Servlet details Russell Beale

Servlet lifecycle The servlet container creates only one instance of each servlet Each use request handled with a separate thread So copes with many users Each thread calls doGet or doPost methods

Behaviour init() method called when servlet first created Each time server receives request for servlet, spawns new thread and calls the service method This checks the HTTP request type (GET, SET, etc.) and calls doGet, doPost, etc. as appropriate destroy() called when server removes previously loaded servlets

init() One-off initialisation code goes here E.g. connection to database Save connection in an instance variable Remember to throw appropriate exceptions (ServletException) Do not call System.exit()

Beware! Since the service() method can be called by multiple clients, it runs in parallel Need to be aware of threading issues and concurrent access May have to sync access to variables, or database, or similar If have database connection opened in init(), then there’s only one connection - may be slow for many users

Sharing information Web componets usually work with other objects to do their thing Share objects that are attributes of a public scope Invoke other web resources Use a database

Scope objects Four scope objects which allow servlets to share information Web Context - javax.servlet.ServletContext Accessible from web compoents within a web context Session - javax.servlet.http.HttpSession Accessible from web components handling a request that belongs to the session Request - javax.servlet.HttpServletRequest Accessible from web components handling the request Page - javax.servlet.jsp.PageContext Accesible from the java server page that creates the object

Scope objects Get attribute values from servlet scope objects using getAttribute method Set attributes using setAttribute method

Invoking other web resources Indirectly Embed a URL in the response that points to another web component Directly Invoked whilst executing Can include the content of another resource Forward the request to another source

How? First, obtain a RequestDispatcher object Using get RequestDispatcher(“URL”)

Including another resource E.g. copyright information, adverts Use include method of RequestDispatcher include(request, response) Sends the request to the included web component, which executes and returns the response in the response object

Passing on control E.g. One servlet does initial processing, another completes it and generates response To transfer control, use forward method of a RequestDispatcher Request URL is set to path of forwarded page Original URI and parts are saved as request attributes Can’t forward on if already accessed PrintWriter object (or ServletOutputStream ) Throws IllegalStateException