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Explain the life cycle of Servlet, i.e. Instantiation, Initialization, Service, Destroy, Unavailable.- The life cycle is managed by the servlet container in which the servlet is deployed,
Life Cycle of a Servlets:
1. Instantiation:
- At the time of starting the web container, it searches for the deployment descriptor (web.xml) for every web application.
- Once the servlet is found in the deployment descriptor, the container creates an instance of the Servlet class, which is considered as the loading of the servlet class.
2. Initialization:
- The init() method of GenericServlet class is invoked by the HttpServlet class which is the sub class of the GenericServlet, meaning that the HttpServlet inherits the init() method of GenericServlet.
- The init() method performs some specific actions like a constructor of a class which initializes an instance at the start up.
- It is automatically be invoked by the servlet container.
- This action causes the parsing by the application context (web.xml).
- The init() method is overloaded with zero parameters and with a parameter that takes a ServletConfig parameter.
- The init() method is invoked only once when the servlet is first loaded.
3. Service:
- The operations of a servlet is performed by the service () method.
- This method has HttpServletRequest and HttpServletResponse parameters.
- The service () method is invoked by the container and is called for each request processed.
- By default the method returns / dispatches the requests to the appropriate methods, usually a developer’s overridden helper methods such as doGet() and doPost().
- You need to author the code to delegate to the helper methods or invoke super. service() method.
4. Destroy:
- The destroy method is invoked when the container is shutting down or if the requests are not sent for a while for the servlet, then the destroy() method may be invoked to ensure the resources are not available for the servlets that are being requested.
- The destroy() method invoked only once before the unloading the servlet.
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