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Figure 7.4 This UML diagram shows a one-to-many relation between the Person and ProductOrder entity beans. This is a relationship that might be conditional on runtime parameters, such as whether the product has been shipped to the customer. These types of relationships are limited for many different reasons.
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This antipattern is trivial, but appears broadly in EJB applications, even on EJB 2.x application servers. We mention it here because it s so prominent. Next, we ll examine a similar, but far more subtle, antipattern application filters.
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7.5 Antipattern: Application Filters
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A filter, in the relational language SQL , is the WHERE clause. Like joins, filters belong on the database when it s possible to put them there. Databases process filters many times faster than applications for several reasons. First, applications that use their own filters instead of database filters retrieve more rows from a database than those that don t. Second, relational databases are optimized to filter, through features like indices and specialized optimizers. For example, an accounting system might retrieve a list of all PurchaseOrder objects that have not cleared and search that list for other criteria. Some application filters will be easy to avoid. Others are much more subtle. In the next section, we ll look at several different types of application filters.
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7.5.1 Understanding the types of application filters
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Object-oriented programming encourages implementation hiding and layered programming. Blissful ignorance makes for cleaner code, but also fertile ground for antipatterns. Our desire to produce elegant, layered software can cause us to produce beautiful code that runs like a crippled sloth. As a rule, filters belong in the database. Let s take a detailed look at the different types of filters that you re likely to encounter. Some, you can work around. In other cases, you ll just have to bite the performance bullet. Dynamic relations. Often, Java developers will attempt to create a relation between persistent objects that is conditional on a runtime parameter. For example, consider a system that has a one-to-many relation between Person and ProductOrder. You can express it in UML, as we have done in figure 7.4.
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Antipattern: Application Filters
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You might decide to parameterize this relation with information about whether or not the ProductOrder has been shipped. In other words, you might want to change the relation accessor method to be
public Collection getProductOrders(boolean shipped)
And you might want the values of this method to differ, depending on the boolean value passed to the method. This is not a relation; this is a filter. Conceptually, you should store relations between classes in a Collection, without any additional runtime information. If the application dictates that a method such as Person.getProductOrders (boolean shipped) is necessary, then you should implement a method to do the filtering and document it as an application filter. Better yet, you should use two separate relations shippedProductOrders and unshippedProductOrders. Even a well-written dynamic relation usually circumvents any caching that an EJB container can do because the conditional processing occurs in the application code domain. Ideally, the application creates the dynamic relation by performing an SQL query that passes the filters through to the relational database as SQL . More often, however, dynamic relations run as an in-memory application filter that operates on the complete relation. So, in our example, the check to see if the product order has shipped might be executed in Java. In that case, the database returns the entire collection of product orders placed by a given person, even if only five of the product orders in question matched the condition. That s far too inefficient. Dynamic queries Dynamic queries are queries defined at runtime that cannot be known in advance. For example, any application that allows a user to search for data by combining boolean filters into a boolean expression will generate SQL that contains a variable number of conditions in the WHERE clause, each joined together by either AND or OR operators. Static SQL queries can use parameterization to allow for runtime modifications, but the query s structure itself cannot be modified. Dynamic queries must be created at runtime. These types of queries are problematic in EJB because EJB QL queries must be defined at deploy time. You cannot assemble an EJB QL statement that satisfies the structural requirements of a dynamic query. You have two possible workarounds at your disposal to allow dynamic querying of CMP entity beans. On the one hand, you can dynamically create an SQL query to determine the primary keys of the entity beans that match the filter. This is not
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