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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Best Practice When choosing a primary key for a new table, especially if you know that the table will be used in an object-oriented system by an ORM tool, use surrogate primary keys whenever possible. Surrogate primary keys are small, simple keys that have no business meaning and are normally not updated or even seen by end users. They provide an audit trail, result in small indexes, make joins easier to write and faster to execute, are more resilient to business changes (because they have no business meaning), and make referential integrity easier to maintain. An example of dangerous business-key usage as a primary key would be using telephone numbers as account identifiers. Because telephone numbers can and do change, maintenance or some sort of mapping would be required when a customer s telephone number changes, even worst in my opinion is the use of composite business/natural primary keys in which many such fields are used in combination as a primary key. The problem is that those fields are normally non-nullable, so there is no easy way (without doing a SELECT to check the row s existence) to determine if the object is already in the database.
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Table 4-2 shows a quick summary of the generator strategies provided by Hibernate.
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Table 4-2. Hibernate s Identifier Generation Strategies
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Strategy
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increment identity sequence hilo
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Generates integer-type identifiers (long, short, and int) using an in-memory scheme. Not recommended for clustered environments. Uses database identity columns for those databases that support it. Use it with an integer-type object property. Uses a database sequence or internal database generator for those databases that support these features. Uses an algorithm to determine the next value based on a column in a given table. Used when native database sequences are not supported and only in non-JTA environments or with user-supplied connections. Uses a named database sequence and the hilo algorithm. Uses an algorithm that generates a string representation of a 128-bit UUID (Universally Unique Identifier). The returned string is hex-encoded. Takes advantage of the capability of certain databases (Microsoft SQL Server and MySQL) to generate Globally Unique Identifiers (GUIDs) Uses the identifier of an associated object. Intelligently chooses the appropriate strategy. It s the recommended strategy for maximum portability. Uses a database trigger or by issuing a SELECT against a unique key tables. Used for applications that use provided business unique keys.
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seqhilo uuid guid foreign native select assigned
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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Choosing the right identity-generation strategy depends on many factors, including performance and portability. Native database strategies are usually more robust and perform better than the equivalent Java-based approach. In most cases I prefer to use simple integers for my primary keys in conjunction with the native database integer key generator.
Saving an Object
Now we need to tell Hibernate how to connect to the database and load the Address.hbm.xml class mapping in the SessionFactory. For this simple example we will do this directly in the Java code. Subsequent examples will use the preferred XML configuration file. Listing 4-4 shows how to create a Hibernate configuration object in code, add a Hibernate mapped class, and retrieve an instance of the SessionFactory from the configuration. When you invoke the addClass method of the Configuration class, passing the class object for the Address class (Address.class), Hibernate will look for a HBM file named Address.hbm.xml in the class path. So the simplest way to ensure that Hibernate will find your mappings is to place them in the same location as the class they map. Once you have a SessionFactory, working with your persistent objects is as simple as obtaining a Session and invoking methods on it. Replace the user name and password with that required for your database. Listing 4-4. Configuring Hibernate in Code Configuration config = new Configuration(). setProperty("hibernate.dialect", "org.hibernate.dialect.MySQLDialect"). setProperty("hibernate.connection.driver_class", "com.mysql.jdbc.Driver"). setProperty("hibernate.connection.url", "jdbc:mysql://localhost/techconf"). setProperty("hibernate.connection.username", "mydbuser"). setProperty("hibernate.connection.password", "mydbpassword"). setProperty("hibernate.show_sql", "true"); config.addClass(Address.class); SessionFactory factory = config.buildSessionFactory(); For example to create an Address object and save it to the database we take the following steps, reflected in Listing 4-5: 1. POJO is created and values are set. 2. SessionFactory is used to create a Session. 3. A Hibernate Transaction bound to the Session is started. 4. The object is saved using the Session persist method. 5. The Transaction is committed. 6. The Session is closed.
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