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Domain models and metadata
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There is no getting around the need for metadata in ORM. However, Hibernate was designed with full awareness of the typical metadata problems. The XML metadata format of Hibernate is extremely readable and defines useful default values. If attribute values are missing, reflection is used on the mapped class to determine defaults. Hibernate also comes with a documented and complete DTD. Finally, IDE support for XML has improved lately, and modern IDEs provide dynamic XML validation and even an autocomplete feature. Let s look at the way you can use XML metadata in Hibernate. You created the Category class in the previous section; now you need to map it to the CATEGORY table in the database. To do that, you write the XML mapping document in listing 3.4.
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Listing 3.4 Hibernate XML mapping of the Category class
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< xml version="1.0" > <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="auction.model.Category" table="CATEGORY">
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<id name="id" column="CATEGORY_ID" type="long"> <generator class="native"/> </id>
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<property name="name" column="NAME" type="string"/> </class> </hibernate-mapping>
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The Hibernate mapping DTD should be declared in every mapping file it s required for syntactic validation of the XML. Mappings are declared inside a <hibernate-mapping> element. You may include as many class mappings as you like, along with certain other special declarations that we ll mention later in the book.
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Object/relational mapping metadata
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The class Category (in the auction.model package) is mapped to the CATEGORY table. Every row in this table represents one instance of type Category. We haven t discussed the concept of object identity, so you may be surprised by this mapping element. This complex topic is covered in the next chapter. To understand this mapping, it s sufficient to know that every row in the CATEGORY table has a primary key value that matches the object identity of the instance in memory. The <id> mapping element is used to define the details of object identity. The property name of type java.lang.String is mapped to a database NAME column. Note that the type declared in the mapping is a built-in Hibernate type (string), not the type of the Java property or the SQL column type. Think about this as the converter that represents a bridge between the other two type systems. We ve intentionally left the collection and association mappings out of this example. Association and especially collection mappings are more complex, so we ll return to them in the second part of the book. Although it s possible to declare mappings for multiple classes in one mapping file by using multiple <class> elements, the recommended practice (and the practice expected by some Hibernate tools) is to use one mapping file per persistent class. The convention is to give the file the same name as the mapped class, appending a suffix (for example, Category.hbm.xml), and putting it in the same package as the Category class. As already mentioned, XML mapping files aren t the only way to define mapping metadata in a Hibernate application. If you use JDK 5.0, your best choice is the Hibernate Annotations based on the EJB 3.0 and Java Persistence standard.
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Annotation-based metadata
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The basic idea is to put metadata next to the information it describes, instead of separating it physically into a different file. Java didn t have this functionality before JDK 5.0, so an alternative was developed. The XDoclet project introduced annotation of Java source code with meta-information, using special Javadoc tags with support for key/value pairs. Through nesting of tags, quite complex structures are supported, but only some IDEs allow customization of Javadoc templates for autocompletion and validation. Java Specification Request (JSR) 175 introduced the annotation concept in the Java language, with type-safe and declared interfaces for the definition of annotations. Autocompletion and compile-time checking are no longer an issue. We found that annotation metadata is, compared to XDoclet, nonverbose and that it
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