CHAPTER 9 Working with objects equivalent with EntityManager (Web proxy server)
CHAPTER 9 Working with objects equivalent with EntityManager instead of Session. Even though we haven t shown you much about these interfaces, the following examples are simple, and you should have no problems understanding the methods we call on the Session. If you request two objects using the same database identifier value in the same Session, the result is two references to the same in-memory instance. Listing 9.1 demonstrates this with several get() operations in two Sessions. Listing 9.1 The guaranteed scope of object identity in Hibernate Session session1 = sessionFactory.openSession(); Transaction tx1 = session1.beginTransaction(); // Load Item with identifier value “1234″ Object a = session1.get(Item.class, new Long(1234) ); Object b = session1.get(Item.class, new Long(1234) ); ( a==b ) // True, persistent a and b are identical tx1.commit(); session1.close(); // References a and b are now to an object in detached state Session session2 = sessionFactory.openSession(); Transaction tx2 = session2.beginTransaction(); Object c = session2.get(Item.class, new Long(1234) ); ( a==c ) // False, detached a and persistent c are not identical tx2.commit(); session2.close(); Object references a and b have not only the same database identity, but also the same Java identity, because they re obtained in the same Session. They reference the same persistent instance known to the persistence context for that unit of work. Once you re outside this boundary, however, Hibernate doesn t guarantee Java identity, so a and c aren t identical. Of course, a test for database identity, a.getId().equals( c.getId() ), will still return true. If you work with objects in detached state, you re dealing with objects that are living outside of a guaranteed scope of object identity. 9.2.3 The identity of detached objects If an object reference leaves the scope of guaranteed identity, we call it a reference to a detached object. In listing 9.1, all three object references, a, b, and c, are equal if we only consider database identity their primary key value. However, they aren t
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