What is a Servlet?
- A servlet is a server-side program which executes inside a Web server, such as Apache.
- It Receives HTTP requests from the client and provides HTTP responses
The Servlet API
Servlet API is used to create servlets.
Servlets use classes and interfaces from two packages: javax.servlet and javax.servlet.http. The javax.servlet package contains classes to support generic, protocol-independent servlets.
These classes are extended by the classes in the javax.servlet.http package to add HTTP-specific functionality.
The top-level package name is javax instead of the familiar java, to indicate that the Servlet API is a standard extension.
Every servlet must implement the javax.servlet.Servlet interface. Most servlets implement it by extending one of two special classes: javax.servlet.GenericServlet or javax.servlet.http.HttpServlet.
What is Tomcat?
Tomcat is a Servlet Container.
- Manages servlet loading/unloading
- Works with the Web server (e.g. Apache) to direct requests to servlets and responses back to clients
How to Create and Run Servlets
1. Download and install Java SE 6 (including JRE)
You can download Java from the link below
http://www.java.com/en/download/index.jsp
Install it in your system
2. Download Tomcat
You can download Tomcat from the link below. The latest version of Tomcat is Tomcat 7.0
Install Tomcat.
3. Configure the server
4. Test set up
- Start tomcat
- Open browser and type http://localhost:8080
The Tomcat Home Page gets displayed if it’s successfully installed
TomcatHomePage
Tomcat Directory/File Structure
Tomcat Directory Structure
This is the Tomcat Directory structure once it is installed in your system.
Tomcat Directories/Files Description
| Directory | Description |
| bin | The binary executables and scripts |
| conf | Configuration files |
| lib | JAR files that contain classes that are available to all Web applications |
| logs | Log files |
| webapps | Web applications |
| work | Temporary files and directories for Tomcat |
Deploy a Web Application in Tomcat
- The application should be under the C:\Tomcat 7.0\webapps directory
- A root directory for the application can be created under the C:\Tomcat 7.0\webapps directory (e.g. C:\Tomcat 7.0\webapps\ContactApp)
- The root directory contains sub-directories, the index file, HTML and JSP files for the application.
- All directories and files relating to our applications should be under this directory
- Applications that use servlets must have the WEB-INF and WEB-INF\classes directories inside their root directories
- Put the Deployment descriptor for the application inside the WEB-INF directory.
Deployment Descriptor
- Deployment descriptor is an XML file with the name web.xml
- It should be in the WEB-INF directory
A Sample Deployment Descriptor
<web-app> <servlet> <!--Name for referring to it elsewhere in the DD --> <servlet-name>HelloServlet</servlet-name> <!-- Full name, including any package details--> <servlet-class>servlets.HelloWorldServlet</servlet-class> </servlet> <servlet-mapping> <!--Name of the servlet again --> <servlet-name>HelloServlet</servlet-name> <!--Client’s URL will have this --> <url-pattern>/HelloServlet</url-pattern> </servlet-mapping> </web-app>
Tomcat: Important Directories for an application in webapps directory
| Directory | Description |
| doument root | Contains sub-directories, the index file, HTML and JSP files for the application. |
| \WEB-INF | Contains a file named web.xml. It can be used to configure servlets and other components that make up the application. |
| \WEB-INF\classes | Contains servlets and other Java classes that are not compressed into a JAR file. If Java packages are used, each package must be stored in a subdirectory that has the same name as the package. |
| \WEB-INF\lib | Contains any JAR files that contain Java classes that are needed by this application, but not by other Web |
Writing your First Servlet
Open any Text Editor and Type the Program below.
HelloWorldServlet.java
package servlets;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class HelloWorldServlet extends HttpServlet
{
public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException
{
res.setContentType("text/html");
PrintWriter out = res.getWriter();
out.println("<html>");
out.println("<head><title>Hello World Servlet</title></head>");
out.println("<body>");
out.println("<h1>Hello World!!!</h1>");
out.println("<p>Welcome to the world of Servlets and JSP!!!</p>");
out.println("</body></html>");
}
}
Compile it like any other Java Program using javac command. The class file should get generated successfully.
Deploying and Running your Servlet
- Create a root directory for the application under the C:\Tomcat 7.0\webapps directory (e.g. C:\Tomcat 7.0\webapps\Servlet1)
- Create a WEB-INF and WEB-INF\classes directories inside the Servlet1 Directory.
- Make a directory called servlets inside WEB-INF\classes directory as our HelloWorld Servlet is inside this package.
- Put the class file of the servlet inside WEB-INF\classes\servlets directory.
- Write the Deployment Descriptor web.xml and put it under the WEB-INF directory.
Deployment Descriptor for Hello World Servlet
<web-app> <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>servlets.HelloWorldServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/HelloServlet</url-pattern> </servlet-mapping> </web-app>
Running Hello World Servlet
- Start tomcat
- Open browser and type http://localhost:8080. The Home Page of Tomcat should open.
- Type http://localhost:8080/Servlet1/HelloServlet in the address bar of your browser.
- The Servlet should run and display the famous greetings in programming
Snapshot of the o/p
Hello World Servlet
Servlets are extremely powerful and can do a lots of work very easily as compared to CGI. They are good in performance and secure. 
