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Client-Server applications Thin Client systems Clients may be “thin”, for example a simple information terminal in a library or perhaps a mobile handheld device Thin client systems assume that the bulk of the work is done at the server A thin client is little more than a screen and user- input device(s)
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Client-Server applications
Introduction to Java Applets
Client-server architectures
Why do Applets exist?
What can an Applet do?
What are they used for?
Some technical details
Conclusion
Client-Server applications
Simple client-server architectureServers may provide different types of services
simultaneously to many clients
Clients are software entities with a network connection to the server
Clients vary in their capabilities – some have very little in the way of resources and are considered “thin”
Client-Server applications
Thin Client systems
Clients may be “thin”, for example a simple information terminal in a library
or perhaps a mobile handheld device
Thin client systems assume that the bulk of the work is done at the server
A thin client is little more than a screen and user-input device(s)
Client-Server applications
Client-Server on the Web
Many clients are not thin
Desktop computer systems with an Internet connection may be more powerful than the server
It is possible to off-load much of the computing work from the server to the client – better for scalability
In such cases, the client downloads and runs an application to do work locally
Client-Server applications
Java client-side solutions
Java is a network-centric programming language
It has a number of solutions for client-side programmingApplets – limited applications that run in a browser
WebStart applications – desktop applications with security restrictions that can be launched from a web page
Low-level programming of completely general, unrestricted network applications
Client-Server applications
Why do Applets exist?
In 1995, Java became available as a network-aware programming language
Applets were part of the language, designed to add complex functionality to Web pages
At the time, Applets were the only way to program multimedia on a Web page!
Now competes with Flash, SVG and others
Client-Server applications
Why do Applets exist?
Applets provide trusted, secure, portable functionality
Cached Applets can be run off-line
Applets allow complex calculations to be run on the client machine, including;encryption and secure communication
rapid data communication
interactive graphics
Client-Server applications
What can an Applet do?
Applets are used for lots of tasks;Online discussions and communication boards
Online document editing
Banking front-ends
Scientific visualisation, e.g. molecule viewers
Advertising (although nearly all Flash now)
Interactive games in 2D and 3D
Client-Server applications
Technical Details
Applets are written in standard Java
Applets are not “complete” applications – they plug into a browser
Applets can be assumed to have graphics capability
Applets run in a “sandbox” security environment, giving them extremely limited access to the resources of the local machine [usually a good thing!]
Client-Server applications
Technical Details
To make an Applet we write a class that extends Applet (or extends Japplet, the more advanced Swing-based version)
Then we write code to override some of the four key methods
These are init(), start(), stop() and destroy()
There is no public static void main(String[] args)
Client-Server applications
Applet methods
Typically we would write code to initialise the Applet in public void init()
When the Applet becomes visible, the browser will invoke public void start(), so this is where we “activate” the program (e.g. start an animation running)
If the Applet is made invisible we have public void stop() which turns off animations, sounds etc.
Client-Server applications
Conclusion
Clients and servers vary in their needs an capabilities
Applets exist to provide rich client capability
Applets can be used as portable, security-limited applications
For more information, see: Wikipedia on Applets, Client-server and also Sun's Applet overview
Client-Server applications
Practical notesYou need to compile the .java files on the schedule
into a .class fileNeed a HTML to call the .class files
Applets have no main method and so cannot run on their own
Two examples follow to demonstrateFree Applets out on the web but beware!
