Transcript Introduction to Component

Introduction to
Component-Based Engineering
Howard Abrams
[email protected]
Object-Oriented Design
 Goals:
• Ease of Maintenance
• Robustness
• Code Reuse
 Primary sources of reuse:
• Libraries and web services
 Marketplace of “object stores” not realized
Go Beyond Objects!
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Nature of
Software Components
}
 Unification of Data and Function
 Data Hiding
 Unique Identity
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Independent
Interactive
Replaceable
Reusable
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}
OOP
Use existing OOP
techniques, but
architect your apps
as components.
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Component Independence
 Objects are extremely sticky.
• Objects as small as possible (object normalization)
• More efficient, but have unnecessary dependencies
 CBD Solution:
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Self contained and independent
Pass only “simple” (widely accepted) variables
Don’t make them too general
Components are slightly larger / Built using OOP
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Component Interaction
 Components are not plugins:
• Reusable in multiple applications
• Components can be connected to each other
 Components built for an architecture:
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Java Beans
Enterprise Java Beans
JSP Tag Libraries
Web Services
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Component Replacement
 Objects are only mildly replaceable.
 Components must implement interfaces.
• Contracts or agreements
• Like a component’s Java Bean specification
 Interfaces “out” are events (callbacks).
 Think carefully about altering contract
• You can upgrade (add features) but don’t change
• Use versioning system or support old interfaces
 When components only talk via interfaces, you
can upgrade/enhance with lower risk.
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Object Replacement?
 Replacing objects
means editing
all references
 System Testing Critical
• Forgotten references
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Component Replacement
 Each “unit” only talks
to another unit via an
interface.
 Goal: Components
can be safely replaced
by unit testing the
component’s interface
 System testing is now
less critical
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Components Replacement
First Name
 Component-to-component
communication not always
possible…
Last Name
Do you convert the
data prior to calling?
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Customer Name
Do you modify the
component to accept
different data?
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Components and Glue Code
 Identify/isolate “glue code” put in controller
 Components more independent/reusable
 Easier to replace components
Control
(App Rules and
Glue Code)
Model
View
(Components)
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(User Interface)
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Component Advantages
 Can be upgraded
• Unit testing concentrates on testing interface
• Lower emphasis on system testing
• You can add, but you can’t take away
 Can be reused
• Components can be made useful
• Less coding as your “Bag o’ Components”
grows
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Component Tips
 Don’t pass around objects. Keep it simple.
 Concentrate more on independence and
reuse than on “object normalization.”
 Create components based on “services”
Function instead of form or features.
 Unit testing is critical and essential.
 Avoid the temptation to put in everything
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Component Tips (con’t)
 Live your component interface:
• Java Bean
• JSP Tag Library
 Create a level of trust in your components
 Pretend others will use your components
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Clean and simple interface
Liberal documentation
Include your JUnit test suite
Include examples of using the component
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Component Tips (con’t)
BeanInfo
Property
Editors
JavaBean
 Add more interfaces to
increase reuse
Component
Functionality
JSP Tags
Tag 1 Tag 2 Tag 3
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 Code to the lowest
deployment requirements
 Non-GUI JavaBean
is most versatile
 JSP TagLib interface
• Properties == Parameters
• Need multiple methods?
Create “tag wrappers”
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XmlQuery JavaBean Example
 Need to retrieve data from XML file
 XML libraries quite complex …
Writing the same marshalling code over and over
 Wrapper around XPath and DOM libraries
 Properties:
• File - XML File
• Query - XPath Expression
• TrimWhiteSpace - boolean
 Method: getValue() returns a String
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XmlQuery is Serializable
public class XmlQuery implements Serializable
{
// Property to hold the XPath query expression…
protected String query = null;
// Property when set will trim all returned values.
protected boolean trimWhitespace = false;
// Property to hold the XML file to parse
protected File file = null;
// Transient factory classes re-established as needed.
transient DocumentBuilder docbuilder = null;
transient DocumentBuilderFactory docfactory = null;
// Support object to generate PropertyChange events.
PropertyChangeSupport changeSupport =
new PropertyChangeSupport(this);
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Serialization Issues
 Transient variables
• Stateless variables
• Those that loose meaning on another system
• Factory methods
 Don’t initialize transients in constructor
• Initialize them as you need them
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XmlQuery Setters/Getters
public String getQuery()
{
if (query == null)
return "";
else
return query;
}
public void setQuery (String newQuery)
{
String oldQuery = query;
query = newQuery;
changeSupport.firePropertyChange("query",
oldQuery, query);
}
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Change Support Methods
public synchronized void
addPropertyChangeListener (PropertyChangeListener listener)
{
if (changeSupport == null)
changeSupport = new PropertyChangeSupport(this);
changeSupport.addPropertyChangeListener(listener);
}
public synchronized void
removePropertyChangeListener (PropertyChangeListener listener)
{
if (changeSupport == null)
changeSupport = new PropertyChangeSupport(this);
changeSupport.removePropertyChangeListener(listener);
}
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Working with Bean Editors
 Need to override the BeanInfo?
• Specify an icon representation
• Give a nicer interface to properties/methods
 Manifest File
org/howardism/xml/XmlQuery.class
• By hand: Name:
Java-Bean: true
• Via Ant: <jar jarfile="${dist-beans}/xmlquery.jar">
<fileset dir="${build}" includes="org/howardism/**"/>
<manifest>
<section name="org/howardism/xml/XmlQuery.class">
<attribute name="Java-Bean" value="true"/>
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Property Editors
 If a property accepts any but basic types,
you’ll want to create an external editor.
 You can create elaborate GUI editors…
 A simple “text bridge” is usually better:
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Property Editors: Example
public class ExceptionEditor extends PropertyEditorSupport
{
public String getAsText()
{
if (getValue() == null)
return "";
else
return ((Throwable) getValue()).getClass().getName();
}
public void setAsText(String value) throws IllegalArgumentException
{
if (value != null && ! value.trim().equals(""))
setValue( Class.forName(value).newInstance());
}
}
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Property Editors: Example
AppComposer
Sun’s BeanBox
Sun’s BeanBuilder
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Alarm JavaBean Example
 Generates an “event” at a particular time
 Properties:
• Time- Particular date or time in the future
• Identity- To label each event
 Methods:
• startAlarm- Must be called
• stopAlarm- To stop event prematurely
 Generates a “AlarmEvent” to listeners
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Alarm Listener Methods
public synchronized void
addAlarmListener (AlarmListener listener)
{
if (alarmListeners == null)
alarmListeners = new java.util.Vector();
alarmListeners.addElement(listener);
}
public synchronized void
removeAlarmListener (AlarmListener listener)
{
if (alarmListeners == null)
return;
alarmListeners.removeElement(listener);
}
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Alarm’s fireAlarm Method
protected void fireAlarm (String identity)
{
java.util.Vector targets = null;
synchronized (this)
{
if (alarmListeners != null)
targets = (java.util.Vector) alarmListeners.clone();
}
if (targets != null)
{
AlarmEvent evt = new AlarmEvent(identity);
for (int i = 0; i < targets.size(); i++)
{
AlarmListener target = targets.elementAt(i);
target.alarmGenerated(evt);
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AlarmListener / AlarmEvent
public interface AlarmListener extends EventListener
{
public void alarmGenerated (AlarmEvent event);
}
public class AlarmEvent extends EventObject
{
public AlarmEvent (String label)
{
super(label == null ? "" : label);
}
}
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Thoughts on Events
 Event generation is the “output interface”
• Equivalent to a “Callback”
• Boils down to a simple function call
 Java’s event mechanism not that nice
• Events in scripting languages generally nicer
• The “clean” event interface is a good thing
 Tools like AppComposer compensate
• Helps client handle event-based communication
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Thanks
www.howardism.org/geekin