Download Programming in Java

Lecture13
Java Applet
Programming in Java
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Objectives
1. Introduction to Applet
2. Running of Applet
3. Applet tags in HTML
4. Environment of Applet running
5. Both Application and Applet
Programming in Java
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Java Technologies
Programming in Java
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How Java Applets work(1)
Server
Brower
class files
Java Virtual Machine
•
•
•
•
•
•
Server delivers HTML data stream (text and tags)
Java applet is referenced in the HTML
Browser parses data and displays on Client
Browser requests applet class file(s)
Browser loads applet into JVM
JVM executes applet code
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How Java Applets work(2)
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Java Applet & Java Application(1)
• Applet
- A Java program that is referenced by a Web page
and runs inside a Java-enabled Web browser
• Application
- A stand-along Java program that runs via the “java”
command
Page
Brower
VM
Applet
VM
Application
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Java Applet & Java Application(2)
Feature
Applet
Application
Starting the Java classFileName Browser Web Page
Browser Open HTML File
(from
a
command
prompt)
Program
Appletviewer htmlFileName
Parameters
from command
prompt(args[0]…)
from HTML file
(PARAM…)
Has a main()
YES
NO
Supports GUI
YES
YES
Security
Restriction
NO
YES
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Java Applet Characteristics
• Some applets possibilities:
Communicate with web server
Dynamic content generation
"Real-time" information
• Advantages:
• Disadvantage:
- Full GUI control possible
- Not all browsers support
all Java levels
- Good security model
- Users may have Java
support disabled
- Complete programming
language
- Component reuse possible
- Class downloads can be
time-consuming
- Security model can be
limiting
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Restrictions and Security(1)
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Restrictions and Security(2)
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What is Sandbox?
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Restrictions and Security(3)
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Restrictions and Security(4)
No built-in
notion of
trusted
code,each
applet runs
with
separate
permissions
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Permissions and Policy Files (1)
• In Java2, an object of the java.security.Policy class now
represents the security policy for all kinds of Java
programs
Permission
Code
Signed Charles applet
R/W /tmp/ and /home/
Unsigned Charles applet
Connect and receive URL
Native program
R/W /tmp/
…
…
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Permissions and Policy Files(2)
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Applet Creation and Running
•Applet
creation
import java.applet.*;
public class Hello extends Applet{ …}
•JApplet
creation
import javax.swing.*;
public class Hello extends JApplet{ …}
• Browers which support Java(HotJava、Netscape
Navigator、IE)
• Applet viewer(\jdk\bin\)
– Applet viewer [option] urls…
– Only can read tags such as OBJECT、EMBED、
APPLET（can’t read other HTML tags）
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Applet Classes
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java.lang.Object
AppletClip
AppletStub
java.awt.Component
AppletContext
java.awt.Container
java.awt.Windows
java.awt.Frame
java.awt.Panel
java.applet.Applet
javax.swing.JFrame
javax.swing.JApplet
Each Applet inherits from java.applet.Applet/javax.swing.JApplet
Programming in Java
Applet Lifecycle(1)
• init()
-Invoked at applet downloaded and implement to
transform paras from HTML to Applet (initialization)
• start()
-Invoked at viewing the page, and could be invoked
again and again
• stop()
-Invoked at leaving the page, and could be invoked
again and again also
• destroy()
-Invoked at closing the browser
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Applet Lifecycle(2)
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• Loading an Applet
Creates an instance by invoking the constructor
Browser invokes init() method
Constructor
Browser invokes start() method
• Leaving and returning to the page
Browser invokes stop() method
Browser invokes start() method
• Reloading the page
stop() - suspend any activity
destroy() - free resources (threads stopped)
Ctor-Create new Applet
(init()
start())
Programming in Java
init
start
stop
destroy
Applet Tags in HTML(1)
Necessary Tags
• Applet’s Tag in HTML
<APPLET CODE=
WIDTH=
HEIGT= >
</APPLET>
•
Three information about Applet
-
Name of Class file
-
Location of Class file
-
How to representation Applet in page
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Applet Tags in HTML(2)
Other Applet’s Tag
<APPLET
[CODEBASE = codebaseURL]
CODE=appletClassFile
[ALT=alternativeText]
[NAME=appletInstanceName]
WIDTH = pixels HEIGHT = pixels
[ALIGN = alignment ]
[VSPACE = pixels] [HSPACE = pixels]>
[<PARAM NAME=AttrName VALUE =AttrValue>]
[<PARAM NAME=AttrName2 VALUE =AttrValue>]
…… [HTML displayed in the absence of Java]
</APPLET>
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Applet Tags in HTML(3)
• ARCHIVE
- list Java jar files,including classes and other resource
 using Swing; but on Java1.1
download Swing patch from network(swing.jar)
<APPLET
CODE = “CalculatorApplet.class”
ARCHIVE = “CalculatorClasses.jar,
swing.jar”
WIDTH = 100
HEIGHT = 150
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Transfer Parameters to Applet from HTML
• Applet can get parameters from html file
• Tag PARAM
– HTML文件
<PARAM NAME=“Param_Name” VALUE=“”>
– APPLET文件
String getParameter(“Param_Name”)
HTML
<APPLET CODE=”Demo.class” WIDTH=400 HEIGHT=300>
<PARAM NAME=”font” VALUE=”TimesRoman”>
<PARAM NAME=”size” VALUE=”20”>
</APPLET>
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Example
Java Code
public class Demo extends Applets {
public void paint(Graphics g){
String fontName=getParameter(“font”);
String sizeName=getParameter(“size”);
int fontSize=Integer.parseInt(sizeName);
Font fnt=new Font(fontName,Font.BOLD,fontSize);
g.setFont(fnt);
g.drawString(“Demo”,40,50);
}
}
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Interaction Between Applets in HTML
• Get the HTML runtime environment
– getAppletContext() [java.applet.Applet]
• Get an Applet
– public Applet getApplet(String name);
• Get all Applets on the same HTML
– public Enumeration getApplets()
<html>
<applet code=“Sender.class” WIDTH=400 HEIGHT=200
name=“Wang”> </applet> <p>
<applet code=“Receiver.class” WIDTH=400 HEIGHT=200
name=“Li”> </applet> <p>
<applet code=“GetApplets.class” WIDTH=400
HEIGHT=200> </applet>
</html>
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Example(1)
import java.applet.*;
import java.awt.*;
import java.awt.event;
import java.util.Enumeration;
import javax.swing;
public class GetApplets extends JApplet implements ActionListener
{ private TextArea textarea;
private String newline;
public void init()
{ container contentPane= getContentPane();
Button b=newButton(“Click to call getApplets()”);
b.addActionListener(this);
contentPane.add(“North”,b);
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Example(2)
textArea = new TextArea(5,40);
textarea.setEditable(false);
contentPane.add(“center”, textArea);
newLine=System.getProperty(“line.separator”);}
public void actionPerformed(ActionEvent e)
{ printApplets(); }
public void printApplets()
{ Enumeration e=getAppletContext().getApplets();
textArea.append(“Result:” + newLine);
while (e.hasMoreElements())
{Applet applet=(Applet)e.nextElement();
textArea.append(“-”+applet.getClass().getName()+
newLine);}
textArea.append(“-”+newLine);
}
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Interaction Between HTML and Applet
• Get Applet URL from browser
– URL getCodeBase()
• Get HTML URL which embedded Applet from browser
– URL getDocumentBase()
• Get PARAM value by name inHTML
– String getParameter(String name)
• Get Applet receiving parameters
– String[] getParameterInfo()
• Get this Applet information,such as applet designer,version
– String[] getParameterInfo()
• Show a new web page in the browser
– Use method ShowDocument() of Class AppletContext
– ShowDocument() have two parameters, first is URL,
second is where display new page.
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Example(1)
import java.applet.*;
import java.awt.*;
import java.net.*;
public class Bookmark extends Applet {
private List links;
public boolean action(Event evt, Object arg) {
if(evt.target==links)
{ try{ AppletContext context=getAppletContext();
URL u = new URL((String)arg);
context.showDocument(u,"right");}
catch(Exception e){showStatus("Erro"+e); }}
else{ return super.action(evt,arg);
}
return true; }
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Example(2)
public void init() {
setLayout(new BorderLayout());
links=new List(5,false);
int i=1;
String s;
while ((s=getParameter("link_"+i))!=null){
links.addItem(s);
i++;
}
add("Center",links);
}
}
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Example(3)
<HTML>
<TITLE>One Simple Bookmark</TITLE>
<FRAMESET COLS-"300,*" cols="42%,*">
<FRAME NAME="left" MARGINHEIGHT=2
MARGINWIDTH=2 SCROLLING="no" NORESIZE
src="URLlist.htm">
<FRAME NAME="right" MARGINHEIGHT=2
MARGINWIDTH=2 SCROLLING="yes" NORESIZE
src="URLshow.htm">
</FRAMESET>
</HTML>
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How are Java Applets Used
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