Servlet - Flow Of Execution

The flow of execution in a servlet defines how a client request is processed and a response is generated in a Java web application. It follows a structured lifecycle managed by the servlet container. This flow ensures efficient handling of requests using methods like init(), service(), and destroy().

Client request is received and passed to the servlet via the service() method.
Based on request type, doGet() or doPost() is executed.
The servlet processes data and sends a response back to the client.

Flow of Execution of Servlets

The flow of execution of servlets describes how a client request is processed and how a response is generated in a Java web application. This process is managed by the servlet container and follows a defined lifecycle.

Client Request :The client (web browser) sends an HTTP request to the web server.
Request Forwarding: The web server forwards the request to the servlet container.
Servlet Mapping: The servlet container identifies the appropriate servlet using URL mapping.
Servlet Loading & Initialization : If the servlet is not already loaded, the container loads it. The init() method is called (executed only once).
Request Processing: The container calls the service() method. Based on the request type, doGet(), doPost(), etc., are invoked.
Response Generation: The servlet processes the request and prepares the response (HTML/JSON).
Response Sent Back: The response is sent back to the client through the servlet container and web server.
Servlet Destruction: When the server shuts down, the destroy() method is called.

Steps to Implement Servlet Execution Example

Step 1: Create Project

Open Eclipse -> Create Dynamic Web Application
Project name: ServletFlow

Step 2: Create index.html

Create an HTML file to take user input
Collect data (user name) from browser
Send request to servlet using form

Create the below form to take the user name as input from the client browser.

HTML

<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Home</title>
</head>
<body>
    <form action="hello" method="post">

        Enter your name and click on submit: 
        <input type="text" name="name" />
        <input type="submit" />

    </form>
</body>
</html>

Step 3: Create Servlet (HelloServlet.java)

Create a servlet class inside src folder
Handle client request
Process data and generate response

Create below servlet to accept the client request, process it, and generate the response.

Java

package com;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
        throws ServletException, IOException
    {

        // Get the client entered data from request object
        String name = request.getParameter("name");

        // set the response content type
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        // Print hello message to the client browser in
        // response object
        out.println("<h3>Hello " + name + "!!</h3>");
        out.close();
    }
}

Step 4: Configure web.xml

Create deployment descriptor inside WEB-INF
Map URL to servlet
Define welcome file

Create deployment descriptor - web.xml file to mention the welcome file and to map the URL to the Servlet class.

XML

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" 
         xmlns="http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html" 
         xsi:schemaLocation="http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
  <display-name>ServletFlow</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
  </welcome-file-list>
  <servlet>
    <servlet-name>HelloServlet</servlet-name>
    <servlet-class>com.HelloServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
</web-app>

Step 5: Run Application

Right-click project -> Run on Server
Server starts and deploys application
Browser opens index.html automatically

Step 1: Deployment Overview

conceptual diagram of the servlet container — Servlet Execution Flow - Step 1

Server Side:

Servlet container runs inside the server
Web application (e.g., ServletFlow) is deployed
WEB-INF contains web.xml and servlet .class files

Client Side:

Browser sends HTTP requests and receives responses
Communication happens via a connection using IP address and port number

Step 2: Server Start & Deployment

Servlet container recognizes and deploys all web applications
Creates a ServletContext object for each application
Locates web.xml inside the WEB-INF folder
Loads and parses the web.xml file
Stores application-level data in the ServletContext object

servlet context object — Servlet Execution Flow - step 2

Now, the container will identify the welcome file page to display on the client browser based on the "welcome file list" mentioned in the "web.xml" file. Here, we specified, "index.html" as a welcome file. So, it displays that HTML page to the browser. URL: http://localhost:8081/ServletFlow/

Once we enter the name and click on submit, then a request will come to protocol with the data entered by the client.

Step 3: Request Processing

Request is divided into header (browser details) and body (user data)
Server validates the request and forwards it to the servlet container

Container identifies the application and resource using URL pattern (/hello)
Mapping is checked in web.xml inside WEB-INF
Corresponding servlet .class file is located
Servlet lifecycle starts after identification

Step 4: Servlet Lifecycle

Servlet Loading: Container loads servlet .class file into memory (if not already loaded)
Instantiation: Creates servlet object and ServletConfig object
Initialization: Calls init() method once to initialize the servlet

Request Handling:
1. Creates HttpServletRequest and HttpServletResponse objects
2. Calls service() → which invokes doPost() / doGet()
3. Request object holds headers, parameters, and attributes
Response Generation: Servlet processes request and prepares response
Response Dispatch: Response is sent to server ->protocol -> client (header + body) . Thread is destroyed after execution

Step 5: Response & Destruction

Response Sent to Client: Protocol sends the response to the browser
Display Output: Browser reads response body and displays result
Connection Closed: Connection between client and server is terminated

Object Cleanup: Container destroys request and response objects
Waiting State: Container waits for new requests
Servlet Destruction: On server shutdown, destroy() method is called:
1. Servlet object and ServletConfig are removed
2. Servlet bytecode is unloaded from memory
3. ServletContext object is destroyed