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Mapping persistent classes
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other instances. Value types, on the other hand, are dependent on a particular entity class. An instance of a value type has a lifecycle bound by its owning entity instance, and it can be referenced by only one entity it has no individual identity. We looked at Java identity, object equality, and database identity, and at what makes good primary keys. You learned which generators for primary key values are built into Hibernate, and how you can use and extend this identifier system. You also learned various (mostly optional) class mapping options and, finally, how basic properties and value-type components are mapped in XML mappings and annotations. For convenience, table 4.2 summarizes the differences between Hibernate and Java Persistence related to concepts discussed in this chapter.
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Table 4.2 Hibernate and JPA comparison chart for chapter 4 Hibernate Core Entity- and value-typed classes are the essential concepts for the support of rich and fine-grained domain models. Java Persistence and EJB 3.0 The JPA specification makes the same distinction, but calls value types embeddable classes. However, nested embeddable classes are considered a nonportable feature. JPA standardizes a subset of 4 identifier generators, but allows vendor extension. JPA standardizes property access through fields or access methods, and strategies can t be mixed for a particular class without Hibernate extension annotations. JPA doesn t include these features, a Hibernate extension is needed.
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Hibernate supports 10 identifier generation strategies out-of-the-box. Hibernate can access properties through fields, accessor methods, or with any custom PropertyAccessor implementation. Strategies can be mixed for a particular class. Hibernate supports formula properties and database-generated values.
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In the next chapter, we ll attack inheritance and how hierarchies of entity classes can be mapped with various strategies. We ll also talk about the Hibernate mapping type system, the converters for value types we ve shown in a few examples.
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Inheritance and custom types
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This chapter covers
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Inheritance mapping strategies The Hibernate mapping type system Customization of mapping types
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Inheritance and custom types
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We deliberately didn t talk much about inheritance mapping so far. Mapping a hierarchy of classes to tables can be a complex issue, and we ll present various strategies in this chapter. You ll learn which strategy to choose in a particular scenario. The Hibernate type system, with all its built-in converters and transformers for Java value-typed properties to SQL datatypes, is the second big topic we discuss in this chapter. Let s start with the mapping of entity inheritance.
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Mapping class inheritance
A simple strategy for mapping classes to database tables might be one table for every entity persistent class. This approach sounds simple enough and, indeed, works well until we encounter inheritance. Inheritance is such a visible structural mismatch between the object-oriented and relational worlds because object-oriented systems model both is a and has a relationships. SQL-based models provide only has a relationships between entities; SQL database management systems don t support type inheritance and even when it s available, it s usually proprietary or incomplete. There are four different approaches to representing an inheritance hierarchy:
Table per concrete class with implicit polymorphism Use no explicit inheritance mapping, and default runtime polymorphic behavior. Table per concrete class Discard polymorphism and inheritance relationships completely from the SQL schema. Table per class hierarchy Enable polymorphism by denormalizing the SQL schema, and utilize a type discriminator column that holds type information. Table per subclass Represent is a (inheritance) relationships as has a (foreign key) relationships.
This section takes a top-down approach; it assumes that you re starting with a domain model and trying to derive a new SQL schema. However, the mapping strategies described are just as relevant if you re working bottom up, starting with existing database tables. We ll show some tricks along the way that help you dealing with nonperfect table layouts.
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