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package ourbeans;
// THIS IS A JAVA BEAN.
import java.util.*;
public class cart
{
Vector
vector1;
public cart()
{
vector1= new Vector();
}
public void add(String
s)
{
vector1.addElement(s);
}
public Vector showitems()
{
return vector1;
}
}
carter.htm
<html>
<body>
<form method=post
action=carter.jsp >
WHICH PLACES DO YOU WANT TO VISIT? <BR>
<select name=combo1>
<option
value="Bombay">Bombay
<option
value="Bangalore">Bangalore
<option
value="Madras">Madras
<option
value="Delhi">Delhi
<option
value="Calcutta">Calcutta
<option
value="Nagpur">Nagpur
</select>
<input type=submit>
</form>
</body>
</html>
The following JSP file is invoked by
carter.htm ( pay particular attention to 'scope' of the bean!)
carter.jsp
<html>
<body>
<jsp:useBean
id="cart1" scope="session" class="ourbeans.cart"
/>
<%
String s=request.getParameter("combo1");
cart1.add(s);
Vector
vector1 =cart1.showitems();
int n=vector1.size();
for(int j=0; j<n;
j++)
{
String a =
(String) vector1.elementAt(j);
out.println(a);
out.println("<br>");
}
%>
</body>
</html>
Copy carter.htm & carter.jsp to c:\tomcat\webapps\root directory.
In Internet Explorer , we type the url as 'http://localhost:8080/carter.htm.
We get the form which asks you what places you would like to visit. You
can select some city and submit. You can go back and select another city
and submit again. We find that city names thus selected are added to the
list and displayed, thereby achieving session-tracking.
While studying EJB, the most important topic in J2EE, there will be
reference to 'stateful session bean'. (ie) a session bean which has
session-tracking feature. ( A stateless session bean does not retain
its state).The greeter bean that we saw earlier is an example of 'stateless'
bean. ( but not in EJB context).
Having thus familairized ourselves with the
essential web-server techniqies, we are now ready to move on to the enterprise
tier, (ie) the remote object server. This RemoteObjectServer may be
either RMI server or IDL server or RMI-IIOP server . Finally, it can
be an EJB server.
The
core of J2EE is EJB . At present , EJB is sought to be
exposed as an XML-WebService. The second group is the essence of J2EE.
Though, four technologies are mentioned there (ie) RMI, IDL,
RMI-IIOP and EJB, the first 3 technologies are a build up towards
EJB ,the culmination.
RMI was introduced in
JDK1.1
(1997)
IDL was introduced
in JDK1.2
(1998)
RMI-IIOP was introduced in
JDK1.3 (1999)
EJB followed closely.
Modern Enterprise Computing , in the final reckoning, is a manifestation
of Distributed Objects Technology. Typically, the Enterprise
Computing world is characterized by great many Operating systems and
equally varied Programming languages. The operating systems may be IBM
Mainframe, IBM Midframe like OS 400, Mini Computer systems (HP) , UNIX
systems ( AIX of IBM, HP-UX of Hewlett-Packard, Solaris of SUN
etc) , Linux & finally Desktop Windows / Mac & Mobile PALM. Such a
situation may well occur even within a single organization, if it is
fairly large and mature.( In other words, even an Intranet may have
a multiplicity of platforms and languages). The earliest Enterprise
language of the Mainframe era was COBOL. In the UNIX systems , C
& later C++ reigned supreme. By 1990, Visual Basic entered the scene
and gradually matured into VB.net, and it now challenges C++! In
1995, Java was released and today it is almost the default language
for Enterprise world. Even in MicroSoft DotNet platform, J# is a an
approved choice and so Java ( pure or tainted) can claim to be
the universal language for Enterprise computing today.( J# is
another incarnation of Visual J++6. but restricted to webapplications
using ASP.net.)
( However, for
works like device drivers, Java is not directly suitable, as it does
not have pointers. Hence C/C++ are preferred for such work. But, the C/C++
routines can be wrapped in Java using JNI (Java Native Interface) and then
released for the Web World. Hence, there is a great need to develop a
standard, where the objects written in different languages & residing
within different machines running different opearting systems can
coopearte to produce the end result. Such a system will be an instance of Distributed
computing.And the objects are Distributed Objects.Before the
advent of Object -Oriented Programming, RPC (Remote Procedure Call) was
used between different machines. This was later modified into ORPC (
Object-Oriented Remote Procedure call).
Microsoft DCOM was based on such a concept.
DCOM was based on COM and COM is language -agnostic.(ie) we can create COM
in any language of our choice in Microsoft platform.( At present
DCOM , has been superceded by DotNet framework) . The drawback of DCOM was
that it was very much platform-dependent. (ie) It worked only where both
the server and client were Windows machines. Even DotNet platform is , as
yet,Windows-specific.(However, DotNet WebService is able to
communicate with other platforms like Unix/Linux etc by using XML
.This is for the first time that it has become possible , with such ease.
XML is textual and non-proprietory. XML parsers are available for all
platforms. Hence the superiority of ASP.net webservice over DCOM.) And
Microsoft has unceremoniuosly jettisoned DCOM.
Java is the only compiled language , so far
,which runs in a great variety of platforms. Unlike C &
C++ programs , we do not have to compile the code and tweak it for each
platform.This feature , besides automatic garbage collection, is the
greatest merit of Java and why J2EE is the preferred choice of big
companies ,which typically use OS like Mainframe, Midframe,
MiniComputer, UNIX & Linux rather than Windows.
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