qr code scaner java app Other JDK-related types Java type Standard SQL built-in type in Java

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Table 5.4 Other JDK-related types Java type Standard SQL built-in type
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Mapping type
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class locale timezone currency
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java.lang.Class java.util.Locale java.util.TimeZone java.util.Currency
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VARCHAR VARCHAR VARCHAR VARCHAR
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The Hibernate type system
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Using mapping types
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All of the basic mapping types may appear almost anywhere in the Hibernate mapping document, on normal property, identifier property, and other mapping elements. The <id>, <property>, <version>, <discriminator>, <index> and <element> elements all define an attribute named type. You can see how useful the built-in mapping types are in this mapping for the BillingDetails class:
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<class name="BillingDetails" table="BILLING_DETAILS"> <id name="id" type="long" column="BILLING_DETAILS_ID"> <generator class="native"/> </id> <discriminator type="character" column="BILLING_DETAILS_TYPE"/> <property name="number" type="string"/> .... </class>
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The BillingDetails class is mapped as an entity. Its discriminator, identifier, and name properties are value typed, and we use the built-in Hibernate mapping types to specify the conversion strategy. It isn t often necessary to explicitly specify a built-in mapping type in the XML mapping document. For instance, if you have a property of Java type java.lang.String, Hibernate discovers this using reflection and selects string by default. We can easily simplify the previous mapping example:
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<class name="BillingDetails" table="BILLING_DETAILS"> <id name="id" column="BILLING_DETAILS_ID"> <generator class="native"/> </id> <discriminator type="character" column="BILLING_DETAILS_TYPE"/> <property name="number"/> .... </class>
Hibernate also understands type="java.lang.String"; it doesn t have to use reflection then. The most important case where this approach doesn t work well is a java.util.Date property. By default, Hibernate interprets a java.util.Date as a timestamp mapping. You need to explicitly specify type="time" or type="date" if you don t wish to persist both date and time information. With JPA annotations, the mapping type of a property is automatically detected, just like in Hibernate. For a java.util.Date or java.util.Calendar property, the Java Persistence standard requires that you select the precision with a @Temporal annotation:
Inheritance and custom types
@Temporal(TemporalType.TIMESTAMP) @Column(nullable = false, updatable = false) private Date startDate;
On the other hand, Hibernate Annotations, relaxing the rules of the standard, defaults to TemporalType.TIMESTAMP options are TemporalType.TIME and TemporalType.DATE. In other rare cases, you may want to add the @org.hibernate.annotations.Type annotation to a property and declare the name of a built-in or custom Hibernate mapping type explicitly. This is a much more common extension as soon as you start writing your own custom mapping types, which you ll do later in this chapter. The equivalent JPA XML descriptor is as follows:
<entity class="auction.model.Item" access="FIELD"> <attributes> ... <basic name="startDate"> <column nullable="false" updatable="false"/> <temporal>TIMESTAMP</temporal> </basic> </attributes> </entity>
For each of the built-in mapping types, a constant is defined by the class org.hibernate.Hibernate. For example, Hibernate.STRING represents the string mapping type. These constants are useful for query parameter binding, as discussed in more detail in chapters 14 and 15:
session.createQuery("from Item i where i.description like :desc") .setParameter("desc", d, Hibernate.STRING) .list();
Note that you may as well use the setString() argument binding method in this case. Type constants are also useful for programmatic manipulation of the Hibernate mapping metamodel, as discussed in chapter 3. Hibernate isn t limited to the built-in mapping types. We consider the extensible mapping-type system one of the core features and an important aspect that makes Hibernate so flexible.
Creating custom mapping types
Object-oriented languages like Java make it easy to define new types by writing new classes. This is a fundamental part of the definition of object-orientation. If we were then limited to the predefined built-in Hibernate mapping types when
Creating custom mapping types
declaring properties of our persistent classes, we would lose much of Java s expressiveness. Furthermore, our domain model implementation would be tightly coupled to the physical data model, because new type conversions would be impossible. Most ORM solutions that we have seen provide support for user-defined strategies for performing type conversions. These are often called converters. For example, the user can create a new strategy for persisting a property of JDK type Integer to a VARCHAR column. Hibernate provides a similar, much more powerful, feature called custom mapping types. First you need to understand when it s appropriate to write your own custom mapping type, and which Hibernate extension point is relevant for you. We ll then write some custom mapping types and explore the options.
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