Introduction to Java Beans

1. Introduction to Java Beans by Anders Børjesson

2. JavaBeans components • JavaBeans are the components in the Java environment • Many classes in J2SE are JavaBeans • Like the Java Swing classes Introduction to JavaBeans

3. Characteristics • Visibility • Some JavaBeans are visual • Buttons, TextFields, etc. • Some JavaBeans are “invisible” • Model components (worker classes) • Size (granularity) • Some JavaBeans are small (in terms of features) • Buttons, TextFields, etc. • Some JavaBeans are big (in terms of features) • SMTP bean, entire applications Introduction to JavaBeans

4. Requirements for JavaBeans classes • Must have a no-arg (no parameters) constructor • More constructors are allowed • Remember: If you add a constructor with parameters then the default no-arg constructor is gone. • Must implements java.io.Serializable • Data should not be public • Data should be accesses via get and set methods • Should be thread safe • Use synchronized carefully Introduction to JavaBeans

5. Non-requirements for JavaBeans • An applet must extend java.lang.Applet • A servlet must extend javax.servlet.Servlet • A thread must implement java.lang.Runnable • Beans does not need to extend or implement anything special • Except for java.io.Serializable • Which is only a marker interface Introduction to JavaBeans

6. JavaBeans terms • Properties • To be read or set by other objects • Get and set methods • Methods • To be called by other objects • Ordinary Java methods • Events • Sent to other objects when something happens to the bean object • Like a property with a new value • This is the interesting part! Introduction to JavaBeans

7. Access levels • 3 access levels • Read-only • Make only a get method • Write-only • Make only a set method • Quite rare • Read / write • Make get and set methods • Most common Introduction to JavaBeans

8. Different kinds of properties • Different kinds of properties • Simple or indexed (array) properties • Bound properties • Event after a new value is set on the property • To update other components (e.g. visual components) • Constrained properties • Event before a new value is set on the property • To check if the new value is legal according to the current business rules Introduction to JavaBeans

9. Simple properties • Represents a single value • A simple property has • 1 name • 1 type • Primitive data type or • Class based data type • Example: size • Name: size Type: int • public void setSize(int size) • public int getSize() • AccountSimple.java Introduction to JavaBeans

10. Example bean: JButton • Some properties of javax.swing.JButton • Text: setText(String), getText() • Font: setFont(Font), getFont() • Icon: setIcon(Icon), getIcon() • Enabled: setEnabled(boolean), isEnabled() • Properties of type boolean has special naming conventions • setXx, isXx Introduction to JavaBeans

11. Programming JavaBeans • JavaBeans can be programmed using • Ordinary text editors • TextPad, even Notepad • Builder tools • NetBeans, Eclipse, JBuilder, etc. • Builder tools often have special features that makes programming JavaBeans easier. Introduction to JavaBeans

12. Exercise: Car bean • You are supposed to program parts of a system for car registration • Program a Car bean • Properties: Registration number, color, etc. • Methods: toString, equals • Constructors: No-arg constructor + maybe other constructors • Implement Serializable • Thread safety • Make a JUnit test to test the Car bean Introduction to JavaBeans

13. Indexed properties • Represents an array of values • Example: Property: address Type: Address • Public void setAddress(int index, Address addr) • Public Address getAddress(int index) • Bulk operations: • Public void setAddresses(Address[] addr) • Public Address[] getAddresses() • AccountIndexedOwner.java Introduction to JavaBeans

14. Exercise: Owner bean • Additional classes for the car registration system • Program a (car) owner bean • Simple properties: Name, address, etc. • Indexed property: Car • One owner may have several cars • The type of this property is Car (from the previous exercise), not String. • Make a JUnit test to test the Owner and Car beans. Introduction to JavaBeans

15. Bound properties • Sends an event to another object after the property is assigned a new value. • The other object is sometimes called: Observer, listener, etc. • The other object may react on the event • Update its own state • Content of an event • propertyName, oldValue, newValue, source Introduction to JavaBeans

16. Examples • AccountBound.java • ScreenLogListener.java • AccountBoundApp.java Introduction to JavaBeans

17. Observer pattern Introduction to JavaBeans

18. Sequence diagram: Firing a property change event Introduction to JavaBeans

19. Sequence diagram: Adding a property change listener to a bean Introduction to JavaBeans

20. Classes and interfaces • Package java.beans • Class PropertyChangeEvent • extends EventObject • Interface PropetyChangeListener • Extends EventListener • An empty (aka. Marker) interface • Like java.io.Serializable • Class PropertyChangeSupport • Helps you to make bound properties Introduction to JavaBeans

21. PropertyChangeListener • Interface PropertyChangeListener extends EventListener • EventListener is a marker interface • No methods to implement • PropertyChangeListener methods • void propertyChange(PropertyChangeEvent evt) Introduction to JavaBeans

22. PropertyChangeEvent • After changing the value of a property • firePropertyChange(propertyChangeName, oldValue, newValue) • Listener called • propertyChange(PropertyChangeEvent ev) • PropertyChangeEvent methods: • String getPropertyName() • Object getOldValue() • Object getNewValue() • Object getSource() Introduction to JavaBeans

23. PropertyChangeSupport • Helps you to make bound properties • Constructor • PropertyChangeSupport(Object sourceBean) • Some methods • addPropertyChangeListener(PropertyChangeListener listener) • removePropertyChangeListener(PropertyChangeListener listener) • addPropertyChangeListener(String propertyName, PropertyChangeListener listener) • removePropertyChangeListener(String propertyName, PropertyChangeListener listener) • firePropertyChange(String propertyName, Object oldValue, Object newValue) • firePropertyChange(String propertyName, int oldValue, int newValue) Introduction to JavaBeans

24. Exercises: Car bean with bound properties • Program a car bean with bound properties • Don’t try to change the Car from the previous exercise. Make a new class with a different name. • Program a PropertyChangeListener • LogListener implements PropertyChangeListener • Every time a property changes the listener writes to the screen the data in the propertyChangeEvent • Source, property name, old value, new value. • Make a Main to try the new Car bean with a propertyChangeListener. • Make a JUnit test to test the bound properties. • Take a look at the test in my BeanBank example before you try to make your own test. • Extra (if you have time): Make a more general log listener that writes to a general stream. Introduction to JavaBeans

25. Event loops • Problem • 2 beans send propertyChangeEvents to each other • Example: User interface objects events a model object, and vice versa • The 2 beans might event each other forever. • Solution • Check if the new value is really a new value • i.e. newValue != oldValue • PropertyChangeSupport.firePropertyChange() does the checking Introduction to JavaBeans

26. Example bean: JButton • Javax.swing.JButton is a bean • Does not use PropertyChangeListener, etc. • addActionListener(ActionListener l) • Listens for button presses • addChangeListener(ChangeListener l) • Listens for changes in the button’s state • Size, font, text, etc. Introduction to JavaBeans

27. Constrained properties • A bean object asks one (or more) observers if an update of a property is legal • Observer may throw PropertyVetoException • Example: account.setBalance(-100) • A property is often bound and constrained • Used to implement pluggable business rules • Examples • AccountConstrained.java • MinimumBalanceChecker.java • MaximumWithdrawChecker.java • AccountConstrainedTest.java • AccountFrame.java + AccountGuiApp.java Introduction to JavaBeans

28. Exercise: Car with constrained properties • Program a car bean with constrained properties • You may either change your “old” car bean or make a new car bean. • Program a few VetoableChangeListeners • ColorChecker • Checks if a color is illegal • Pink, orange, etc. are considered illegal colors of a car • RegistrationNumberChecker • Checks if the registration number is legal • A proper registration number must have the structure AB12345, i.e. 2 letter followed by 5 digits. • Make a JUnit test to test the Car bean with the new listeners. • Extra (if you have time) • Program a configurable ColorChecker as a bean + VetoableChangeListener • Illegal colors is an indexed property Introduction to JavaBeans

29. Dynamic vs. static characteristics on properties • Static characteristic • Follows the object for its life time. • Dynamic characteristic • Changes over time during the objects life time • Characteristics may be (de)aggregated using constrained properties. • Example: AccountAdminFrame.java Introduction to JavaBeans

30. Low coupling • The event mechanism in JavaBeans provides low coupling • The bean object doesn’t know much about the listeners • Except that listeners must implement PropertyChangeListener or PropertyVetoListener • The listeners doesn’t know much about the bean • It’s just an Object • Which we sometimes have to typecast to the Bean class (which is very bad) Introduction to JavaBeans

31. Using JavaBeans in builder tools • JavaBeans can be used in a builder tool like • NetBeans • Eclipse • Borland JBuilder • etc. • The bean must be “installed” in the builder • The builder usually helps you to during installation Introduction to JavaBeans

32. Packing JavaBeans in JAR files • JAR = Java Archive • The standard package format in Java • JAR = ZIP file with a manifest • A JAR file may contain • Class files • Icons, etc. • Manifest file describing the contents of the JAR file Introduction to JavaBeans

33. Introspection • The builder tool unzip’s the JAR file to get the class files. • The builder tool “learns” about the beans’ properties, methods, and events using the introspection API • Package java.beans • Class Introspector • Interface BeanInfo • Class PropertyDescriptor • Class MethodDescriptor • Class EventSetDescriptor • Example: beanbank/introspection/IntrospectionApp.java Introduction to JavaBeans

34. Using JavaBeans • JavaBeans can be used as ordinary classes in • Java applications, applets, servlets • JSF(JavaServer Faces) Introduction to JavaBeans

35. JavaBeans vs. Enterprise JavaBeans • JavaBeans • Are relatively simple • Part of Java Standard Edition (J2SE) • Can be used in an ordinary Java application / applet • Enterprise JavaBeans • More advanced • Part of Java Enterprise Edition (J2EE) • Middle tier: Between (web)application and database. • Can only be used in an enterprise Java application • Requires an J2EE server to run the application Introduction to JavaBeans

36. Job types using JavaBeansand other types of components • Designing components • Programming components • Documenting components • Technical writer • Design using components • Programming using components • Marketing / selling components • Testing components • Correctness, performance, security [Trojan horses, etc.] Introduction to JavaBeans

37. References • Niemeyer & KnudsenLearning Java, 3rd edition, O’Reilly 2005 • 22. JavaBeans, page 751-786 Introduction to JavaBeans