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The Life Cycle of a Servlet

This tutorial explains the Servlet interface and Servlet life cycle. A servlet example is given at the end which demonstrates the life cycle of a servlet.

Servlets are managed components. They are managed by web container. Of the various responsibilities of web container, servlet life cycle management is the most important one. A servlet is managed through a well defined life cycle that defines how it is loaded, instantiated ad 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. Most of the servlet you develop will implement it by extending HttpServlet class.

The servlet life cycle methods defined in Servlet interface are init(), service() and destroy(). The life cycle starts when container instantiates the object of servlet class and calls the init() method, and ends with the container calling the destroy() method.

The signature of this methods are shown below. 

public void init(ServletConfig config) throws ServletException<br />
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException<br />
public void destroy()

The servlet life cycle consists of four steps, instantiation, initialization, request handling and end of service. Each of these steps is explained below.

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

During initialization stage of the Servlet life cycle, 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

Understanding the Servlet life cycle with an example

We will create a Servlet that will help you in better understanding the life cycle of a servlet. 

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");
	}	
 
}

Above ServletLifeCycleExample Servlet extends HttpServlet and overrides init() Service() and destroy() methods. The servlet logs a message into server log file when servlet is initialized and sets counter to 0. Every time the service method is called, it increments the counter by 1 and displays the current value of counter to user. finally when destroy method is called, it logs a message to server log file.

Compile above class and put it into WEB-INF/classes directory. Define the servlet and servlet mapping into web.xml file. If you do not know how to define the servlet in web.xml see this servlet example.

Deploy the application and start the server. Call the servlet by opening the URL that you specified as url-pattern in web.xml. See the server log file. Hit the URL multiple times and you will see that a message is logged every time the service() method is called. The current value of counter will be displayed in browser.

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