Servlets

Servlets are a fundamental part of Java's web technology stack. They are server-side components that extend the capabilities of web servers and facilitate the creation of dynamic web applications. Servlets process client requests and generate responses, making them a key player in building web applications. In this guide, we'll cover the basics of servlets with suitable examples.

Prerequisites

Before diving into servlets, ensure that you have the following prerequisites:

1. Java: Basic knowledge of Java programming.

2. Eclipse IDE (or any Java IDE of your choice).

3. Apache Tomcat: A web server or servlet container. Download and install it from the Apache Tomcat website (https://tomcat.apache.org/).

Creating a Simple Servlet

In this section, we'll create a basic servlet and deploy it on the Apache Tomcat server.

1. Open Eclipse and create a new dynamic web project:

   • File > New > Dynamic Web Project.

   • Enter a project name, e.g., "SimpleServletDemo."

2. Create a Servlet Class:

   • Right-click on the src folder within your project.

   • Select New > Servlet.

   • Provide a package name (e.g., com.example) and a class name (e.g., SimpleServlet).

   • Click Next, then Finish.

3. Edit the Servlet Class:

   In the SimpleServlet.java class that was generated, override the doGet method to handle HTTP GET requests:

   import java.io.IOException;
   import javax.servlet.ServletException;
   import javax.servlet.annotation.WebServlet;
   import javax.servlet.http.HttpServlet;
   import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;
   
   @WebServlet("/SimpleServlet")
   public class SimpleServlet extends HttpServlet {
       protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           response.getWriter().println("Hello, World from SimpleServlet!");
       }
   }

4. Configure the Deployment Descriptor:

   • Open the web.xml file in the WEB-INF folder of your project.

   • Add the following servlet and servlet-mapping entries:

   <servlet>
       <servlet-name>SimpleServlet</servlet-name>
       <servlet-class>com.example.SimpleServlet</servlet-class>
   </servlet>
   <servlet-mapping>
       <servlet-name>SimpleServlet</servlet-name>
       <url-pattern>/SimpleServlet</url-pattern>
   </servlet-mapping>

5. Run the Application:

   • Right-click on your project.

   • Select Run As > Run on Server.

   • Choose the Tomcat server and click Finish.

   • Your servlet will be accessible at http://localhost:8080/SimpleServletDemo/SimpleServlet.

Handling HTTP Requests

Servlets can handle different HTTP methods like GET, POST, and more. Here's how to handle a POST request in a servlet:

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // Process POST request here
}

Request and Response Objects

Servlets use HttpServletRequest and HttpServletResponse objects to interact with the client:

• HttpServletRequest: Represents the request made by the client and provides methods to access request parameters, headers, and more.

• HttpServletResponse: Represents the response that will be sent to the client. You can set response content, headers, and status codes using this object.

Example: Reading Request Parameters

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String name = request.getParameter("name");
    response.getWriter().println("Hello, " + name + "!");
}

Sending Redirects

You can use the sendRedirect method to redirect a client to another URL:

response.sendRedirect("newpage.jsp");

Session Management

Servlets can maintain session data using HttpSession:

// Create or access a session
HttpSession session = request.getSession();

// Store data in the session
session.setAttribute("username", "john_doe");

// Retrieve data from the session
String username = (String) session.getAttribute("username");

Servlets are a powerful tool for building dynamic web applications in Java. They handle HTTP requests and responses, allowing you to create interactive and data-driven web applications. This guide provides a foundational understanding of servlets, and further exploration and practice will enhance your web development skills.