References

Tanenbaum 3rd edition section 7.6.4: Java

Comer chapter 29 Java Technology for Active Web documents

Active Web pages

Recall Web page classification

• static
• dynamic
• active

Active pages more dynamic than "dynamic" web pages.

Internal interaction at client - not dependent on server for interaction

Interaction with server costs

Examples:

• document contains data for viewing in different ways

• active forms: document is for collecting data with internal consistency

• document needs continuous update - server push expensive

Other examples? from Web experience:........................

Basic mechanism for active Web pages

The browser requests a Web document in the usual way - HTTP request- URL

The server delivers in the usual way - HTTP response - a document containing an
executable specification of actions.

Because of the context of the browser's request, the browser chooses to interpret that document as actions - rather than HTML for display or like other content types
(even though it is labelled as text/plain)

Example: in HTML an applet tag identifies an active code element.

It can be given parameters when it executes.

It executes in the context of the document that contains the applet invocation, within a window in the browser window.

<h3>Question 2</h3>

For a second chance at the money, tell us the name of the 4th layer in the OSI model.

<applet
codebase="http://cs.anu.edu.au/people/Chris.Johnson" code="Multichoice2" width=600 height=650>

<param name=answer1 value="Session layer">

<param name=answer2 value="TCP/IP">

<param name=answer3 value="Transport layer">

<param name=answer6 value="Datalink layer">

<param name=answer4 value="Physical layer">

<param name=answer5 value="Heaviside layer">

<param name=correct value="3">

<param name=goodprompt value="Right! the layers are 1: Physical, 2: Datalink 3: network (and as you said) 4: Transport"

<param name=prompt value="Hint:you should have a mnemonic for the layer names">

<param name=badprompt value="That turns out to be incorrect. Try again, please.">

</applet>

The function of the applet in this case was designed to display a multichoice question with a button for each answer, reacting to the selection of each with colour changes and response messages (badprompt), until the right answer or a limit on attempts is reached.

Active Web pages - advantages

• decreased server load - in requests and in CPU work
• decreased network traffic
• faster response to user
• an active document may be smaller to download than its resulting display (whether essentially static in content, or active)
• the page itself does not change - cacheable

Active Web pages - by using Java

Java Technology

Java is a collection of strongly linked technologies from Sun Microsystems
- proprietary system but openly usable

The Java technologies attempt to provide active Web technology
[and apparently the kitchen sink as well]

Three parts to the technology:

Java programming language

Java runtime environment

including security and network awareness

Java libraries

Java language

The language is derived from C++ - a recent 3rd-generation language.

Like many recent languages it is a core language with a set of defined libraries for many of its important functions.

Many C++ features have been eliminated, others tidied up and strengthened.

Not a small language.

Unexpected complexities in definition and implementation - as with all programming languages.

Programming language acceptors and Java

A language acceptor is a mechanism for evaluating a program in that language.

Classical programming language acceptors
distinguish between compilers and interpreters.

Compiler

Interpreter - simple (e.g. early versions of BASIC)

Interpreter with compiler

Compiler - bytecode - interpreter

The abstract machine code (bytecode)

• makes object code small in storage
  and small to transport over the network on request
• can be optimised
• can be further compiled to native machine code
• is completely portable
  - runs anywhere that there is an interpreter for that abstract machine

The Java Virtual Machine -- is the interpreter.

In Java the modules are classes.

The linking of the various classes in a program is lazy
- delayed until needed at execution time

Java Runtime Environment

The interpreter

• is written to interact with a host operating system, file system, network communications
• one form is built in to Web browsers
• other versions are standalone

The interpeter provides a PL run time system

• efficient storage management including runtime garbage collection
• multithreaded execution
• graphics support for user interaction
• Internet access

and implements a safety policy.

Java technology - the Web advantages

Performance

Size and speed of Java bytecode vs C ( a tiny program!)

Web safety

Portability - platform independence

Java Libraries

The Java libraries are mainly in bytecode, some are in native code.

Provide the program interface to services of the interpreter runtime system and other added features

Many of these are essential but not built in to the language.

The library mechanism (through collections of pre-compiled classes) provides a very powerful means of extending the language for whole areas of applications:

has commercial possibilities (re-usable code, packages can be sold as commodity or designed for specific purposes)

... perhaps...

Java safety features

In the language:

• no pointers, unlike C and C++ - no pointer arithmetic
  yes references
• strong typing
• strong static type checking
• active array bounds checking, null pointer checking
• exception handling model
• object-oriented (? safety? some say so)

In the runtime environment:

• interpreter is sensitive to mode of running
  - can block actions which are outside allowed policy
• interpreter checks validity of downloaded bytecodes
  - prevents doctored bytecodes from being executed

Java - and the kitchen sink

Very popular - more for promise than present delivery

Very appropriate for small-medium scale programs - especially applets
- an unknown in large scale programs (and slow speed may tell)

Not easy to program - nothing really is.

Object code is portable.

Provides dynamic documents economically.

An open standard?

A universal standard?