CHAPTER 5 n AN OVERVIEW OF MAPPING (Web hosting providers) n
CHAPTER 5 n AN OVERVIEW OF MAPPING n AN OVERVIEW OF MAPPING These files can be created directly with a text editor or with the help of various tools created by the Hibernate team and others. We discuss the details of XML mapping files in Chapter 8. Hibernate now also supports the Annotations feature introduced in Java 5. This permits the use of a special syntax to include metadata directly in the source code for the application. While this allows the core features of Hibernate to be controlled, many of the additional features cannot be specified in annotations. There is therefore something of a trade-off between the advantages of maintaining the mapping information directly within the associated source code, and the more flexible features available from the XML-based mappings. We discuss the details of annotation-based mapping in Chapter 6. Other Information Represented in Mappings While Hibernate can determine a lot of sensible default values for the mappings, most of these can be overridden by one or both of the file- and XML-based approaches. Some apply directly to mapping; others, such as the foreign key names, are really only pertinent when the mapping is used to create the database schema. Lastly, some mappings can also provide a place to configure some features that are perhaps not mappings in the purest sense. The final sections of this chapter discuss the features that Hibernate supports in addition to those already mentioned. Specification of (Database) Column Types and Sizes Java provides the primitive types and allows user declaration of interfaces and classes to extend these. Relational databases generally provide a small subset of standard types, and then provide additional proprietary types. Restricting yourself to the proprietary types will still cause problems, as there are only approximate correspondences between these and the Java primitive types. A typical example of a problematic type is java.lang.String (treated by Hibernate as if it were a primitive type since it is used so frequently), which by default will be mapped to a fixed-size character data database type. Typically, the database would perform poorly if a character field of unlimited size were chosen but lengthy String fields will be truncated as they are persisted into the database. By overriding the default type mappings, the developer can make appropriate trade-offs between storage space, performance, and fidelity to the original Java representation. The Mapping of Inheritance Relationships to the Database There is no SQL standard for representing inheritance relationships for the data in tables; and while some database implementations provide a proprietary syntax for this, not all do. Hibernate provides several configurable ways in which to represent inheritance relationships, and the mapping file permits users to select a suitable approach for their model.
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