Servlet lifecycle

Servlets are managed by web container.Life cycle defines how servlet is loaded, instantiated and initialized, handles requests from clients and how it is taken out of service.The servlet life cycle methods are defined in the javax.servlet.Servlet interface of the Servlet API that all Servlets must implement directly or indirectly by extending GenericServlet or HttpServlet abstract classes.
The servlet life cycle methods defined in Servlet interface are init(), service() and destroy().
The signature of these methods are shown below.

public void init(ServletConfig config) throws ServletException

public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException

public void destroy()

First the url collected from the user generates an http request, this request is mapped with the appropriate servlet and loaded in the address space of the server. once the servlet is loaded the following 3 phases starts
Init() – initializes the servlets initialization information with the configuration parameters.
Service – business logic i.e. request and response services goes here.
Destroy() – used to destroy all variables and objects used by the servlet.
Loading and instantiation during this step, web container loads the servlet class and creates a new instance of the servlet. The container can create a servlet instance at container startup or it can delay it until the servlet is needed to service a request.

Initialization
The web container initializes the servlet instance by calling the init() method. The container passes an object implementing the ServletConfig interface via the init() method. This configuration object allows the servlet to access name-value initialization parameters from the web application Request handling After a servlet is properly initialized, the servlet container may use it to handle client requests. Requests are represented by request objects of type ServletRequest. The servlet fills out response to requests by calling methods of a provided object of type ServletResponse. These objects are passed as parameters to the service method of the Servlet interface. In the case of an HTTP request, the objects provided by the container are of types HttpServletRequest and HttpServletResponse.
End of service

When servlet container determines that the servlet should be removed from the service, it calls the destroy() method of the servlet to allow servlet to release any resources it is using (eg. database connections or file handles). Before calling the destroy() method, the container allows any request threads that are currently running in the service method to complete execution within a defined time limit. Once the servlet is removed out of service, container will not send any requests to the servlet. If the servlet needs to be put in service again, the container will create a new servlet instance and the life cycle begins from the initialization phase.

Understanding the Servlet life cycle with an example

import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
public class ServletLifeCycleExample extends HttpServlet {
 
	private int count;
 
	@Override
	public void init(ServletConfig config) throws ServletException {
		super.init(config);
		getServletContext().log("init() called");
		count=0;
	}
 
	@Override
	protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		getServletContext().log("service() called");
		count++;
		response.getWriter().write("Incrementig the count: Count = "+count);
 
	}
 
	@Override
	public void destroy() {
		getServletContext().log("destroy() called");
	}	

Architecture Digram:
The following figure depicts a typical servlet life-cycle scenario.

First the HTTP requests coming to the server are delegated to the servlet container.
The servlet container loads the servlet before invoking the service() method.
Then the servlet container handles multiple requests by spawning multiple threads, each thread executing the service() method of a single instance of the servlet.