04 Data Access

Data Access with Hibernate Persistent objects and persistence contexts

Object state transitions and Session methods Transient Persistent Detached new garbage garbage delete() save() saveOrUpdate() get() load() find() iterate() etc. evict() close()* clear()* update() saveOrUpdate() lock() * affects all instances in a Session

Transient objects ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Persistent objects ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Detached objects ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

The scope of object identity ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

The Hibernate identity scope Session session1 = sf.openSession(); Transaction tx1 = session.beginTransaction(); Object a = session1.load(Category.class, new Long(1234) ); Object b = session1.load(Category.class, new Long(1234) ); if ( a == b ) System.out.println("a and b are identicial and the same database identity."); tx1.commit(); session1.close(); Session session2 = sf.openSession(); Transaction tx2 = session.beginTransaction(); Object b2 = session2.load(Category.class, new Long(1234) ); if ( a != b2 ) System.out.println("a and b2 are not identical."); tx2.commit(); session2.close();

Outside of the identity scope ,[object Object],[object Object],[object Object]

Identity and equality contracts ,[object Object],[object Object],[object Object],[object Object],Session session1 = sf.openSession(); Transaction tx1 = session.beginTransaction(); Object itemA = session1.load(Item.class, new Long( 1234 ) ); tx1.commit(); session1.close(); Session session2 = sf.openSession(); Transaction tx2 = session.beginTransaction(); Object itemB = session2.load(Item.class, new Long( 1234 ) ); tx2.commit(); session2.close(); Set allObjects = new HashSet(); allObjects.add(itemA); allObjects.add(itemB); System.out.println(allObjects.size()); // How many elements?

Implementing equals() and hashCode() ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],We need a business key .

Using business keys for equality ,[object Object],[object Object],[object Object],public class Item { public boolean equals(Object other) { if (this == other) return true; if (!(other instanceof Item)) return false; final Item item = (Item) other; if (! getSummary() .equals(item.getSummary())) return false; if (! getCreated() .equals(item.getCreated())) return false; return true; } public int hashCode() { int result; result = getSummary() .hashCode(); return 29 * result + getCreated() .hashCode(); } }

The Hibernate Session ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Making an object persistent ,[object Object],[object Object],[object Object],User user = new User(); user.getName().setFirstName("John"); user.getName().setLastName("Doe"); Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); session.save(user); tx.commit(); session.close();

Updating a detached instance ,[object Object],[object Object],[object Object],user.setPassword("secret"); Session sessionTwo = sessions.openSession(); Transaction tx = sessionTwo.beginTransaction(); sessionTwo.update(user); user.setLoginName("jonny"); tx.commit(); sessionTwo.close();

Locking a detached instance ,[object Object],[object Object],Session sessionTwo = sessions.openSession(); Transaction tx = sessionTwo.beginTransaction(); sessionTwo.lock(user, LockMode.NONE); user.setPassword("secret"); user.setLoginName("jonny"); tx.commit(); sessionTwo.close();

Retrieving objects ,[object Object],Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); int userID = 1234; User user = session.get (User.class, new Long(userID)); // "user" might be null if it doesn't exist tx.commit(); session.close();

Making a persistent object transient ,[object Object],[object Object],[object Object],Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); int userID = 1234; User user = session.get(User.class, new Long(userID)); session.delete(user); tx.commit(); session.close();

Making a detached object transient ,[object Object],[object Object],Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); session.delete(user); tx.commit(); session.close();

Persistence by reachability ,[object Object],Transient Persistent Persistent by Reachability Electronics: Category Cell Phones: Category Computer: Category Desktop PCs: Category Monitors: Category

Association cascade styles ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Association cascade styles ,[object Object],[object Object],[object Object],<class name=“Category” … > … <many-to-one name=“parentCategory” column=“PARENT_ID” cascade=“none” /> <set name=“childCategories” cascade=“save-update” … > <key column=“PARENT_ID”/> <one-to-many class=“Category”/> </set> </class>

Adding a new category to object graph Computer: Category Laptops : Category Electronics: Category Cell Phones: Category Desktop PCs: Category Monitors: Category

Association cascade styles ,[object Object],[object Object],[object Object],Session session = sessions.openSession(); Transaction tx = session.begnTransaction(); Category computer = (Category)session.get(Category.class,computerID); Category laptops = new Category(“Laptops”); Compter.getChildCategories().add(laptops); Laptops.setParentCategory(computer); tx.commit(); session.close(); The computer instance is persisted(attached to a session), and the childCategoryies assoication has cascade save enabled. Hence, this code results in the new laptops category becoming persistent when tx.commit() is called, because Hibernate cascades the dirty-checking operation to the children of computer. Hibrnate executes an insert statement

Association cascade styles ,[object Object],[object Object],Category computer = …. // Loaded in a previous session Category laptops = new Category(“Laptops”); computer.getChildCategories().add(laptops); laptops.setParentCategory(computer); The detached computer object and any other detached objects it refers to are now associated with the new transient laptops object(and vice versa).

Association cascade styles ,[object Object],[object Object],[object Object],[object Object],[object Object],Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); //persist one new category and the link to its parent category session.save(laptops); tx.commit(); session.close();

Automatic save or update for detached object graphs ,[object Object],[object Object],[object Object],Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); // Let Hibernate decide whats new and whats detached session.saveOrUpdate(theRootObjectOfGraph); tx.commit(); session.close();

Detecting transient instances ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],<class name="Category" table="CATEGORY">  <id name="id" unsaved-value="0" > <generator class="native"/> </id> .... </class>

04 Data Access

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04 Data Access