SatyaCodes

Servlets- Servlet Lifecycle

Servlet Lifecycle

First we see the example, then we can understand the Lifecycle. For every Servlet program contains following strcuture

C:\Users\kaveti_S\Desktop\tmp.png

Here Servlets is Application name


Example 1: Using Servlet Interface

public class HelloServlet implements Servlet{
	ServletConfig config = null;
	@Override
	public void init(ServletConfig config) throws ServletException {
 this.config = config;
 System.out.println("1.Init...");
	}
	@Override
	public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
 System.out.println("2.Service ...");
	PrintWriter pw =	res.getWriter();
	pw.write("<h1>Hello, World!</h1>");
	}
	@Override
	public void destroy() {
 System.out.println("3.Destroy ..");
	}
	@Override
	public ServletConfig getServletConfig() {
 System.out.println("4.getServletConfig ..");
 return config;
	}
	@Override
	public String getServletInfo() {
 return "getServletInfo";
	}
}

web.xml
---------------
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
	<servlet>
 <servlet-name>hello</servlet-name>
 <servlet-class>demo.HelloServlet</servlet-class>
	</servlet>

	<servlet-mapping>
 <servlet-name>hello</servlet-name>
 <url-pattern>/hello</url-pattern>
	</servlet-mapping>

	<welcome-file-list>
 <welcome-file>index.jsp</welcome-file>
	</welcome-file-list>	 
</web-app>

Flow of Excecution

  1. When ever we deploys the application, container loads the application & creates ServletContext Object & waits for the Request

  2. if we give <load-on-startup>1</load-on-startup> container will creates ServletConfig Object when the time of Deploying application

  3. when we give the url : http://localhost:8080/Servlets/hello , request goes to container, and it searches for /hello url pattern in web.xml

  4. web.xml searches for /hello , in <servlet-mapping> and gets Servelt-name

  5. container loads demo.HelloServlet class and creates creates ServletConfig Object and calls inti() method

  6. for every request it will calls service(req,res) method, for 100 requests it will execute 100 times

  7. destroy() method will be called before servlet is removed from the container, and finally it will be garbage collected as usual.

In above <load-on-startup>1</load-on-startup> we may give (1,2..10). based up on priority order it will creates the ServletConfig Object

` `

  • If we want to make any page/servlet as Homepage we have to specify in this tag

  • If it contains more then 1 file, it will give priority by the Order


Example 2: Using GenericServlet

public class HelloServlet extends GenericServlet  {
	public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
 res.setContentType("text/html");
 PrintWriter pw = res.getWriter();
 pw.write("Hello, Generic Servlet"); 
	}
}


Example 3: Using HttpServlet

public class HelloServlet extends HttpServlet  {	
	@Override
	public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
 System.out.println("Public Service ........");
	}
	
	@Override
	protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
 System.out.println("Protecd Service ........");
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 System.out.println("doGet() ....");
	}
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 // TODO Auto-generated method stub
 System.out.println("doPost() ....");
	}
}
-----------------------
INFO: Reloading Context with name [/Servlets] is completed
Public Service........

  • Container first calls public Service(req,res) method

  • Public Service() method internally calls protected Service(req,res) method

  • Protected Service() method will internally calling doGet() or doPost() or doXXX() depends on the type of http method used by the client

  • If the client is not specifying the type of Http method then Http protocol by default consider GET method,

  • so finally the client request is processed at doGet() method

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