The Servlet API

The example application in Figure 1 uses a Java servlet invoked by URL request http://domainOrIP/servlet/Seminar. The Seminar servlet dynamically builds the HTML response with the latest product information. That HTML response includes a form whose action is handled by the same Seminar servlet (although it can be a different servlet). The Seminar servlet is Java built atop the generic HttpServlet class. HttpServlet provides code implementations for common services, two of which I’ve already mentioned: doGet and

doPost. HttpServlet’s code implementations for doGet and doPost do nothing worthwhile. They’re in HttpServlet as placeholders to assure the servlet engine that all servlets, if only by order of object-oriented (OO) inheritance, have these methods. This is important because the servlet engine calls the doGet method based on a URL request and calls the doPost based on an HTML form action request.

If your servlet is to process an HTTP GET request from a URL, you must define a custom implementation of HttpServlet’s doGet method, and if your servlet is to process HTML forms, you must code a doPost.

HttpServlet also has a method called init. The servlet engine invokes a servlet’s init method the first time that servlet is requested, like an initialization subroutine in RPG. As you know from RPG programs, this initial processing can be handy. For servlets, you may want to code your init method to load a Java Database Connectivity (JDBC) driver and initialize JDBC connections or, if you prefer record-level access, open files as global fields declared in a Java class but not in a method of that class. Those JDBC connections or record-level access file opens then become available to all remote requests, saving the time required to connect to the database.

The Example

Code for the Seminar Java servlet is given in Figure 3. At Label A, the Seminar class extends the HttpServlet class. At Label B, the global fields of the Seminar class are declared. The values of these fields are accessible to all clients. The init method, at Label C, first calls its dad’s implementation of init. When you override HttpServlet’s init method, you need to invoke the parent’s version of init because it does some important initial processing that you need not code yourself. The Seminar class’s custom version of the init method then creates a vector (a dynamically sizable array) to hold the registrants. This servlet is coded for simplicity, so pretend that the init function has gone to DB2/400 to obtain the latest seminar locations and counts. Otherwise, I may as well code this information in a static HTML file. That’s the point of servlets: to present dynamic information over the Internet.

As Good as It Gets

The doGet method on Label D in Figure 3 has two parameters: HttpServletRequest and HttpServletResponse. The first parameter contains information about the request; the second contains the dynamically generated HTML response. I suggest you review the Java documentation for these two classes, as they have some powerful capabilities. Both of them, for instance, have methods that make using HTTP cookies a breeze. The request contains any parameter on the URL, but because the Seminar application uses no URL parameters, its implementation of doGet ignores the request parameter. At Label E, however, the response parameter is used. Think of HttpServletResponse as a handle to an HTML file containing the response to the user’s request. At Label E, the document resp.setContentType (“text/html”) tells HTTP that this file is to be transferred to that browser as text-based HTML. The next line at Label E receives a handle to the output stream. It’s like setting a record format in an RPG screen program.

The doGet method on Label F in Figure 3 dynamically constructs an HTML file. All HTML files are enclosed by and tags. They also have a head and body , so these obligatory tags are inserted into the response buffer. Label F then builds the body of the HTML file, inserting the .gif file for MC’s coffee cup and listing the fall dates for my Java seminars. Again, in a real application, this information comes from a database. At Label G, the input form is constructed. The input form contains a drop-down selection for the city of choice and a text field for the registrant’s name. The doGet code ends by closing the print writer, at which point the servlet engine sends the HTML response to the browser (see Figure 2).

Postage Due

Once the user selects a city, keys in a name, and clicks Register, the ACTION option of the HTML form tag instructs the Seminar servlet to process the HTTP POST request. Code for the doPost method is given in Label H of Figure 3. The doPost method has the same two parameters as doGet. Again, the content type is set, and a handle to the output writer is obtained. At Label I, the city and name parameters specified by the client are retrieved. Then, at Label J, after stuffing the obligatory HTML, HEAD, and BODY tags, doPost’s code checks whether registration is full. If seats are still available, doPost increments the array element of the global count field associated with the selected city. (A real application would update a database at this point.) Figure 4 shows the dynamic HTML content generated by doPost when Billy Briggs registers, and Figure 5 shows how it looks on Billy’s browser.

You Must Pool Your Resources

Obviously, Internet applications are more complex than my Seminar servlet, and industrial- strength servlets have more requirements. They must have some form of transaction control to ensure data integrity. For performance, they should have some form of database connection pooling. They also need a strategy to maintain state across the stateless HTTP. Transaction control can be enabled with JDBC transactions, record-level access commitment control, or Enterprise JavaBeans (EJB) transactions. You can code your own connection pool logic or use the connection pooling features included with most Web application servers, including IBM’s WebSphere. For such things as the Internet’s popular shopping cart metaphor, you can use either HTTP 1.1’s Session API or HTTP cookies. You can also add elegant state management features by employing EJB with your servlets. You may ask, “Isn’t embedding HTML into a Java servlet the same thing as describing screens in RPG?” That is a problem. It’s always better to separate screen design from program design. It’s even more important with Web applications because HTML design is becoming a profession unto itself. Business programmers can easily present information logically with HTML but don’t often have the time or training to make it pretty with graphics. Recently, Sun developed Java Server Pages (JSP), which separates programming logic from screen presentation, and because JSP is relatively new, it’s fortunate that WebSphere supports JSP.

Related Materials

• “HTTP Undercover,” Teresa Pelkie, MC, July 1999
• “WebSphere for AS/400 Setup,” Lisa Wellman, AS/400 NetJava Expert Web Edition, www.midrangecomputing.com/anje/99/02

Figure 1: When a Web server sees the servlet qualification on a URL, it invokes the named Java servlet.








Fall '99 Seminar Dates

• September 22-24 -- Las Vegas
  
• October 18-22 -- Kansas City
  
• November 15-19 -- Dallas
  
• December 6-10 -- Phoenix
  

Online Registration

City:


Las Vegas
Kansas City
Dallas
Phoenix
 
Name:

Figure 2: HTML forms can associate a Java servlet to process the user request.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

public class Seminar extends HttpServlet

{

A G H

I

J

String cities[] = {"Las Vegas", "Kansas City", "Dallas", "Phoenix"};
int count[] = {57, 48, 99, 100};
Vector registrants[] = null;

public void init(ServletConfig sc)
throws ServletException
{
super.init(sc);
registrants = new Vector[cities.length];
for (int i = 0; i < cities.length; i++) {
registrants[i] = new Vector();

}

B D

E

F

C

Figure 3: The Seminar servlet creates a registration form in its doGet method and processes that form in its doPost method.






Registration Confirmation

Figure 4: The HTML response from a servlet contains dynamic information.

resp.setContentType("text/html");
PrintWriter out = new PrintWriter(resp.getOutputStream());

String name = req.getParameter("name");
String city = req.getParameter("city");

out.println("");
out.println("");
out.println("");
out.println("

Registration Confirmation

int iCity = 0;
while (cities[iCity].equals(city) != true)
iCity++;
if (count[iCity] >= 100) {
out.println("Sorry, the seminar is full in "+cities[iCity]);
} else {
count[iCity]++;
registrants[iCity].addElement(name);
out.println("Thank you, "+name+". Your registration is confirmed.");
}

out.println("");
out.println("");
out.close();

}

public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException

{

resp.setContentType("text/html");
PrintWriter out = new PrintWriter(resp.getOutputStream());

out.println("");
out.println("");
out.println("");
out.println("

"ALIGN=""Top"" WIDTH=500 HEIGHT=200>
");

out.println("

Fall '99 Seminar Dates

");
out.println("

");
out.println("
• September 22-24 -- Las Vegas");
  out.println("
• October 18-22 -- Kansas City");
  out.println("
• November 15-19 -- Dallas");
  out.println("
• December 6-10 -- Phoenix");
  out.println("

");

out.println("

Online Registration

");

out.println(""); out.println("City: ");
out.println("");
out.println("");
for (int i = 0; i < cities.length; i++)
out.println("" + cities[i]);
out.println(" ");
out.println("Name: ");
out.println(""SIZE=20 MAXLENGTH=50>");
out.println("

");
out.println("

"VALUE=""Register"">");

out.println("");
out.println("");
out.close();

}

public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException

{ }

}






Figure 5: Whether you use Java servlets or RPG CGI, the point is to interact with the user.