Servlet Lifecycle in Java | Complete Beginner’s Guide

Introduction

Every Java program has a beginning and an end.
Similarly, a Servlet follows a well-defined lifecycle that is managed by the Servlet container (such as Apache Tomcat).

Understanding the Servlet Lifecycle is extremely important because:

It explains how servlets are created, used, and destroyed
It helps you write efficient and bug-free web applications
It is a frequently asked interview topic

This guide explains the lifecycle step by step, using simple language, diagrams, and examples.

What Is the Servlet Lifecycle?

Simple Definition:

The Servlet Lifecycle is the sequence of steps that a servlet goes through from the moment it is loaded until it is destroyed.

These steps are fully controlled by the Servlet Container, not by the developer.

Who Manages the Servlet Lifecycle?

The Servlet Container (like Tomcat):

Loads the servlet class
Creates the servlet object
Calls lifecycle methods
Handles multiple requests
Destroys the servlet when needed

Developers do not manually create or destroy servlets.

Main Phases of the Servlet Lifecycle

The Servlet lifecycle consists of three main phases:

Initialization – init()
Request Processing – service()
Destruction – destroy()

Servlet Lifecycle Diagram (Text-Based)

Servlet Loaded
      |
      ↓
   init()
      |
      ↓
 service()
 (doGet / doPost)
      |
      ↓
   destroy()
      |
      ↓
Servlet Unloaded

Phase 1: Initialization (init())

What Happens in init()?

Called only once
Executed when servlet is first loaded
Used to perform startup tasks

Example Uses:

Database connection setup
Loading configuration data
Initializing resources

Method Signature:

public void init() throws ServletException {
    // Initialization code
}

init() is called only once during the servlet’s lifetime.

Phase 2: Request Handling (service())

What Happens in service()?

Called for every request
Handles client requests
Delegates to:
- doGet()
- doPost()
- doPut()
- doDelete()

Flow:

Client Request → service() → doGet() / doPost()

Example:

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

    response.getWriter().println("Hello from Servlet!");
}

This is the most frequently executed phase.

Phase 3: Destruction (destroy())

What Happens in destroy()?

Called only once
Executed before servlet is removed
Used to release resources

Example Uses:

Closing database connections
Releasing memory
Logging shutdown messages

Method Signature:

public void destroy() {
    // Cleanup code
}

After destroy(), the servlet is no longer available.

Complete Lifecycle Example

public class LifeCycleServlet extends HttpServlet {

    public void init() {
        System.out.println("Servlet Initialized");
    }

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

        response.getWriter().println("Servlet is processing request");
    }

    public void destroy() {
        System.out.println("Servlet Destroyed");
    }
}