Coursehero >> Pennsylvania >> UPenn >> CIT 597

Course Hero has millions of student submitted documents similar to the one below including study guides, homework solutions, papers, exam answer keys and textbook solutions.

for MVC Servlets Apr 10, 2009 MVC One of the most common Design Patterns is ModelView-Controller (MVC) The model does all the computational work It is input/output free All communication with the model is via methods User input goes to the controller The view can get results from the controller, or The view can get results directly from the model 2 The controller tells the model what to do The view shows results; it is a window into the model Advantages of MVC One advantage is separation of concerns Computation is not intermixed with I/O Consequently, code is cleaner and easier to understand The GUI (if one is used) can be completely revamped without touching the model in any way The same model used for a servlet can equally well be used for an application or an applet (or by another process) Another advantage is flexibility Another big advantage is reusability MVC is widely used and recommended 3 MVC for servlets The model, as usual, does all the computational work, and no I/O The model can consist of multiple classes The servlet class (the one that extends HttpServlet) acts as the controller The servlet gives any relevant information from the user request to the model The servlet takes the results and passes them on to the view The view that is, the HTML page that is returned to the user is frequently created by JSP 4 Web applications A web application typically consists of: Some (Java) class, acting as the controller, that extends HttpServlet The model code (also Java) The view code (ultimately Java, but we write it as JSP) Plus, of course, the web.xml file That s what the rest of this lecture is (mostly) about All these parts need to communicate with one another 5 web.xml servlet Apr 10, 2009 Servlet life-cycle methods public void init() Called after the servlet is constructed but before the servlet is placed into service As the name implies, a good place to do initializations public void service(ServletRequest request, ServletResponse response) Called when a servlet request is made The HttpServlet service method will dispatch the request to doGet, doPost, or one of the other service methods Called when a servlet is terminated Can be used to clean up any resources (files, databases, threads, etc.) public void destroy() 7 ServletConfig You can override public void init() Servlet has the methods: public ServletConfig getServletConfig() You will probably use this if you override init() public String getServletInfo() By default, returns an empty string; override to make it useful The main purpose of ServletConfig is to provide initialization information to the servlet ServletConfig has these methods: public java.lang.String getServletName() public ServletContext getServletContext() public Enumeration getInitParameterNames() public String getInitParameter(String name) Our interest will be in getting initialization parameters 8 Servlet init parameters Where does a servlet get its initialization information? From the web.xml file, of course! Inside <servlet>: <init param> <param name>myName</param name> <param value>myValue</param value> </init param> In the servlet code: String myValue = getServletConfig().getInitParameter("myName"); 9 web.xml entire web application Apr 10, 2009 Multiple servlets A web application can consist of multiple servlets We just saw how to send configuration information to a single servlet Context init parameters can send configuration information to all servlets in a web application Not inside a particular <servlet> tag: <context param> <param name>myName</param name> <param value>myValue</param value> </context param> String myValue = getServletContext().getInitParameter("myName"); 11 In any servlet: Servlet vs. context init parameters Servlet init parameters are: Context init parameters are: Defined within a <servlet> tag Written within an <init param> tag Retrieved from a ServletConfig object, which you get by calling getServletConfig() Read from the ServletConfig object by calling getInitParameter(name) Defined outside all <servlet> tags Written within a <context param> tag Retrieved from a ServletContext object, which you get by calling getServletContext() Read from the ServletContext object by calling getInitParameter(name) 12 Public ServletContext methods String getInitParameter(String name) Enumeration getInitParameterNames() Object getAttribute(String name) Enumeration getAttributeNames() void setAttribute(String name, Object object) void removeAttribute(String name) String getRealPath(String path) RequestDispatcher getRequestDispatcher(String path) 13 servlet JSP Apr 10, 2009 The ServletRequest object You ve seen these methods of the ServletRequest object: public Enumeration getParameterNames() public String getParameter(String name) public String[] getParameterValues(String name) public Enumeration getAttributeNames() public Object getAttribute(String name) public void setAttribute(String name, Object object) ServletRequest also has these methods: You can use attributes to send information to the JSP 15 Dispatching to the JSP request.setAttribute(name, object) Notice that we put the information on the request RequestDispatcher view = request.getRequestDispatcher("result.jsp"); We ask the request object for a dispatcher We supply, as a String, a path to the JSP file If the path begins with a slash, it is relative to the current context root Otherwise, it is relative to the servlet location view.forward(request, response); Having added the result information to the HttpRequest object, we forward the whole thing to the JSP The JSP does the rest it will send out the HTML page 16 Aside: redirect vs. forward The previous slide showed how a servlet could a forward request to JSP (or to another servlet) This is all done on the server side response.sendRedirect(URL) sends a response back to the browser that says, in effect, I can t handle this request; you should go to this URL instead. You cannot use this method if you have already written something to the response The URL can be relative to the location of this servlet 17 Attributes Apr 10, 2009 Parameters are not attributes You can get parameters from the Deployment Descriptor: getServletConfig().getInitParameter(name); getServletContext().getInitParameter(name); You cannot set these parameters You can get request parameters request.getParameter(String name) Parameter values are always Strings Attribute values are always Objects When you get an attribute, you have to cast it to the type you want 19 Attribute scopes Servlets can access three scopes: Application scope All servlets in the web application have access Attributes are stored in the ServletContext object Available for the lifetime of the servlet Available to servlets that have access to this specific session Attributes are stored in the HttpSession object Available for the life of the session Available to servlets that have access to this specific request Attributes are stored in the ServletRequest object Available for the life of the request (until your doGet or doPost method completes) Session scope Request scope 20 Attribute methods ServletContext objects, ServletRequest objects, and HttpSession objects all have the following methods: Object getAttribute(String name) void setAttribute(String name, Object object) void removeAttribute(String name) Enumeration getAttributeNames() 21 Thread safety Thread problems can occur when: One Thread is writing to (modifying) an object at the same time another Thread is reading it Two (or more) Threads are trying to write to the same object at the same time Thread problems cannot (in general) be detected by the Java runtime system Instead, thread problems cause random, mysterious, non-replicable corruption of data There are simple steps that you can take to avoid many threading problems However, threading is very error-prone and can be extremely difficult to ensure that you have it right 22 Thread safety in servlets Tomcat starts a new Thread for every new request Each request, and therefore each Thread, has its own request and response objects Therefore, these are inherently Thread-safe Local variables (including parameters) of your service methods are also thread-safe Instance variables are not thread-safe You don t have multiple servlet objects you have multiple Threads using the same servlet object Application (context) scope is shared by all servlets Therefore, context attributes are inherently Thread-unsafe It is possible to have multiple simultaneous requests from the same session Session attributes are not completely Thread-safe 23 Thread safety in class assignments In reality, the servlets you write for this course are not going to service thousands of requests per second You (and my TA) will enter a few requests manually, with billions of nanoseconds in between You are not going to have threading problems I m trying to teach real world programming Therefore, you have to pretend that thread safety is a real issue in your programming assignments If I had lots of spare time (which I don t!), I could write a program to send your servlet thousands of requests per second However... Even if I did that, my program could not reliably catch problems Bottom line: Try your best to make your servlets thread-safe, even though we can t test them for thread safety 24 Protecting context attributes To protect context attributes, synchronize on the ServletContext object Example (from Head First Servlets & JSP): synchronized(getServletContext()) { getServletContext().setAttribute("foo", "22"); getServletContext().setAttribute("bar", "42"); out.println(getServletContext().getAttribute("foo")); out.println(getServletContext().getAttribute("bar")); } This will protect you from any other code that also synchronizes on the ServletContext It will not protect you from code that doesn t so synchronize But this is the best we can do 25 Protecting session attributes To protect session attributes, synchronize on the HttpSession object Example (from Head First Servlets & JSP): HttpSession session = request.getSession(); synchronized(session) { session.setAttribute("foo", "22"); session.setAttribute("bar", "42"); out.println(session.getAttribute("foo")); out.println(session.getAttribute("bar")); } This will protect you from any other code that also synchronizes on the HttpSession 26 Getting init parameters in JSP You can get servlet and context init parameters in your JSP Step 1: Specify in your DD that you want them: <servlet> <servlet name>SomeServletName</servlet name> <jsp file>/UseServletInit.jsp</jsp file> <init param> ... </init param> <init param> ... </init param> ... </servlet> <%! public void jspInit() { // use getServletConfig() and getServletContext() as usual } %> 27 Step 2: Override jspInit() (must be done in a JSP declaration): PageContext In JSP, pageContext is an implicit object (like request and response) of type PageContext PageContext has these methods (among others): Object getAttribute(String name) // uses page scope Object getAttribute(String name, int scope) Enumeration getAttributeNamesInScope(int scope) Object findAttribute(String name) Searches in the order: page context, request scope, session scope, application scope void setAttribute(String name, Object value) void setAttribute(String name, Object value, int scope) Where scope can be one of PageContext.APPLICATION_SCOPE, PageContext.PAGE_SCOPE, PageContext.REQUEST_SCOPE, or PageContext.SESSION_SCOPE So you can access a lot of information from a PageContext object! 28 The End 29

Find millions of documents here - Study Guides, Homework Solutions, Papers, Exam Answer Keys and more. Course Hero has millions of course related materials that will enable you to learn better, faster and get an A in all your courses.
Below is a small sample set of documents: