Antipattern: Persistent Problems in Java

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8.5 Antipattern: Persistent Problems
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Few decisions stay with an application longer than the choice of a persistence framework. Unfortunately, many developers and managers make this decision by default, choosing a persistence framework based on the persistence solution provided by their application server. An unstated rule seems to guide many developers: EJB persistence must occur through entity beans. However, the overhead and inflexibility of the entity bean architecture makes the framework impractical for many implementations. In reality, you re likely to spend far less by choosing a persistence framework based on an application s needs, even if you have to buy additional software or training to make it happen. Software development costs
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simply dwarf shrink-wrapped software costs in all but the most extreme instances. You ll want to consider the shortcomings of EJB entity beans. We re not saying that the EJB entity frameworks are completely inadequate for any task. We just believe that if you do decide to use EJB entity beans, you should do so with your eyes wide open.
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8.5.1 Generic entity bean weaknesses
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Let s review a few arguments we Invoice made in chapter 2. The entity bean specification provides four major services: declarative and disPerson Address tributed transactions, declarative security, scalability and fail-over support, and data persistence. When you look closely, you ll notice a significant mismatch between the way persistence is used and the way the other three Line Item Part services are typically used the latter are fine, and the former is coarse. Figure 8.3 shows an example. You ll probably want coarseFigure 8.3 This invoice is a coarse-grained object, grained services for an invoice, but composed of many fine-grained objects. The invoice contains a person with a billing and shipping address, fine-grained persistence for the and a list of invoice items, each with a part. You want objects within the invoice. In fact, persistence at the fine-grained level, but other you ll probably often want your services such as distribution and transaction integrity at a higher level. coarse-grained services at an even higher level. This imposes a number of architectural limitations inherent in the entity bean specification. These limitations are at the root of most entity bean antipatterns, including those discussed in chapter 7.
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8.5.2 Inheritance and polymorphism
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As you know, inheritance is the capability of an object-oriented programming language for one class to specialize or extend a base class. Polymorphism is the capability of an object-oriented programming language for a subclass to override a method s behavior in one of its superclasses. Java supports both inheritance and polymorphism. Many architectures require these capabilities as well. However, the
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Antipattern: Persistent Problems
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entity bean specification does not properly support these concepts. Writing an entity bean that extends a parent entity bean is possible. However, when executing an entity bean finder method, the container returns only objects of the same class as the entity bean s object interface. In our boat rental example, the domain object model comprises three classes: Boat, Kayak, and Canoe. Kayak and Canoe both extend Boat. You may want to define a findAllBoats method that can provide a list of all Boat objects in the database. Entity beans can t represent this relatively simple domain object model the Boat entity bean cannot define a finder method that returns anything other than Boat entity beans. Worse, Boat is not actually a full-fledged entity bean in the way that we ve emulated inheritance in our examples. No entry exists in the deployment descriptor for Boat because we do not want Boat objects to be created; it s an abstract class. One common workaround to this problem is to establish a convention of writing a method that finds one list of all Kayak objects and another list of all Canoe objects, and returns the combination of these lists. This workaround presents three major problems:
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The method that does this work must know about all its subclasses This means that every time you add a new subclass of Boat, you must also modify this method to add the new type lookup. This technique does not scale well from a development standpoint and requires that anyone extending Boat has access to the source for this method. Returned results are grouped by subclass It may be that you want to sort all Boat objects regardless of subclass by storage location. To do this in an architecture that uses the subclass-assembly finder technique described earlier, you would have to perform a costly application sort, which would be better performed in the database than in the Java application. Results in higher cost and higher incident of defects The complexity of this solution increases the likelihood of bugs and the cost of both maintenance and initial development.
In our example, the code for this workaround might look like that in listing 8.14:
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