Writing Servlets
Hello, Again!
The third and last servlet we discuss here adds the capability of maintaining state using a session object.
Web Servers have no built-in concept of the state of an application between the invocation of servlets.
Thus, if we want this feature, we must provide some mechanism of passing state when we write web applications.
Fortunately, the Servlet API provides this capability for free.
When you create a session object in the Servlet API, a cookie is presented to the user's web browser which is used to tie the user to a unique session id.
This session id is then presented as a cookie each time the user goes back to the servlets related to the application on that Web Server.
Meanwhile, the Java Servlet engine maintains the data that you placed inside a session object.
This session data is maintained between distinct invocations of servlets related to this application.
In other words, by storing information in the session object when a servlet is executed, a subsequent call to that servlet by the same user can allow the data to be retrieved.
A session object is tied to a single user.
We will modify the HelloUser servlet to keep track of the name entered on the form using the session object.
Using this method, if the user goes back to the same servlet that they had previouslny entered a name for, the HTML form asking for the name will not display.
Instead, the servlet will know that it can obtain the name data from the session object instead.
The code to HelloSession.java appears below:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloSession extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
HttpSession session = request.getSession(true);
String name = (String)session.getValue("name");
if (name == null || name.equals("")) {
name = request.getParameter("name");
}
if (name == null || name.equals("")) {
out.println("<HTML>");
out.println("<HEAD>");
out.println("<TITLE>Please Enter Your Name</TITLE>");
out.println("</HEAD>");
out.println("<BODY BGCOLOR=\"WHITE\">");
out.println("<H1>Please Enter Your Name</H1>");
out.println("<FORM>");
out.println("Full Name:");
out.println("<INPUT TYPE=\"TEXT\" NAME=\"name\"><P>");
out.println("<INPUT TYPE =\"SUBMIT\"><P>");
out.println("</FORM>");
out.println("</BODY>");
out.println("</HTML>");
} else {
session.putValue("name",name);
name = "Hello, " + name + "!";
out.println("<HTML>");
out.println("<HEAD>");
out.println("<TITLE>" + name + "</TITLE>");
out.println("</HEAD>");
out.println("<BODY BGCOLOR=\"WHITE\">");
out.println("<H1>" + name + "</H1>");
out.println("</BODY>");
out.println("</HTML>");
}
}
}
Adding session code is not that difficult an extension.
As you can see, most of the change to add state handling involved adding the following code:
HttpSession session = request.getSession(true);
String name = (String)session.getValue("name");
if (name == null || name.equals("")) {
name = request.getParameter("name");
}
A session object is created using the request object.
When the request object's getSession method is called with an arguement of true, then a brand new session will be created if it does not already exist from a prior call.
Obtaining data from the session is easy.
The getValue method returns an object that matches a string value.
This is similar to the Hashtable object in the JDK 1.0.2 and 1.1 APIs.
Unfortunately, like the Hashtable, we need to coerce the return value.
Since a generic object is returned, we must coerce it into a String class which is what we want to assign to the name variable.
If the name is still not defined after probing the session object, we try to get the name out of any HTML form values that might have been filled into an HTML form by the user.
Although this ends our discussion of the code snippet above, our modifications to HelloUser.java do not end here.
In addition to the code listed above, we also added a putValue method in the area of the script that prints the name in an HTML page.
The following is the line of code that places the name value inside of the Servlet engine's session object.
session.putValue("name",name);
NEXT
Gunther Birznieks contributes to JavaBoutique's Web/Networking column.
Gunther currently works for Barclays Capital in London, one of the leading global investment banks in Europe and has previously worked as a senior computer scientist in the Human Genome Project.
Gunther is also known for writing several books on Web Programming (Perl, CGI, Java) as well as for co-creating Extropia with Selena Sol.
Extropia is one of the best known public domain web programming archives
Email: gunther@extropia.com.
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