Servlet Lifecycle init, service, destroy Methods: Tutorial, Examples, FAQs & Interview Tips

Servlet Lifecycle Overview

The Servlet tl-container manages the complete lifecycle of a Servlet. The lifecycle consists of three main phases:

Initialization - init() is called once when the Servlet is first loaded
Request Handling - service() is called for every client request
Destruction - destroy() is called once when the Servlet is removed from service

The tl-container creates only one instance of each Servlet and handles concurrent requests using multiple threads. This is why Servlet instance variables must be thread-safe.

Servlet Lifecycle Methods

package com.example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.WebServlet;
import java.io.*;
import java.util.concurrent.atomic.AtomicInteger;

@WebServlet("/lifecycle")
public class LifecycleServlet extends HttpServlet {

    // Use AtomicInteger for thread-safe counter
    private AtomicInteger requestCount = new AtomicInteger(0);
    private String initMessage;

    // ===== PHASE 1: INITIALIZATION =====
    // Called ONCE when servlet is first loaded (or at startup if load-on-startup is set)
    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        // Read init parameters
        initMessage = config.getInitParameter("message");
        if (initMessage == null) initMessage = "Default message";
        System.out.println("[INIT] LifecycleServlet initialized. Message: " + initMessage);
    }

    // ===== PHASE 2: REQUEST HANDLING =====
    // service() dispatches to doGet/doPost/etc. based on HTTP method
    // You can override service() directly, but it's better to override doGet/doPost
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        System.out.println("[SERVICE] Handling request #" + requestCount.incrementAndGet());
        super.service(req, resp); // Delegates to doGet/doPost/etc.
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.setContentType("text/html;charset=UTF-8");
        PrintWriter out = resp.getWriter();
        out.println("<h2>Servlet Lifecycle Demo</h2>");
        out.println("<p>Init message: " + initMessage + "</p>");
        out.println("<p>Total requests: " + requestCount.get() + "</p>");
        out.println("<p>Thread: " + Thread.currentThread().getName() + "</p>");
    }

    // ===== PHASE 3: DESTRUCTION =====
    // Called ONCE when servlet is removed (server shutdown or undeploy)
    @Override
    public void destroy() {
        System.out.println("[DESTROY] LifecycleServlet destroyed. Total requests served: "
                + requestCount.get());
        // Release resources: close DB connections, stop threads, etc.
    }
}

load-on-startup and Lazy Loading

By default, Servlets are loaded lazily - on the first request. You can force eager loading at startup using load-on-startup:

Negative value (default): Lazy loading - loaded on first request
0 or positive: Eager loading - loaded at tl-container startup. Lower numbers load first.

load-on-startup Configuration

// loadOnStartup = 1 means load at startup, priority 1 (loads before priority 2, 3, etc.)
@WebServlet(urlPatterns = "/app", loadOnStartup = 1)
public class AppInitServlet extends HttpServlet {
    @Override
    public void init() throws ServletException {
        // This runs at server startup
        System.out.println("Application initialized at startup");
        // Initialize shared resources: DB connection pool, caches, etc.
        getServletContext().setAttribute("appStartTime", System.currentTimeMillis());
    }
}

<servlet>
    <servlet-name>AppInitServlet</servlet-name>
    <servlet-class>com.example.AppInitServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>

<servlet>
    <servlet-name>DataServlet</servlet-name>
    <servlet-class>com.example.DataServlet</servlet-class>
    <load-on-startup>2</load-on-startup>
</servlet>

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Servlet Lifecycle init, service, destroy Methods: Tutorial, Examples, FAQs & Interview Tips

Servlet Lifecycle Overview

load-on-startup and Lazy Loading

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