servlet in brief

1. What is a Servlet?

A Java Servlet is a Java program that runs on a web server and handles HTTP requests and responses. Servlets are used to create dynamic web content.

2. Tomcat Server

Apache Tomcat is an open-source servlet container that provides an environment to run Java Servlets and JSPs.

• Acts as a web server and servlet container

• Deploy .war files to run web apps

3. Servlet Lifecycle

The servlet lifecycle is managed by the servlet container and includes:

1. Loading – Servlet class is loaded.

2. Instantiation – Object of the servlet is created.

3. Initialization – init() method is called once.

4. Request handling – service() method is called for each request.

5. Destruction – destroy() method is called before the servlet is removed.

public class MyServlet extends HttpServlet {
    public void init() throws ServletException {
        // Initialization code
    }
 
    public void service(HttpServletRequest req, HttpServletResponse res) throws IOException {
        // Handle request
    }
 
    public void destroy() {
        // Cleanup code
    }
}

4. Servlet Class Hierarchy

Object
  ↳ GenericServlet
       ↳ HttpServlet

• GenericServlet is a protocol-independent base class.

• HttpServlet is commonly used for HTTP-specific servlets.

5. Handling GET and POST Requests

Servlets use doGet() and doPost() methods to handle HTTP requests.

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

6. HTML to Servlet Communication

HTML form data can be sent to a servlet using GET or POST.

HTML Example:

<form action="MyServlet" method="post">
  Name: <input type="text" name="username">
  <input type="submit" value="Submit">
</form>

Servlet:

String name = request.getParameter("username");

7. Session Tracking

Used to maintain state between multiple requests from the same user.

a. Cookies

Small text files stored in the client’s browser.

Cookie ck = new Cookie("name", "John");
response.addCookie(ck);

b. HTTP Session

Stores data on the server.

HttpSession session = request.getSession();
session.setAttribute("user", "John");

8. RequestDispatcher

Used to forward or include a request from one resource to another.

Forwarding:

RequestDispatcher rd = request.getRequestDispatcher("nextServlet");
rd.forward(request, response);

Including:

RequestDispatcher rd = request.getRequestDispatcher("header.html");
rd.include(request, response);

(Shall I continue with Java Server Pages (JSP) next in this level of detail?)