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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Listing 4-5. Saving an Address Address address = new Address(); ... Session session = null; Transaction tx = null; try { session = factory.openSession(); tx = session.beginTransaction(); session.persist(address); tx.commit(); } catch (Exception e) { if (tx != null) tx.rollback(); } finally { session.close(); } The code in Listing 4-5 follows the pattern you ll use when interacting with Hibernate mapped objects in a simple J2SE application. Behind the scenes the Hibernate Session just opened is given a JDBC connection for the database set in the SessionFactory configuration (this connection might be a new connection or might come from a pool of existing connections available to Hibernate). Once the Session is opened it acts as cache for mapped object entities, that is, any object you retrieve or save via the opened Session will now be in the Session cache. So, if you were to load the same object twice, in the first call Hibernate will issue a SQL statement to load the state of the object, but in any subsequent calls using the same Session, the object would be retrieved from the Session s internal cache and no database access would occur. If the code in Listing 4-5 executes successfully, the Address instance that was passed to the persist method will now have its Id field populated with the identifier generated by the database and there will be a new row in the ADDRESS table. Hibernate s Session object provides several methods that persist the state of an object to the database, including persist, save, update, and saveOrUpdate. If an object s primary key attribute isn t set, Hibernate will detect this and automatically generate a primary key for the object based on the strategy selected on the id element. This will make the underlying operation a SQL INSERT.
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Note Hibernate detects that an object hasn t been saved by checking the primary key value for null, which is the default of the unsaved-value attribute. If your class required a value other than null, set this value in the unsaved-value attribute.
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In the case of the saveOrUpdate method, if the primary key is set then the operation becomes a SQL UPDATE if the object exists and a SQL INSERT doesn t, while in the case of the persist, save and update methods, the semantics are clearly defined; a call to persist or save will always result in a SQL INSERT while a call to update will result in a SQL UPDATE.
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CHAPTER 4 OBJECT RELATIONAL MAPPING WITH HIBERNATE
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Another important piece of information about Hibernate s functioning is that Hibernate interacts with the database in a transparent write-behind fashion. That means that you might not see SQL statements being executed against the database the moment the persist method is executed but possibly sometime after. This is a common technique employed by ORM tools to allow for runtime optimizations in the interaction with the database. For example, the actions in your code might result in several back-to-back identical SQL UPDATE statements which the ORM tool might combine into a single SQL UPDATE. In Hibernate, the moment when the Session is synchronized with the database is referred to as flushing. The Hibernate Session class also provides an explicit flush method which you can invoke. Typically you don t need to manually flush the Session, but in complex scenarios it might be useful for debugging purposes. Finally, notice that in Listing 4-5 we wrapped the block of code saving the Address object in a try-catch-finally. In the finally clause we ensure that we invoke the close method of the Session. This ensures that any resources (like database connections) are released. In the catch clause the Hibernate transaction is rolled back in the case of an exception to ensure the integrity of the operation. Note that the usage of the Hibernate transaction is not required, but it is recommended that you always use it since it insulates you from having to deal with the underlying JDBC connection (you could call the commit method of the JDBC connection associated with the Session). The Hibernate transaction API is a very simple wrapper to whatever the underlying transaction mechanism might be for the environment your code is operating under. In the simple example above, the Transaction object represents the underlying JDBC transaction, while the same code running on a JTA-enabled environment would use the container JTA user transaction available in the context of the method. The end result is that you have consistent code that works in all environments, allowing you to test your POJOs outside of a container. Listing 4-6 shows the complete listing for the example. Listing 4-6. Simple Hibernate Test for Saving an Address public static void main(String[] args) { 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/test"). setProperty("hibernate.connection.username", "root"). setProperty("hibernate.connection.password", "valencia"). setProperty("hibernate.show_sql", "true"); config.addClass(Address.class); SessionFactory factory = config.buildSessionFactory(); Address address = new Address(); address.setStreetAddress("1835 73rd Ave NE"); address.setCity("Medina"); address.setState("WA"); address.setZipCode("98039");
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