1) Loading of Servlet 2) Creating instance of Servlet 3) Invoke init() once 4) Invoke service() repeatedly for each client request 5) Invoke destroy()
Step 1: Loading of Servlet When the web server (e.g. Apache Tomcat) starts up, the servlet container deploy and loads all the servlets.
Step 2: Creating instance of Servlet Once all the Servlet classes loaded, the servlet container creates instances of each servlet class. Servlet container creates only once instance per servlet class and all the requests to the servlet are executed on the same servlet instance.
Step 3: Invoke init() method Once all the servlet classes are instantiated, the init() method is invoked for each instantiated servlet. This method initializes the servlet. Note: The init() method is called only once during the life cycle of servlet.
Step 4: Invoke service() method Each time the web server receives a request for servlet, it spawns a new thread that calls service() method. If the servlet is HttpServlet then service() method receives the request and dispatches it to the correct handler method based on the type of request. For example if its a Get Request the service() method would dispatch the request to the doGet() method by calling the doGet() method with request parameters. Similarly the requests like Post, Head, Put etc. are dispatched to the corresponding handlers doPost(), doHead(), doPut() etc. by service() method of servlet.
Step 5: Invoke destroy() method When servlet container shuts down(this usually happens when we stop the web server), it unloads all the servlets and calls destroy() method for each initialized servlets. Servlet Life Cycle
Note: Unlike init() and destroy() that are called only once, the service() method can be called any number of times during servlet life cycle. As long as servlet is not destroyed, for each client request the service() method is invoked. Out of all the 5 steps in life cycle, this is the only step that executes multiple times.
