Lecture 14 from the IAG0040 Java course in TTÜ. See the accompanying source code written during the lectures: https://github.com/angryziber/java-course

1. Java course - IAG0040
Java Beans,
Applets & GUI
Anton Keks 2011

2. Java Beans
● JavaBeans is a component technology (like CORBA, ActiveX, etc)
– JavaBeans API allows creation of reusable, self-contained, cross-
platform components.
– Java components are called “beans”
– Beans can be used in Applets, applications, or other Beans.
– Beans are usually UI components, but it is not a requirement
●
There are many JavaBeans-compatible visual tools
● Formerly, there was the BDK (Bean Development Kit), which
contained BeanBox. Now it is superseded by BeanBuilder.
● Nowadays, the concept of “beans” is used also outside of JavaBeans
(not using java.beans API), e.g. in many server-side frameworks.
Sometimes these beans are called POJOs (Plain Old Java Objects)
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3. Bean basics
● Beans can expose their
– properties, which can be modified at design time
– actions (methods to do something)
– events
● java.beans.Introspector analyses Java Bean classes
– Generally automatically using Reflection API
– Or using the provided BeanInfo implementation (optional)
●
it must be named XyzBeanInfo for bean named Xyz
● Introspector.getBeanInfo(Xyz.class) will return a
BeanInfo instance, describing the Xyz bean
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4. How to make a Bean?
● A Java bean is a Java class that
– follows certain rules (conventions, design patterns), which
enable dynamic discovery of its features
– is Serializable (not strictly enforced)
– has a default constructor (parameter-less)
– can extend any class (no restrictions), but usually they extend
some GUI container classes
– has properties defined by corresponding getters and setters
(getXxx(), isXxx() and setXxx() public methods, where xxx is the
property name)
– has public void action methods
● methods can throw any exceptions
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5. More features
●
Bean properties can have PropertyEditors assigned
●
More complex editing is possible using the Customizer
interface (it can customize the whole bean at once)
● Aside from properties, Beans can have events
– event listeners must implement an interface (e.g.
ActionListener)
– Bean must provide two methods: addXXX() and removeXXX()
● addActionListener(ActionListener listener)
● removeActionListener(ActionListener listener)
– The interface must define a method, taking the event object
● actionPerformed(ActionEvent e)
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6. Persistence
●
Every bean is Serializable, hence can be easily
serialized/deserialized
– using ObjectOutputStream and ObjectInputStream
●
Long-term bean-specific serialization to XML is
also possible
– using XMLEncoder and XMLDecoder
– these enforce Java bean convention very strictly
– smart enough to persist only required (restorable)
properties, i.e. read-write properties with non-
default values
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7. Warning: Java Beans ≠ EJB
●
EJB are Enterprise Java Beans
● EJB are part of Java EE (Enterprise Edition)
● EJB and JavaBeans have very few in common
● EJB = bad thing (heavy-weight)
– at least before EJB 3.0
– even EJB architects at Sun agree on that now
●
Don't confuse yourself
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8. Bean task
1. Write a simple CommentBean
with String property comment
2. Try using the Introspector on it
3. Make it a GUI bean by extending java.awt.Canvas
4. Make it display text: override the paint() method, use
g.drawString()
5. Make the comment text scroll from right to left by using a Timer or
a manually written Thread
6. Tip: run it temporarily with this code in the main() method
Frame frame = new Frame(); frame.add(new CommentBean());
frame.setSize(w, h); frame.setVisible(true);
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9. Java GUI toolkits
●
Most Java GUI toolkits are cross-platform, as Java itself
●
The most popular ones are
– AWT (Abstract Widgets Toolkit), java.awt – the first GUI toolkit for
Java, the most basic one, sometimes may look ugly.
● The principle of LCD (least common denominator)
– JFC Swing, javax.swing – pure Java, supports pluggable look-and-
feels, more widgets, more powerful.
●
Included in JRE distribution
– SWT (Standard Widget Toolkit), org.eclipse.swt – developed for
Eclipse, can be used stand-alone.
● Provides native look-and-feel on every platform.
● Implemented as thin layer on native libraries for many platforms
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10. Java 1.6 desktop additions
● Cross-platform system tray support
– SystemTray.getSystemTray();
– tray.add(new TrayIcon(img, “Hello”));
●
Cross-platform java.awt.Desktop API
– Desktop.getDesktop();
– desktop.browse() - opens a web browser
– desktop.mail() - opens a mail client
– open(), edit(), print() - for arbitrary documents
– all this uses file/URL associations in the OS
● These may not be supported on each platform
– use isSupported() methods to check
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11. Java Applets
● Applets were the killer-app for Java
● In short, Applets are GUI Java applications, embedded in HTML
pages, and distributed over the Internet
●
Convenient to deploy centrally, convenient to run
●
Built-in security
●
Nowadays not as popular, because of Servlets, AJAX, Flash,
and aggressiveness of Microsoft (Java is no longer shipped with
Windows by default)
●
Applets are created by extending one of these classes:
– java.applet.Applet – older, AWT-based API
– javax.swing.JApplet – newer, Swing-based API (extends Applet itself)
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12. Applet API
● The applet API lets you take advantage of the close relationship
that applets have with Web browsers. See both Applet and
AppletContext (obtainable with getAppletContext())
● Applets can use these APIs to do the following:
– Be notified by the browser of state changes: start(), stop(), destroy()
– Load data files specified relative to the URL of the applet or the page in
which it is running: getCodeBase(), getDocumentBase(), getImage()
– Display short status strings: showStatus()
– Make the browser display a document: showDocument()
– Find other applets running in the same page: getApplets()
– Play sounds: getAudioClip(), play()
– Get parameters specified by the user in the <APPLET> tag:
getParameter(), getParameterInfo()
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13. Applets and Security
● The goal is to make browser users feel safe
● SecurityManager is checking for security violations
● SecurityException (unchecked) is thrown if something is not allowed
● In general, the following is forbidden:
– no reading/writing files on local host
– network connections only to the originating host
– no starting of programs, no loading of libraries
– all separate applet windows are identified with a warning message
– some system properties are hidden
●
Trusted Applets can be allowed to do otherwise forbidden things
– They are digitally signed applets, which can ask user if he/she allows
to do something. See the keytool program in JDK.
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14. Deployment of Applets
● The special <applet> HTML tag is used
– <applet code=”MyApplet.class” width=”10” height=”10”>
<param name=”myparam” value=”avalue”/>
</applet>
– Additional attributes:
● codebase – defines either relative of absolute URL where class files
are located
● archive – can specify jar file(s), where to load classes and other
files from
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15. Applet task
● Create CommentApplet
● Use CommentBean there
● Use Applet parameters for customization of
background color and comment
● Create an text field and use it for changing the
comment String at runtime
● Display the java-logo.gif within the Applet by using
getImage(getCodeBase(), “filename”) and
g.drawImage(img, 0, 0, this)
● Deploy applet and view using a web browser
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16. JFC
●
JFC = Java Foundation Classes
●
Includes
– Swing GUI Components
– Pluggable look-and-feel support
– Accessibility API
– Java2D API
– Drag-and-drop support
– Internationalization
●
JFC/Swing currently is the most popular GUI toolkit
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17. Hello, Swing!
● public class HelloSwing {
private static void createAndShowGUI() {
JFrame frame = new JFrame("HelloSwing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JLabel label = new JLabel("Hello, Swing!");
frame.add(label);
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
}
} }
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18. Swing concepts
●
Containers contain other components
– Top-level (JApplet, JDialog, JFrame), they have contentPane
(e.g. JPanel) and optional JMenuBar
– General-purpose (JPanel, JScrollPane, JSplitPane, JTabbedPane,
JToolBar, etc)
– containers provide add() methods
● Layouts control positions of child components
● Most noncontainer components have optional Model interfaces (e.g.
ButtonModel), which can store their state (think of MVC pattern)
●
The overall design follows JavaBeans conventions, including the
event handling mechanism
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19. Swing & Concurrency
● Most of the API is not thread-safe
– Thread-safe parts are documented so
●
Swing and AWT use their own event dispatch thread
– most interactions with GUI components should happen
there
– SwingUtilities class provides invokeLater() and
invokeAndWait()
– event handling code must be as short as possible
●
Longer running code must be in separate threads
– this allows GUI to always stay responsive, avoids freezing
– Java 1.6 introduced SwingWorker to simplify this
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20. Swing Tips
●
JOptionPane provides various simple dialog boxes
– showMessageDialog – shows a message box with an OK button
– showConfirmDialog – shows a confirmation dialog with Yes,
No, Cancel, etc buttons
– showInputDialog – shows a dialog for entering text
● Look-and-feel is controlled by the UIManager
– UIManager.setLookAndFeel(“com.sun.java.swing.plaf.”
+ “motif.MotifLookAndFeel");
– UIManager.setLookAndFeel(UIManager.
getSystemLookAndFeelClassName());
– UIManager.setLookAndFeel(UIManager.
getCrossPlatformLookAndFeelClassName());
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21. SWT
●
SWT == Standard Widget Toolkit
● Fast, portable, native (uses native “themes”)
●
Implemented in Java using native Java adapters
●
API is a bit less flexible than Swing, not 100% JavaBean-compatible
● UI access is strictly single-threaded
●
Not included in standard distribution, must be deployed manually
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22. Hello, SWT!
● public class HelloSWT {
public static void main (String[] args) {
Display display = new Display();
Shell shell = new Shell(display);
Label label = new Label(shell, SWT.BORDER);
label.setText(“Hello, SWT!”);
shell.pack();
shell.open();
while (!shell.isDisposed()) {
if (!display.readAndDispatch())
display.sleep();
}
display.dispose ();
}
}
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23. SWT concepts
●
Containers contain other components
– Top-level container is Shell, which is a Composite
– Widget constructors take parent Composite as a parameter. No
relocations or multiple parents.
– All widgets take style bits in constructors, which can be
composed
Button btn = new Button(shell, SWT.PUSH | SWT.BORDER);
● Display class provides the environment
● Layouts control positions of child components, each control can have its
LayoutData assigned
● Not all API conforms to the JavaBeans conventions; event handling
mechanism is pretty standard
● All widgets must be manually dispose()d! Parent disposes its children.
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24. SWT & Concurrency
●
The Thread that creates the display becomes
the user-interface Thread (aka event-
dispatching thread)
– other threads cannot access UI components
– Display provides methods to ease this task
● asyncExec, syncExec, timerExec – they all execute
provided Runnable implementation in the UI thread
● Event loop must be executed manually
– Display.readAndDispatch() in a loop
●
processes events on native OS's queue
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25. GUI task
● Write a simple CalculatorApplication
using either Swing or SWT
● It must have number buttons from 0 to 9, +, -, *,
/, =, and Clear. Label must be used for
displaying the current number or the result.
● Note: IDEA's Frame Editor can help :-)
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