Understanding the JavaBeans Java Class
A JavaBean is a Java class that uses setter and getter methods, as a means of introspection of the class attributes. Introspection, like reflection, is the ability to query class attributes by using getXXX(), where XXX is one of the class attributes. A JavaBean can save its state through persistence. Therefore, all of its persistent properties must be serializable. You use a JavaBean to encapsulate data mapping between a form's parameters and a database. It can be used as a data container (DataBean) to transfer information between the Web tier and the EJB tier. A JavaBean can encapsulate data for display purposes (ViewBean) and manage data flow between applets and applications. JavaBeans use events to interact with other beans. The most powerful use of a JavaBean is to automatically map and populate its properties from a form.
One example of a JavaBean is the StudentDataBean to transfer data between the UserManager EJB and the Web tier:
import java.io.Serializable;
public class StudentDataBean implements java.io.Serializable {
private String userId;
private String password;
private String firstName;
private String lastName;
public StudentDataBean (String userId, String password,
String firstName, String lastName) {
this.userId = userId;
this.password = password;
this.firstName = firstName;
this.lastName = lastName;
}
public void setFirstName(String n){ firstName = n;}
public String getFirstName(){ return firstName;}
public void setLastName(String n){ lastName = n;}
public String getLastName(){ return lastName;}
public void setUserId(String n){ userId = n;}
public String getUserId(){ return userId;}
public void setPassword(String n){ password = n;}
public String getPassword (){ return password;}
}
JSP can use a JavaBean to automatically capture the form data of the client tier. Here, we use the standard tag <jsp:useBean> to process the form:
<%@ page import="day07.* %>
<jsp:useBean id="myBean" scope="session"
class="day07.StudentDataBean" property="*" />
<html>
<head>
<title>Using Student Bean</title> </head>
<body>
<h1> Using Automatic Mapping of Student Bean</h1>
<%
String userName = request.getParameter( "Login Name" );
out.print(userName + " : " + myBean.getUserId());
%>
</body>
</html>
In the preceding example, the <jsp:useBean> tag creates a JavaBean component by instantiating the StudentDataBean class, and automatically populates all its properties, as indicated by "*".
Servlets can use JavaBeans as parameters passed to their method invocations when calling an EJB. The EJB populates the JavaBean properties with result data after processing the servlet's request from a database or other entity EJBs. For example, the following doPost() method displays all courses for which a student has enrolled:
public void doPost (HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
String studentId = req.getParameter("Login name");
PrintWriter out = res.getWriter();
try{
Collection coll = userManager.getCourses(studentId);
while (coll.hasNext){
EnrollmentDataBean enroll = (EnrollmentDataBean) coll.getNext();
out.write("Course id: " + enroll.getCourseId() + "<br>");
}catch{
e.printStackTrace();
}
}