Chapter 34 Java Technology for Active Web Documents

1
Chapter 34 Java Technology for Active Web
Documents

• methods used to provide continuous Web updates
  to browser
• Server push
• Active documents

2
Browser updates via Server Push

• a server is responsible to continuously run a
  dynamic document program to produce new versions
  of the document which is sent to the browser.
• This method causes the server to be overloaded
  whenever there are too many browser clients.
• Delays arise from limits in available CPU and
  network bandwidth. I

3
Browser updates viaActive documents

• browser downloads an active document which is
  then run locally on the browser.
• CPU resources on the server is kept to a minimum
• network bandwidth usage is kept to a minimum

4
Active Document Representations (fig 34.1)

• active document in source form is used by
  programmer which is then compiled into active
  document in binary (object) form or machine
  independent(portable) Java bytecode
  representation which is stored on Web server
• active document in executable form is used by
  browser. It is generated by resolving remaining
  references and linking the the active document in
  binary form to library routines and system
  functions.
• Java is a technology produced by Sun Microsystems
  for generating active documents

5
Java Components

• programming language
• a run-time environment
• a library of classes
• The Java library and run-time system provide a
  graphics toolkit (eg. to create buttons,
  pull-down menus, dialog boxes, scroll bar) to
  make graphics programming easier

6
Java Language Characteristics

• high level general purpose
• object oriented
• objects contain data and methods that operate on
  data
• Similar to C
• Dynamic
• an instance of an object is created dynamically
  at run-time
• strongly typed
• operations tied to data of particular type
• Concurrent
• program can have multiple threads of control
• threads execute concurrently ie. appear to run at
  same time

7
Java Run-time Environment

• interpretative execution
• Java compiler translates a java program into a
  machine-independent binary representation known
  as the java bytecode representation.
• Java interpreter reads this code an interprets
  the instructions
• automatic garbage collection
• program need not call free to free up memory
• multithreaded execution
• run-time system has parts of operating system to
  handle scheduling and context switching
• internet access
• run-time system includes a socket library that a
  client program can use to contact a remote server
  via TCP or UDP
• graphics support
• create windows graphics

8
Uses of Java Library of Classes

• graphics manipulation
• allows applet to display text, graphics, or
  dialog boxes
• low-level network I/O
• easy access to sockets for UDP or TCP connections
• interaction with a Web server
• applet can access static/dynamic Web documents or
  other applets
• run-time system access
• allows applet to create a thread
• file I/O
• conventional data structures
• a dictionary class allows storage of items
• event capture
• such as mouse clicks or keyboard entry
• exception handling

9
Java Applet

• an active Java document
• A programmer creates an applet by writing a
  source program in Java programming language (fig
  34.2)
• A java compiler (eg. javac on server
  pluto.calstatela.edu) is then used to translate
  the source program (with extension .java) into
  a bytecode representation ( extension .class),
  and places the bytecode representation on a web
  server.
• When a browser downloads this program, the Java
  interpreter in the browser executes the applet
  (fig 34.3)
• To make java applets portable across multiple
  hardware platforms, the java graphics toolkit
  does not manipulate the display hardware.
• Instead, the run-time system maps Java graphics
  methods to equivalent operations( eg. X windows)
  that use the computers window system.

10
Methods used to invoke an Applet

• user supplies the URL of an applet (extension
  .class) to a browser that is java enabled
• eg. http//www.calstatela.edu/example/bbb.class
• embed an applet tag within an HTML document.
• eg. ltapplet codebasewww.calstatela.edu/example
  codebbb.classgt
• When browser encounter the applet tag, a copy of
  the class file will be requested and downloaded
  from the server
• an instance of class bbb will be created, and the
  init method will be invoked.

11
JavaScript

• A variation of Java which allows a programmer to
  store applets in source form in HTML page (fig
  34.7).
• Pro no compiler is needed.
• Con slower to execute.