J2EE Tutorial - Java Bean
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 : 0
> javac greeterbean.java
We get greeterbean.class
We move this class file to: 1
c:\tomcat\webapps\root\web-inf\classes\ourbeans
*****************************************************************
We will now create greeter.htm & greeter.jsp and place them in: 2
c:\tomcat\webapps\root
greeter.htm
<html> 3
<body>
<form method=post action=greeter.jsp>
<input type=text name='text1'> 4
<input type=submit>
</form>
</body> 5
</html>
greeter.jsp
<html> 6
<body>
<jsp:useBean id="greeter1" class="ourbeans.greeterbean" />
<% 7
String s = request.getParameter("text1");
String a = greeter1.greetme(s);
out.println(a); 8
%>
</body>
</html> 9
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.