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CHAPTER 10 WORKING WITH OBJECTS
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state of the book object is managed by the persistence context. Note that persist() doesn t return the serializable ID of the newly created record.
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Retrieving a Persistent Object
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In Hibernate, you need an instance of the session to retrieve an entity by its ID. Hibernate provides two methods for this: get() and load(). Both methods take the entity class and the ID as input parameters: Session session = getSession(); Transaction tx = session.beginTransaction(); //Book book = (Book) session.load(Book.class, new Long(294912)); Book book1 = (Book) session.get(Book.class, new Long(294912)); tx.commit(); session.close(); System.out.println(book1); The get() method makes a call to the database and does a lookup for the primary key. If it finds the primary key, it retrieves and returns the object. If it can t, it returns null. The load() method, on the other hand, looks in the current persistence context. If it can find the entity, it returns the entity object. If the object isn t being managed in the current persistence context, then a proxy placeholder is returned without hitting the database. This means if you request an entity object that isn t in the database (or, of course, the current persistence context), then the load() method returns a proxy. You get an org.hibernate.ObjectNotFoundException when you try to read/retrieve properties other than the primary key within the same persistence context, and you get an org.hibernate.LazyInitializationException when you try to read/retrieve properties after the session is closed (the persistence context has ended). The load() method is useful when you only need a proxy and don t need to make a database call. You need just a proxy when, in a given persistence context, you need to associate an entity before persisting. In JPA, you use the EntityManager to read/retrieve persistent entities. JPA provides two methods: find() and getReference(). Both methods take the entity class and the ID as input parameters: EntityManager manager = SessionManager.getEntityManager(); EntityTransaction tran = manager.getTransaction(); tran.begin(); Book book = manager.find(Book.class, new Long(294912)); //Book book1 = manager.getReference(Book.class, new Long(294912)); tran.commit(); manager.close(); The find() method is similar to the get() method in Hibernate. A call to the database is made when this method is invoked. If it doesn t find the persistent entity, the find() method returns null. Note that you haven t cast the return value from the find() method. It isn t required, because find() is a generic method, and the first parameter is set as the return type. The getReference() method is similar to the load() method in Hibernate. You should use it when you only need a proxy.
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CHAPTER 10 WORKING WITH OBJECTS
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Modifying a Persistent Object
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You can modify any persistent object that is associated with a session and a persistence context. Its state is then synchronized with the database: Session session = getSession(); Transaction tx = session.beginTransaction(); Book book = (Book) session.get(Book.class, new Long(294912)); book.setName("Book Name - hibernate 2"); tx.commit(); session.close(); The entity is retrieved from the database and modified in the same transaction. The modifications are propagated to the database when the commit() method is called. The persistence context identifies the changes that have been made to the entity s state and synchronizes with the database. This is called automatic dirty checking. The implementation in JPA is as follows: EntityManager manager = SessionManager.getEntityManager(); EntityTransaction tran = manager.getTransaction(); tran.begin(); Book book = manager.find(Book.class, new Long(294912)); book.setBookName("Book Name - hibernate 22"); tran.commit(); manager.close();
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Deleting a Persistent Object
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You can delete from the database any persistent object that is associated with a session and a persistence context. To do so, you call the delete() method: Session session = getSession(); Transaction tx = session.beginTransaction(); Book book = (Book) session.get(Book.class, new Long(294912)); session.delete(book); tx.commit(); session.close(); The entity is in the removed state after the call to delete(). The database record is deleted after the transaction is committed, and the object is in the transient state after the session is closed. In JPA, the remove() method performs the same function as the delete() method: EntityManager manager = SessionManager.getEntityManager(); EntityTransaction tran = manager.getTransaction(); tran.begin(); Book book = manager.find(Book.class, new Long(294912)); manager.remove(book); tran.commit(); manager.close();
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