Writing Servlets
Hello, Browser!
The most basic feature of servlets is the the capability of communicating information back to a web browser.
Our first script will take advantage of this by sending some data as HTML so that a browser can display a "Hello" message.
|
Selena Sol's Introduction To Java provides a good overview of the Java language.
If you are unfamiliar with Java, you will find this to be a useful companion reference to the Java Networking series.
|
The following is the source code to the "HelloBrowser.java" file.
This Servlet is one of the simplest you will ever write.
It basically pulls information from the Web Server Request Header about the browser that called this servlet and says 'Hello, [Browser]!' where [Browser] is the browser type.
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloBrowser extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String title = request.getHeader("User-Agent");
if (title == null || title.equals("")) {
title = "Unknown Browser";
}
title = "Hello, " + title + "!";
out.println("<HTML>");
out.println("<HEAD>");
out.println("<TITLE>" + title + "</TITLE>");
out.println("</HEAD>");
out.println("<BODY BGCOLOR=\"white\">");
out.println("<H1>" + title + "</H1>");
out.println("</BODY>");
out.println("</HTML>");
}
}
All Servlets for a Web Server extend HttpServlet.
The servlet can then be extended by overriding default methods such as doGet and doPost to perform actions based on whether the server is being called using GET or POST parameters.
Because we are calling this servlet as a straight URL without any parameters, we know that we are using the GET method of passing CGI Form variables.
Therefore, we extend the doGet method of the HttpServlet class in our HelloBrowser class.
The doGet method takes two parameters: HttpServletRequest and HttpServletResponse.
The request variable contains information about parameters passed from an HTML form as well as HTTP headers that were passed in via the browser, while the the response variable allows us to print out HTML code to the user's browser.
The response variable also allows us to manipulate the HTTP headers that would pass information such as cookies and content type information to the user's browser.
For the purposes of the HelloBrowser program, we use the response variable to set the content type to "HTML", and get an output stream where we can print the HTML to.
The request variable is used to get the information about the "User-Agent".
This "User-Agent" name is the name of the browser that was sent along with the other HTTP headers when this servlet was invoked.
The following snippet of code was used to get the "User-Agent" value from the HTTP header:
String title = request.getHeader("User-Agent");
The rest of the servlet is then spent generating the HTML and printing it to the out variable.
We created the out variable as the output stream from the response object by using the following code:
PrintWriter out = response.getWriter();
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.
New on the Java Boutique:
New Review:
Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling
API boasts simplicity, ease-of-integration, a well-rounded feature
set, and it's free!
New Applet:
Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
| 
