Further Reading in Java

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Further Reading
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[Alur et al] give a detailed description of how to implement Front Controller in Java They also describe Intercepting Filter, which goes very well with Front Controller
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A number of Java Web frameworks use this pattern An excellent example appears in [Struts]
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Example: Simple Display (Java)
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Here's a simple case of using Front Controller for the original and innovative task of displaying information about a recording artist We'll use dynamic commands with a URL of the form http://localhost:8080/isa/music name=barelyWorks&command=Artist The command parameter tells the Web handler which command to use
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Figure 143 The classes that implement Front Controller
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We'll begin with the handler, which I've implemented as a servlet
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class FrontServlet public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { FrontCommand command = getCommand(request); commandinit(getServletContext(), request, response); commandprocess(); } private FrontCommand getCommand(HttpServletRequest request) { try { return (FrontCommand) getCommandClass(request)newInstance(); } catch (Exception e) { throw new ApplicationException(e); } } private Class getCommandClass(HttpServletRequest request) { Class result; final String commandClassName = "frontController" + (String) requestgetParameter("command") + "Command"; try { result = ClassforName(commandClassName); } catch (ClassNotFoundException e) { result = UnknownCommandclass; } return result; }
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The logic is straightforward The handler tries to instantiate a class named by concatenating the command name and "Command" Once it has the new command it initializes it with the necessary information from the HTTP server I've passed in what I need for this simple example You may well need more, such as the HTTP session If you can't find a command, I've used the Special Case (496) pattern and returned an unknown
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command As is often the case, Special Case (496) allows you to avoid a lot of extra error checking
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Commands share a fair bit of data and behavior They all need to be initialized with information from the Web server
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class FrontCommand protected ServletContext context; protected HttpServletRequest request; protected HttpServletResponse response; public void init(ServletContext context, HttpServletRequest request, HttpServletResponse response) { thiscontext = context; thisrequest = request; thisresponse = response; }
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They can also provide common behavior, such as a forward method, and define an abstract process command for the actual commands to override
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class FrontCommand abstract public void process()throws ServletException, IOException ; protected void forward(String target) throws ServletException, IOException { RequestDispatcher dispatcher = contextgetRequestDispatcher(target); dispatcherforward(request, response); }
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The command object is very simple, at least in this case It just implements the process method, which involves invoking the appropriate behavior on the model objects, putting the information needed for the view into the request, and forwarding to a Template View (350)
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class ArtistCommand public void process() throws ServletException, IOException { Artist artist = ArtistfindNamed(requestgetParameter("name")); requestsetAttribute("helper", new ArtistHelper(artist)); forward("/artistjsp"); }
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The unknown command just brings up a boring error page
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class UnknownCommand public void process() throws ServletException, IOException { forward("/unknownjsp"); }
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Template View
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Renders information into HTML by embedding markers in an HTML page
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Writing a program that spits out HTML is often more difficult than you might imagine Although programming languages are better at creating text than they used to be (some of us remember character handling in Fortran and standard Pascal), creating and concatenating string constructs is still painful If there isn't much to do, it isn't too bad, but a whole HTML page is a lot of text manipulation
With static HTML pages those that don't change from request to request you can use nice WYSIWG editors Even those of us who like raw text editors find it easier to just type in the text and tags rather than fiddle with string concatenation in a programming language Of course the issue is with dynamic Web pages those that take the results of something like database queries and embed them into the HTML The page looks different with each result, and as a result regular HTML editors aren't up to the job
The best way to work is to compose the dynamic Web page as you do a static page but put in markers that can be resolved into calls to gather dynamic information Since the static part of the page acts as a template for the particular response, I call this a Template View