1. Applications
Active Web Documents
Active Web Documents

2. Introduction
Active documents consist of code executed on computer running browser
Java language allows development of active document programs
Programs called applets
Platform independent
Secure
Active Web Documents

3. Continuous Update: Server Push
Some servers will send new versions of document: server push.
Or clients may request new versions periodically: client pull.
Each push or pull requires dedicated server resources.
May not scale with number of clients.
Also requires network bandwidth for each update.
Active Web Documents

4. Active Documents
Shifts the responsibility for updates to browser client…computation on client now.
Work scales with number of active documents on client
Active document can, in fact, incur less server overhead than dynamic document
Active Web Documents

5. Active Documents
Requires programming language; what should that language look like?
How should the language be represented for execution? Text, binary or compressed?
Multiple implementations possible; standards necessary for coordination
Active Web Documents

6. Active Documents
Active Web Documents

7. Java
Technique for writing, compiling, downloading and executing active documents
Includes:
Programming language
Runtime environment
Class library
Active Web Documents

8. Java: Language Resembles C++ Characteristics: Object-oriented
Some C++ stuff removed or restricted
Characteristics:
High level, general purpose, object oriented
Dynamic memory allocation
Strongly typed, static type checking
Concurrent
Active Web Documents

9. Java: Runtime Environment
Interpretive execution - Java language compiled into bytecodes
Automatic garbage collection
Multithreaded execution
Internet access
Graphics
Active Web Documents

10. Java: Portability Java must be platform-independent
Run-time environment clearly defined and has no implementation dependencies
Bytecode representation is platform independent
Java Virtual Machine. (JVM)
Active Web Documents

11. Java: Library
Library provides collection of common functions for Java programs in the form of classes.
Classes for:
Graphics
Low-level network I/O (socket-level)
Web server interaction
Run-time system calls
File I/O
Data structures
Event capture - user interaction
Exception handling
Active Web Documents

12. AWT Graphics
Java graphics library called Abstract Window Toolkit (AWT)
Includes high-level and low-level facilities
Windows with components - scrollbars, buttons
Blank rectangular area with object drawing
Run-time environment implementation maps AWT functions to window system-specific functions.
Active Web Documents

13. Java Programs Two types of Java programs:
Stand alone Java programs.
Java applets.
What does a Java standalone program look like?
How is it compiled & run?
Active Web Documents

14. Compiling Java Programs
javac translates Java source code into bytecodes
Checks for correct syntax
Imports classes from library
Writes bytecode program to filename.class
Other development environments exist
May include source code management
Active Web Documents

15. Java Applets
Java applet is compiled in the usual way: javac filename results in filename.class.
filename.class is put in HTML file and run.
An applet is run using an appletviewer or a browser. E.g. appletviewer filename.html.
An applet has a panel associated with it.
Java applet has a method init() to initialize the panel associated with it.
Active Web Documents

16. Java & Browsers Browser must include Java interpreter
Java interpreter works through browser
Graphics
HTML
Interpreter also works through native operating system
File I/O
Network operations
Active Web Documents

17. A Java Applet
Example
Active Web Documents

18. Running Java Example
<applet code="clickcount.class" height=150 width=400> </applet>
Active Web Documents

19. Other Alternatives JavaScript VBScript.
Interpreted scripting language
Like csh for browser
VBScript.
Other languages compiled into Java bytecodes.
Other programming technologies - Inferno
Active Web Documents

20. Active Web Documents

21. Summary Active documents execute code in browser on user's computer
Java is most widely used active document technology
Java consists of:
Programming language
Run-time environment
Class library
Tags in browser for Java program invocation
Active Web Documents