Monday, August 15, 2011

Servlet Life Cycle

1. Loading and instantiating a servlet

When we start up a servlet container, it looks for deployment descriptors (web.xml) which includes an entry for each of the servlets it uses. An entry specifies the name of the servlet and a Java class name for the servlet. The servlet container creates an instance of the given servlet class using the method Class.forName(className).newInstance().

2. Initializing a servlet

Once the container creates the servlet instance, it calls the init(ServletConfig) method on this newly created instance. The ServletConfig object contains all the initialization parameters that we specify in the deployment
descriptor of the web application. The framework guarantees that the servlet container will call the init() method once and only once on a servlet instance.

3. Servicing client requests

After the servlet instance is properly initialized, it is ready to service client requests. When the servlet container receives requests for this servlet, it will dispatch them to the servlet instance by calling the Servlet.service(ServletRequest, ServletResponse) method.

4. Destroying a servlet

If the servlet container decides that it no longer needs a servlet instance, it calls the destroy() method on the servlet instance. Once this method is called, the servlet instance will be out of service and the container will never call the service() method on this instance. The servlet container cannot reuse this instance in any way. From this state, a servlet instance may only go to the unloaded state. Before calling the destroy() method, the servlet
container waits for the remaining threads that are executing the servlet’s service() method to finish.

A servlet container may destroy a servlet if it is running low on resources and no request has arrived for a servlet in a long time. Similarly, if the servlet container maintains a pool of servlet instances, it may create and destroy the instances from time to time as required. A servlet container may also destroy a servlet if it is shutting down.

5. Unloading a servlet

Once destroyed, the servlet instance may be garbage collected, in which case the servlet instance is said to be unloaded. If the servlet has been destroyed because the servlet container is shutting down, the servlet class will also be unloaded.

Before a servlet can service the client requests, a servlet container must take certain steps in order to bring the servlet to a state in which it is ready to service the requests. The first step is loading and instantiating the servlet class; the servlet is now considered to be in the loaded state. The second step is initializing the servlet instance. Once the servlet is in the initialized state, the container can invoke its service() method whenever it receives a request from the client. There may be times when the container will call the destroy() method on the servlet instance to put it in the destroyed state. Finally, when the servlet container shuts down, it must unload the servlet instance.