I then use the script to sum up based on the result set passed back from the gateway in Java

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I then use the script to sum up based on the result set passed back from the gateway
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class RecognitionService public Money recognizedRevenue(long contractNumber, MfDate asOf) { Money result = Moneydollars(0); try { ResultSet rs = dbfindRecognitionsFor(contractNumber, asOf); while (rsnext()) { result = resultadd(Moneydollars(rsgetBigDecimal("amount"))); } return result; }catch (SQLException e) {throw new ApplicationException (e); } }
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When the calculation is as simple as this, you can replace the in-memory script with a call to a SQL statement that uses an aggregate function to sum the amounts
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For calculating the revenue recognitions on an existing contract, I use a similar split The script on the service carries out the business logic
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class RecognitionService
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public void calculateRevenueRecognitions(long contractNumber) { try { ResultSet contracts = dbfindContract(contractNumber); contractsnext(); Money totalRevenue = Moneydollars(contractsgetBigDecimal("revenue")); MfDate recognitionDate = new MfDate(contractsgetDate("dateSigned")); String type = contractsgetString("type"); if (typeequals("S")){ Money[] allocation = totalRevenueallocate(3); dbinsertRecognition (contractNumber, allocation[0], recognitionDate); dbinsertRecognition (contractNumber, allocation[1], recognitionDateaddDays(60)); dbinsertRecognition (contractNumber, allocation[2], recognitionDateaddDays(90)); }else if (typeequals("W")){ dbinsertRecognition(contractNumber, totalRevenue, recognitionDate); }else if (typeequals("D")) { Money[] allocation = totalRevenueallocate(3); dbinsertRecognition (contractNumber, allocation[0], recognitionDate); dbinsertRecognition (contractNumber, allocation[1], recognitionDateaddDays(30)); dbinsertRecognition (contractNumber, allocation[2], recognitionDateaddDays(60)); } }catch (SQLException e) {throw new ApplicationException (e); } }
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Notice that I'm using Money (488) to carry out the allocation When splitting an amount three ways it's very easy to lose a penny
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The Table Data Gateway (144) provides support on the SQL First there's a finder for a contract
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class Gateway public ResultSet findContract (long contractID) throws SQLException{ PreparedStatement stmt = dbprepareStatement(findContractStatement); stmtsetLong(1, contractID); ResultSet result = stmtexecuteQuery(); return result; } private static final String findContractStatement = "SELECT * " + "FROM contracts c, products p " + "WHERE ID = AND cproduct = pID";
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And secondly there's a wrapper for the insert
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class Gateway public void insertRecognition (long contractID, Money amount, MfDate asof) throws SQLException { PreparedStatement stmt = dbprepareStatement(insertRecognitionStatement); stmtsetLong(1, contractID); stmtsetBigDecimal(2, amountamount()); stmtsetDate(3, asoftoSqlDate()); stmtexecuteUpdate(); } private static final String insertRecognitionStatement = "INSERT INTO revenueRecognitions VALUES ( , , )";
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In a Java system the recognition service might be a regular class or a session bean
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As you compare this to the example in Domain Model (116), unless your mind is as twisted as mine, you'll probably be thinking that this is much simpler The harder thing to imagine is what happens as the rules get more complicated Typical revenue recognition rules get very involved, varying not just by product but also by date (if the contract was signed before April 15 this rule applies ) It's difficult to keep a coherent design with Transaction Script once things get that complicated, which is why object bigots like me prefer using a Domain Model (116) in these circumstances
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Domain Model
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An object model of the domain that incorporates both behavior and data
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At its worst business logic can be very complex Rules and logic describe many different cases and slants of behavior, and it's this complexity that objects were designed to work with A Domain Model creates a web of interconnected objects, where each object represents some meaningful individual, whether as large as a corporation or as small as a single line on an order form
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Putting a Domain Model in an application involves inserting a whole layer of objects that model the business area you're working in You'll find objects that mimic the data in the business and objects that capture the rules the business uses Mostly the data and process are combined to cluster the processes close to the data they work with
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An OO domain model will often look similar to a database model, yet it will still have a lot of differences A
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Domain Model mingles data and process, has multivalued attributes and a complex web of associations, and uses inheritance
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As a result I see two styles of Domain Model in the field A simple Domain Model looks very much like the database design with mostly one domain object for each database table A rich Domain Model can look different from the database design, with inheritance, strategies, and other [Gang of Four] patterns, and complex webs of small interconnected objects A rich Domain Model is better for more complex logic, but is harder to map to the database A simple Domain Model can use Active Record (160), whereas a rich Domain Model requires Data Mapper (165)
Since the behavior of the business is subject to a lot of change, it's important to be able to modify, build, and test this layer easily As a result you'll want the minimum of coupling from the Domain Model to other layers in the system You'll notice that a guiding force of many layering patterns is to keep as few dependencies as possible between the domain model and other parts of the system
With a Domain Model there are a number of different scopes you might use The simplest case is a single-user application where the whole object graph is read from a file and put into memory A desktop application may work this way, but it's less common for a multitiered IS application simply because there are too many objects Putting every object into memory consumes too much memory and takes too long The beauty of objectoriented databases is that they give the impression of doing this while moving objects between memory and disk
Without an OO database you have to do this yourself Usually a session will involve pulling in an object graph of all the objects involved in it This will certainly not be all objects and usually not all the classes Thus, if you're looking at a set of contracts you might pull in only the products referenced by contracts within your working set If you're just performing calculations on contracts and revenue recognition objects, you may not pull in any product objects at all Exactly what you pull into memory is governed by your database mapping objects
If you need the same object graph between calls to the server, you have to save the server state somewhere, which is the subject of the section on saving server state (page 81)
A common concern with domain logic is bloated domain objects As you build a screen to manipulate orders you'll notice that some of the order behavior is only needed only for it If you put these responsibilities on the order, the risk is that the Order class will become too big because it's full of responsibilities that are only used in a single use case This concern leads people to consider whether some responsibility is general, in which case it should sit in the order class, or specific, in which case it should sit in some usage-specific class, which might be a Transaction Script (110) or perhaps the presentation itself
The problem with separating usage-specific behavior is that it can lead to duplication Behavior that's separated from the order is harder to find, so people tend to not see it and duplicate it instead Duplication can quickly lead to more complexity and inconsistency, but I've found that bloating occurs much less frequently than predicted If it does occur, it's relatively easy to see and not difficult to fix My advice is not to separate usage-specific behavior Put it all in the object that's the natural fit Fix the bloating when, and if, it becomes a problem Java Implementation
There's always a lot of heat generated when people talk about developing a Domain Model in J2EE Many of the teaching materials and introductory J2EE books suggest that you use entity beans to develop a domain model, but there are some serious problems with this approach, at least with the current (20) specification
Entity beans are most useful when you use Container Managed Persistence (CMP) Indeed, I would say there's little point in using entity beans without CMP However, CMP is a limited form of object-relational mapping, and it can't support many of the patterns that you need in a rich Domain Model
Entity beans can't be re-entrant That is, if you call out from one entity bean into another object, that other object (or any object it calls) can't call back into the first entity bean A rich Domain Model often uses reentrancy, so this is a handicap It's made worse by the fact that it's hard to spot re-entrant behavior As a result, some people say that one entity bean should never call another While this avoids re-entrancy, it very much cripples the advantages using a Domain Model
A Domain Model should use fine-grained objects with fine-grained interfaces Entity beans may be remotable (prior to version 20 they had to be) If you have remote objects with fine-grained interfaces you get terrible performance You can avoid this problem quite easily by only using local interfaces for your entity beans in a Domain Model
To run with entity beans you need a container and a database connected This will increase build times and also increase the time to do test runs since the tests have to execute against a database Entity beans are also tricky to debug
The alternative is to use normal Java objects, although this often causes a surprised reaction it's amazing how many people think that you can't run regular Java objects in an EJB container I've come to the conclusion that people forget about regular Java objects because they haven't got a fancy name That's why, while preparing for a talk in 2000, Rebecca Parsons, Josh Mackenzie, and I gave them one: POJOs (plain old Java objects) A POJO domain model is easy to put together, is quick to build, can run and test outside an EJB container, and is independent of EJB (maybe that's why EJB vendors don't encourage you to use them)
My view on the whole is that using entity beans as a Domain Model works if you have pretty modest domain logic If so, you can build a Domain Model that has a simple relationship with the database: where there's mostly one entity bean class per database table If you have a richer domain logic with inheritance, strategies, and other more sophisticated patterns, you're better off with a POJO domain model and Data Mapper (165), using a commercial tool or with a homegrown layer
The biggest frustration for me with the use of EJB is that I find a rich Domain Model complicated enough to deal with, and I want to keep as independent as possible from the details of the implementation environment EJB forces itself into your thinking about the Domain Model, which means that I have to worry about both the domain and the EJB environment