c# create barcode image Understanding the Hibernate type system in Java

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Understanding the Hibernate type system
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types when executing data manipulation language (DML). For database-specific DDL generation, Hibernate translates from the ANSI-standard type to an appropriate vendor-specific type, using the built-in support for specific SQL dialects. (You usually don t have to worry about SQL data types if you re using Hibernate for data access and data schema definition.) Date and time mapping types Table 6.2 lists Hibernate types associated with dates, times, and timestamps. In your domain model, you may choose to represent date and time data using either java.util.Date, java.util.Calendar, or the subclasses of java.util.Date defined in the java.sql package. This is a matter of taste, and we leave the decision to you make sure you re consistent, however!
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Table 6.2 Date and time types Java type java.util.Date or java.sql.Date java.util.Date or java.sql.Time java.util.Date or java.sql.Timestamp java.util.Calendar java.util.Calendar Standard SQL built-in type DATE TIME TIMESTAMP TIMESTAMP DATE
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Mapping type date time timestamp calendar calendar_date
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Large object mapping types Table 6.3 lists Hibernate types for handling binary data and large objects. Note that none of these types may be used as the type of an identifier property.
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Table 6.3 Binary and large object types Java type byte[] java.lang.String any Java class that implements java.io.Serializable java.sql.Clob java.sql.Blob Standard SQL built-in type VARBINARY (or BLOB) CLOB VARBINARY (or BLOB) CLOB BLOB
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Mapping type binary text serializable clob blob
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Advanced mapping concepts
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java.sql.Blob and java.sql.Clob are the most efficient way to handle large objects in Java. Unfortunately, an instance of Blob or Clob is only useable until the JDBC transaction completes. So if your persistent class defines a property of java.sql.Clob or java.sql.Blob (not a good idea anyway), you ll be restricted in
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how instances of the class may be used. In particular, you won t be able to use instances of that class as detached objects. Furthermore, many JDBC drivers don t feature working support for java.sql.Blob and java.sql.Clob. Therefore, it makes more sense to map large objects using the binary or text mapping type, assuming retrieval of the entire large object into memory isn t a performance killer. Note you can find up-to-date design patterns and tips for large object usage on the Hibernate website, with tricks for particular platforms. Various JDK mapping types Table 6.4 lists Hibernate types for various other Java types of the JDK that may be represented as VARCHARs in the database.
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Table 6.4 Other JDK-related types Java type java.lang.Class java.util.Locale java.util.TimeZone java.util.Currency Standard SQL built-in type VARCHAR VARCHAR VARCHAR VARCHAR
Mapping type class locale timezone currency
Certainly, <property> isn t the only Hibernate mapping element that has a type attribute.
6.1.2 Using mapping types
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. (There are certain limitations on which mapping basic types may function as an identifier or discriminator type, however.) You can see how useful the built-in mapping types are in this mapping for the BillingDetails class:
Understanding the Hibernate type system
<class name="BillingDetails" table="BILLING_DETAILS" discriminator-value="null"> <id name="id" type="long" column="BILLING_DETAILS_ID"> <generator class="native"/> </id> <discriminator type="character" column="TYPE"/> <property name="number" type="string"/> ... </class>
The BillingDetails class is mapped as an entity. Its discriminator, identifier, and number properties are value typed, and we use the built-in Hibernate mapping types to specify the conversion strategy. It s often not 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 will discover this using reflection and select string by default. We can easily simplify the previous mapping example:
<class name="BillingDetails" table="BILLING_DETAILS" discriminator-value="null"> <id name="id" column="BILLING_DETAILS_ID"> <generator class="native"/> </id> <discriminator type="character" column="TYPE"/> <property name="number"/> .... </class>
The most important case where this approach doesn t work well is a java.util.Date property. By default, Hibernate interprets a Date as a timestamp mapping. You d need to explicitly specify type="time" or type="date" if you didn t wish to persist both date and time information. For each of the built-in mapping types, a constant is defined by the class net.sf.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 chapter 7:
session.createQuery("from Item i where i.description like :desc") .setParameter("desc", desc, Hibernate.STRING) .list();
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