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5.2.3. Custom Value Types

It is relatively easy for developers to create their own value types. For example, you might want to persist properties of type java.lang.BigInteger to VARCHAR columns. Hibernate does not provide a built-in type for this. Custom types are not limited to mapping a property, or collection element, to a single table column. So, for example, you might have a Java property getName()/setName() of type java.lang.String that is persisted to the columns FIRST_NAME, INITIAL, SURNAME.
To implement a custom type, implement either org.hibernate.UserType or org.hibernate.CompositeUserType and declare properties using the fully qualified classname of the type. View org.hibernate.test.DoubleStringType to see the kind of things that are possible. 
<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
    <column name="first_string"/>
    <column name="second_string"/>
</property>
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Notice the use of <column> tags to map a property to multiple columns.
The CompositeUserType, EnhancedUserType, UserCollectionType, and UserVersionType interfaces provide support for more specialized uses.
You can even supply parameters to a UserType in the mapping file. To do this, your UserType must implement the org.hibernate.usertype.ParameterizedType interface. To supply parameters to your custom type, you can use the <type> element in your mapping files. 
<property name="priority">
    <type name="com.mycompany.usertypes.DefaultValueIntegerType">
        <param name="default">0</param>
    </type>
</property>
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The UserType can now retrieve the value for the parameter named default from the Properties object passed to it.
If you regularly use a certain UserType, it is useful to define a shorter name for it. You can do this using the <typedef> element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type is parameterized. 
<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero">
    <param name="default">0</param>
</typedef>
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<property name="priority" type="default_zero"/>
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It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping.
Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use a custom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your application. For example, a MonetaryAmount class is a good candidate for a CompositeUserType, even though it could be mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would be protected against changes to the way monetary values are represented.
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