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To bring out the differences between the different
appraoaches, we will consider a simple case where the user types his
name in the textbox(text1) in the html form and submits it to the
webserver. The JSP then invokes the greeter bean in the webserver . The
greeter bean encapsulates the 'Business Logic' ,and so it
appends a greeting message to the name thus submitted and sends it back to
the user.
What is a Javabean? A Javabean
is just a java class with the following requirements.
i) It has a
public no-args constructor
ii) It has 'set'
and 'get' methods for its properties.
iii) It may have
any general functions.
iv) If required it
must be Serializable.
However,
It is not
necessary always that a bean should have properties.
If there are no
properties, we need not provide 'set' & 'get' methods either.
( Even the no-args constructor
is provided by the compiler by default!)
If the bean uses library
classes alone, it is automatically serializable.
In that case, it becomes
just a class for encapsulating some functionality (ie) business logic.
Such a bean , may or may not have a
visual representation. As the field has moved away from the
desktop, to webserver, there is no point in providing a visual
represenation for a component which works in server side as the user
cannot interact with it directly. (Such a bean used by jsp in
webserver, is exactly similar to ASP component). ( So no worry about
jar file ,BDK, BeanBox etc traditionally associated with JavaBean)
Let us now create such a bean.
The Tomcat server searches for any classes in c:\tomcat\webapps\root\web-inf\classes
folder. We create a subfolder under classes folder and name it 'ourbeans'.
Therefore, we specify the
package information in the first line of our jspbean as :
package
ourbeans;
*************************
package ourbeans;
public class greeterbean
{
public greeterbean()
{ }
public String
greetme(String s)
{
return "How
are you?...."+s;
}
}
*****************************************************************
To compile the above file :
> javac greeterbean.java
We get greeterbean.class
We move this class file to:
c:\tomcat\webapps\root\web-inf\classes\ourbeans
*****************************************************************
We will now create greeter.htm &
greeter.jsp and place them in:
c:\tomcat\webapps\root
greeter.htm
<html>
<body>
<form method=post action=greeter.jsp>
<input type=text
name='text1'>
<input type=submit>
</form>
</body>
</html>
greeter.jsp
<html>
<body>
<jsp:useBean
id="greeter1" class="ourbeans.greeterbean"
/>
<%
String s = request.getParameter("text1");
String
a = greeter1.greetme(s);
out.println(a);
%>
</body>
</html>
In the above example, the
business logic was invoked through an instance of 'greeter' bean class. So,
it is object-oriented approach. Where was the method invoked? It was
invoked in the Webserver.Where was the object available? The object was
available in webserver itself.(ie) 'locally'.
This point is worth noting carefully. For
this reason, the jspbean method cannot be classified as
'Distributed Object ' Technology. Because, in Distributed Object
Technology, the object on which the method is invoked locally, should be
available in a remote server and not locally .( Why is this distinction so
important? Is it important atall? If we are able to answer these questions
to ourselves convincingly, we have truly grasped the essence of Enterprise
computing but not otherwise. That is why this point is
being laboured.
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