The impedance mismatch in Java

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8.1 The impedance mismatch
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The term impedance mismatch refers to the differences in the OO and relational paradigms and difficulties in application development that arise from these differences. The persistence layer where the domain model resides is where the impedance mismatch is usually the most apparent. The root of the problem lies in the differing fundamental objectives of both technologies. Recall that when a Java object holds a reference to another, the actual referred object is not copied over into the referring object. In other words, Java accesses objects by reference and not by value. For example, two different Item objects containing the same category instance variable value really point to the same Category object in the JVM. This fact frees us from space efficiency concerns in implementing domain models with a high degree of conceptual abstraction. If this were not the case, we d probably store the identity of the referred Category object (perhaps in an int variable) inside the Item and materialize the link when necessary. This is in fact almost exactly what is done in the relational world.
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Object-relational mapping
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The JVM also offers the luxury of inheritance and polymorphism (by means that are very similar to the object reference feature) that does not exist in the relational world. Lastly, as we mentioned in the previous chapter, a rich domain model object includes behavior (methods) in addition to attributes (data in instance variables). Databases tables, on the other hand, inherently encapsulate only rows, columns, and constraints, and not business logic. These differences mean that the relational and OO model of the same conceptual problem look very different, especially for an appropriately normalized database created by an experienced DBA. Table 8.1 summarizes some of the overt mismatches between the object and relational worlds.
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Table 8.1 The impedance mismatch: obvious differences between the object and relational worlds OO Model (Java) Object, classes Attributes, properties Identity Relationship/reference to other entity Inheritance/polymorphism Methods Code is portable Table, rows Columns Primary key Foreign key Not supported Indirect parallel to SQL logic, stored procedures, triggers. Not necessarily portable, depending on vendor Relational Model
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In the following sections, we ll crystallize the object-relational mismatch a little more by looking at a few corner cases while saving a persistence layer domain model into the database. (As you ll recall from chapter 2, a corner case is a problem or situation that occurs only outside normal operating parameters.) We ll also discuss problems in mapping objects to database tables and provide a brief overview of ORM.
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8.1.1 Mapping objects to databases
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The most basic persistence layer for a Java application could consist of saving and retrieving domain objects using the JDBC API directly. To flush out the particularly rough spots in the object-relational mismatch, we ll assume automated ORM does not exist and that we are following the direct JDBC route to persistence. Later we ll see that the EJB 3 Persistence API irons out these rough spots through simple configuration. Scott Ambler has written an interesting article that discusses the
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The impedance mismatch
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problem of mapping objects to a relational database (www.agiledata.org/essays/ mappingObjects.html). One-to-one mapping As we discussed in the previous chapter, one-to-one relationships between entities, though rare in applications, make a great deal of sense in the domain-modeling world. For example, the User and BillingInfo objects represent two logically separate concepts in the real world (we assume) that are bound by a one-to-one relationship. Moreover, we also know that it does not make very much sense for a BillingInfo object to exist without an associated User. The relationship could be unidirectional from User to BillingInfo. Figure 8.1 shows this relationship, and listing 8.1 implements it.
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Listing 8.1 One-to-one relationship between User and BillingInfo
public class User { protected String userId; protected String email; protected BillingInfo billing; }
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