Servlet - Life Cycle

First Request:

1. Servlet Loading
2. Servlet Instantiation
3. init() Method Execution
4. service() Method Execution

Second Request Onwards:

• service() Method Execution

At Server Startup or Application Deployment:

1. Servlet Loading
2. Servlet Instantiation
3. init() Method Execution

First Request (and onwards):

• service() Method Execution

From Servlet 2.5 Version onwards we can define multiple url-patterns for the same servlet in web.xml:

<web-app>
  .....
  <servlet-mapping>
    <servlet-name>DemoServlet</servlet-name>
    <url-pattern>/test</url-pattern>
    <url-pattern>/demo</url-pattern>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
</web-app>

All three URLs will invoke the same servlet:

• http://localhost:7777/advapps1A/test
• http://localhost:7777/advapps1A/demo
• http://localhost:7777/advapps1A/hello

1. Client (Browser) sends a request to the server
2. Server checks whether the request is for static or dynamic information based on URL
3. If static: server searches for the file → provides it or returns 404
4. If dynamic: web server forwards the request to the web container
5. Web container identifies the corresponding servlet class based on URL pattern and web.xml
6. Web container checks whether servlet object is available or not:
   • If not available: loads servlet class (by Class.forName()), creates instance (by newInstance()), executes init()
   • If available: skips to step 7
7. Web container calls service() method which provides the required response
8. Finally web container calls destroy() method to perform cleanup (at undeployment or shutdown)

Key points:

• Steps 1-6 (loading, instantiation, init) happen at the time of first request (without load-on-startup)
• If <load-on-startup> is configured, these steps happen at server startup or deployment
• destroy() is called at application undeployment or server shutdown

At the time of first request:

1. Servlet Loading
2. Servlet Instantiation
3. init() Method Execution
4. service() Method Execution

Second Request Onwards: service() Method Execution only.

At server startup or application deployment:

1. Servlet Loading
2. Servlet Instantiation
3. init() Method Execution

First Request onwards: service() Method Execution only.

Note: At the time of application undeployment or server shutdown, destroy() method will be executed.