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To write programs that use a WWW browser as their user interface, two
main possibilities exist:
- CGI
- (``Common Gateway Interface'') programs
can be written in any programming language that is available on the
web-server. The CGI program can be called upon specific events by the
web-server. Very often, such programs are simple shell or Perl
scripts. The CGI provides a common interface between the server
and the program by passing arguments inside environment
variables. The program in return has to create HTML code as output
which will be sent to the web browser of the user who originated the
event that caused the CGI program to start.
Since standard HTML code is generated, programs that use the CGI
technique can be executed using all kinds of web-browsers. It
suffices that the browser is capable of displaying the generated HTML
code.
- Java
- programs
are, in contrary to CGI programs, executed inside the Java
Virtual Machine (VM) [25] of the users web browser. To
make this possible, Java source code is compiled into so called
``class files'' in a special Java byte code format. After beeing
transferred to the web browser on the client machine, the Java byte
code is verified for security problems and then executed within the Java
Virtual Machine. The same byte code format is used on all
implementations of the Java VM, therefore an application written in
Java is executable on all kinds of platforms for which a Java VM is
available. The VM is included in all recent versions of the Netscape
Web browser, therefore Java programs can be executed by virtually
everybody who uses the World-Wide-Web.
From the users view, Java programs when compared to CGI have the
advantage that they can continue running when the web page is already
loaded in the browser. With CGI programs this is not possible. The only
way to change/modify data that is displayed by a program using CGI is to
restart the program. This could be triggered by another user event, or
as it is the case in programs like mrtg, by a periodic refresh
of the whole page(equivalent to the user pressing the ``Reload'' button
of the web browser).
Figure 4.4:
Conceptual Difference between Java and CGI Programs
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8/4/1997