Front Controller in Java

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Front Controller
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A controller that handles all requests for a Web site
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In a complex Web site there are many similar things you need to do when handling a request These things include security, internationalization, and providing particular views for certain users If the input controller behavior is scattered across multiple objects, much of this behavior can end up duplicated Also, it's difficult to change behavior at runtime
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The Front Controller consolidates all request handling by channeling requests through a single handler object This object can carry out common behavior, which can be modified at runtime with decorators The handler then dispatches to command objects for behavior particular to a request
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A Front Controller handles all calls for a Web site, and is usually structured in two parts: a Web handler and a command hierarchy The Web handler is the object that actually receives post or get requests from the Web server It pulls just enough information from the URL and the request to decide what kind of action to initiate and then delegates to a command to carry out the action (see Figure 142)
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Figure 142 How the Front Controller works
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The Web handler is almost always implemented as a class rather than as a server page, as it doesn't produce any response The commands are also classes rather than server pages and in fact don't need any knowledge of the Web environment, although they're often passed the HTTP information The Web handler itself is usually a fairly simple program that does nothing other than decide which command to run
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The Web handler can decide which command to run either statically or dynamically The static version involves parsing the URL and using conditional logic; the dynamic version usually involves taking a standard piece of the URL and using dynamic instantiation to create a command class
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The static case has the advantage of explicit logic, compile time error checking on the dispatch, and lots of flexibility in the look of your URLs The dynamic case allows you to add new commands without changing the Web handler
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With dynamic invocation you can put the name of the command class into the URL or you can use a properties file that binds URLs to command class names The properties file is another file to edit, but it does make it easier to change your class names without a lot of searching through your Web pages
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A particularly useful pattern to use in conjunction with Front Controller is Intercepting Filter, described in [Alur et al] This is essentially a decorator that wraps the handler of the front controller allowing you to build a filter chain (or pipeline of filters) to handle issues such as authentication, logging, and locale identification Using filters allows you to dynamically set up the filters to use at configuration time
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Rob Mee showed me an interesting variation of Front Controller using a two stage Web handler separated into a degenerate Web handler and a dispatcher The degenerate Web handler pulls the basic data out of the http parameters and hands it to the dispatcher in such a way that the dispatcher is completely independent of the Web server framework This makes testing easier because test code can drive the dispatcher directly without having to run in a Web server
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Remember that both the handler and the commands are part of the controller As a result the commands can (and should) choose which view to use for the response The only responsibility of the handler is in choosing
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which command to execute Once that's done, it plays no further part in that request
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The Front Controller is a more complicated design than its obvious counterpart, Page Controller (333) It therefore needs a few advantages to be worth the effort
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Only one Front Controller has to be configured into the Web server; the Web handler does the rest of the dispatching This simplifies the configuration of the Web server, which is an advantage if the Web server is awkward to configure With dynamic commands you can add new commands without changing anything They also ease porting since you only have to register the handler in a Web-server-specific way
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Because you create new command objects with each request, you don't have to worry about making the command classes thread-safe In this way you avoid the headaches of multi-threaded programming; however, you do have to make sure that you don't share any other objects, such as the model objects
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A commonly stated advantage of a Front Controller is that it allows you to factor out code that's otherwise duplicated in Page Controller (333) To be fair, however, you can also do much of this with a superclass Page Controller (333)
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There's just one controller, so you can easily enhance its behavior at runtime with decorators [Gang of Four] You can have decorators for authentication, character encoding, internationalization, and so forth, and add them using a configuration file or even while the server is running ([Alur et al] describe this approach in detail under the name Intercepting Filter)