Input Controller Patterns in Java

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Input Controller Patterns
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There are two patterns for the input controller The most common is an input controller object for every page on your Web site In the simplest case this Page Controller (333) can be a server page itself, combining the roles of view and input controller In many implementations it makes things easier to split the input controller into a separate object The input controller can then create appropriate models to do the processing and instantiate a view to return the result Often you'll find that there isn't quite a one-to-one relationship between Page Controllers (333) and views A more precise thought is that you have a Page Controller (333) for each action, where an action is a button or link Most of the time the actions correspond to pages, but occasionally they don't such as a link that may go to a couple of different pages depending some condition
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With any input controller there are two responsibilities handling the HTTP request and deciding what to do with it and it often makes sense to separate them A server page can handle the request, delegating a separate helper object to decide what to do with it Front Controller (344) goes further in this separation by having only one object handling all requests This single handler interprets the URL to figure out what kind of request it's dealing with and creates a separate object to process it In this way you can centralize all HTTP handling within a single object, avoiding the need to reconfigure the Web server whenever you change the action structure of the site
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Further Reading
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Most books on Web server technologies provide a chapter or two on good server designs, although these are often buried in the technological descriptions An excellent discussion of Java Web design is 9 of [Brown et al] The best source for further patterns is [Alur et al]; most of these patterns can be used in nonJava situations I stole the terminology on separating input and application controllers from [Knight and Dai]
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5 Concurrency
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by Martin Fowler and David Rice
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Concurrency is one of the most tricky aspects of software development Whenever you have multiple processes or threads manipulating the same data, you run into concurrency problems Just thinking about concurrency is hard since it's difficult to enumerate the possible scenarios that can get you into trouble Whatever you do, there always seems to be something you miss Furthermore, concurrency is hard to test for We're great fans of a large body of automated tests acting as a foundation for software development, but it's hard to get tests to give us the security we need for concurrency problems
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One of the great ironies of enterprise application development is that few branches of software development use concurrency more yet worry about it less The reason enterprise developers can get away with a naive view of concurrency is transaction managers Transactions provide a framework that helps avoid many of the most tricky aspects of concurrency in an enterprise application As long as you do all your data manipulation within a transaction, nothing really bad will happen to you
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Sadly, this doesn't mean we can ignore concurrency problems completely, for the primary reason that many interactions with a system can't be placed within a single database transaction This forces us to manage concurrency in situations where data spans transactions The term we use is offline concurrency, that is, concurrency control for data that's manipulated during multiple database transactions
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The second area where concurrency rears its ugly head for enterprise developers is application servers supporting multiple threads in an application server system We've spent much less time on this because dealing with it is much simpler Indeed, you can use server platforms that take care of much of it for you
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Sadly, to understand these issues, you need to understand at least some of the general concurrency concepts So we begin this chapter by going over these issues We don't pretend that this chapter is a general treatment of concurrency in software development for that we'd need at least a complete book What this chapter does is introduce concurrency issues for enterprise application development Once we've done that we'll introduce the patterns for handling offline concurrency and say our brief words on application server concurrency
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In much of this chapter we'll illustrate the ideas with examples from an area that we hope you are very familiar with the source code control systems used by teams to coordinate changes to a code base We do this because it's relatively easy to understand as well as well as familiar After all, if you aren't familiar with source code control systems, you really shouldn't be developing enterprise applications
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