java.util.EventObject
• public class VisitEvent extends EventObject
• protected VisitEvent(Object source, int type,
  Object visited, Object data)
• super(source)
• fieldType type
• fieldVisited visited
• fieldData data
• private transient int fieldType 0

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Visitor kinds

• Traversal visitors
• manipulate the visitee part and pass data without
  looking at it
• Computational visitors
• pass the visitee untouched and manipulate only
  the data

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Adaptive Bean visitor style
strategy
in
Local adaptive beans
out
nodes edges enter exit
terminate node edge
Traversal status events (contain objects
traversed)