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An Early Form of Continuous Update
• Problems with “Dynamic” web documents.
– They are not good enough for all applications.
– Information in the document is fixed when the
Active Web Document Technologies document is created.
(Java, JavaScript) – They would not work for stock price updates.
• Server push technique.
– Server periodically sends new copies of a document.
Chapter 37
– Server program must run continuously putting a
large load on the server platform.
– Server must run a separate copy of the generator
program for each client.
– Each client must maintain an active TCP connection.
Active Documents and Server Overhead Active Document Representation
• Active Document technique.
– The computation is moved from the server to the
browser (client).
– An active document is really a computer program
that is run locally by the browser. It is delivered to
the browser just like a static document.
– It is stored in a file on the server and changed only
by programmer intervention.
– The active document can be held in cache.
– The browser must have the ability to run the code.
– The active document may be translated into a
compressed form to use less space.
Java Technology Java Programming Language
• Java was developed by Sun Microsystems. • High Level
• A Java Applet is an active document using Java • General Purpose
technology. • Similar to C++
– Programming Language -- Source code for Java – No operator overloading, no multiple inheritance, no
applets. extensive automatic data coercions.
– Runtime Environment -- Provides the facilities • Object Oriented
needed to run a Java program. • Dynamic - objects created at run time.
– Class Library -- Large library of software already • Strongly Typed
written to handle many common chores.
• Statically Type Checked - at compile time.
• Concurrent
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Java Run-Time Environment Java Class Library
• Interpretative Execution • Graphics Manipulation
– A Java “compiler” translates a Java program into a – Text, graphics, images, dialog boxes.
machine-independent binary representation know as • Low-Level Network I/O (Sockets)
the Java Bytecode representation. • Interaction with a Web Server
– An “interpreter” reads the bytecode and interprets the • Run-Time System Access
instructions for a specific target machine.
– Create another thread
• Automatic Garbage Collection • File I/O on the local machine
– Automatic malloc() and free().
• Conventional Data Structures
• Multithreaded Execution
• Event Capture
• Internet Access – Mouse clicks, etc.
• Graphics Support • Exception Handling
• Machine Independence and Portability
Graphics Toolkit Using Java Graphics on a Particular
• Together the run-time graphics support and the Machine
graphics library are known as a graphics toolkit. • A Java run-time environment include an
Specifically this one is called the Abstract intermediate layer of software.
Window Toolkit (AWT). – Each function in this layer uses the computer’s own
– The toolkit has classes for both low-level and high- window system to implement one specific Java
level manipulation. graphics operation.
– Before the applet executes, the run-time environment
established a mapping between each Java graphics
method and the appropriate intermediate function.
Java Interpreters and Browsers Compiling a Java Program
• A browser can contain one or more interpreters. • The Java compiler is called javac.
• A browser that runs Java has at least two of • Java source code has extension .java.
these: HTML and Java. • Java bytecode has extension .class.
• A Java interpreter is a virtual machine with • A source file must contain exactly one public
instruction pointer, registers, etc. class, which must use the same name as the
• The Java interpreter must be able to source file prefix.
communicate with the HTTP client and the
HTML interpreter in order to display objects on
the screen.
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Example Java Applet Example Java Applet (2)
import java.applet.*;
import java.awt.*;
public class clickcount extends Applet {
int count;
TextField f;
public void init() {
count = 0;
add( new Button( “Click Here” ));
f = new TextField(“Button not clicked.”);
f.setEditable( False );
add( f );
}
public boolean action( Event e, Object arg ) {
if((( Button ) e.target ) .getLabel() == “Click Here” ) {
count += 1;
f.setText( “Button clicked “ + count + “ times.” );
} • Screen display before the button is clicked and
return true;
} after it is clicked once.
}
Invoking an Applet Interaction with a Browser
• User supplies URL of the applet to a browser • An applet can interact with both the HTTP client
that understands Java. and the HTML interpreter in a browser.
– http://www.server.edu/example/bbb.class • Here is an example that uses the browser’s
• HTML document contains an applet tag. HTTP client to retrieve documents and the
– Specifies the location of the .class file. browser’s HTML interpreter to display pages of
– <applet codebase=“www.server.edu/example” information.
code=“bbb.class”>
Buttons.java Example Buttons.java Example (2)
import java.applet.*; catch ( Exception ex ) {
import java.net.*; // note: code to handle the exception goes here //
import java.awt.*; }
}
public class buttons extends Applet { else if ((( Button ) e.target ).getLabel() == "Yang“ ) {
try {
public void init() { getAppletContext().showDocument( new URL (
setBackground( Color.white ); "http://www.netbook.cs.purdue.edu/cs363/lecture_notes/chap29/yang.html"));
add(new Button( "Ying“ )); }
add(new Button( "Yang“ )); catch ( Exception ex ) {
} // note: code to handle the exception goes here //
}
public boolean action( Event e, Object arg) { }
if ((( Button ) e.target ).getLabel() == "Ying“ ) { return true;
try { }
getAppletContext().showDocument( new URL ( }
"http://www.netbook.cs.purdue.edu/cs363/lecture_notes/chap29/ying.html“));
}
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Errors and Exception Handling JavaScript Technology
• In Java, all errors produce exceptions. • Instead of compiling an applet, JavaScript
• An applet is required to handle such exceptions provides a scripting language, and arranges for a
explicitly. browser to read and interpret a script in source
• The applet uses the try-catch construct. form.
• try {s} catch {e} {h} • JavaScript can be intermixed with HTML.
– s = code to execute while looking for an exception.
– e = the exception to look for.
– h = the handler to invoke should e happen.
– If the handler is not specified, all exceptions are
ignored.
JavaScript Example
<html> <head> <title> JavaScript counter </title>
<body onLoad=“return incrementcount(document.demo);”>
<script>
var n;
n = -1;
function incrementcount(form) {
n = n + 1;
if(n==0) {
s = “The button has not been clicked.”;
}
else {
s = “The button has been clicked” + n + “times.”;
}
form.elements[“OutputArea”].value = s;
}
</script>
<form name=“demo”>
<input type=“button” name=“ButtonCLick” value=“Click here”
onCLick=“incrementcount(this.form);”
<textarea name=“OutputArea” rows=1 cols=42 </textarea>
</form>
</body> </html>
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