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For a typical web or enterprise application, transaction-scoped identity is preferred. Process-scoped identity offers some potential advantages in terms of cache utilization and the programming model for reuse of instances across multiple transactions; however, in a pervasively multithreaded application, the cost of always synchronizing shared access to persistent objects in the global identity map is too high a price to pay. It s simpler, and more scalable, to have each thread work with a distinct set of persistent instances in each transaction scope. Speaking loosely, we would say that Hibernate implements transaction-scoped identity. Actually, the Hibernate identity scope is the Session instance, so identical objects are guaranteed if the same persistence manager (the Session) is used for several operations. But a Session isn t the same as a (database) transaction it s a much more flexible element. We ll explore the differences and the consequences of this concept in the next chapter. Let s focus on the persistence lifecycle and identity scope again. If you request two objects using the same database identifier value in the same Session, the result will be two references to the same in-memory object. The following code example demonstrates this behavior, with several load() operations in two Sessions:
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Session session1 = sessions.openSession(); Transaction tx1 = session1.beginTransaction(); // Load Category with identifier value "1234" 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 identical."); } tx1.commit(); session1.close(); Session session2 = sessions.openSession(); Transaction tx2 = session2.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();
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Object references a and b not only have the same database identity, they also have the same Java identity since they were loaded in the same Session. Once outside this boundary, however, Hibernate doesn t guarantee Java identity, so a and b2
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The persistence lifecycle
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aren t identical and the message is printed on the console. Of course, a test for database identity a.getId().equals ( b2.getId() ) would still return true. To further complicate our discussion of identity scopes, we need to consider how the persistence layer handles a reference to an object outside its identity scope. For example, for a persistence layer with transaction-scoped identity such as Hibernate, is a reference to a detached object (that is, an instance persisted or loaded in a previous, completed session) tolerated
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4.1.5 Outside the identity scope
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If an object reference leaves the scope of guaranteed identity, we call it a reference to a detached object. Why is this concept useful In web applications, you usually don t maintain a database transaction across a user interaction. Users take a long time to think about modifications, but for scalability reasons, you must keep database transactions short and release database resources as soon as possible. In this environment, it s useful to be able to reuse a reference to a detached instance. For example, you might want to send an object retrieved in one unit of work to the presentation tier and later reuse it in a second unit of work, after it s been modified by the user. You don t usually wish to reattach the entire object graph in the second unit of of work; for performance (and other) reasons, it s important that reassociation of detached instances be selective. Hibernate supports selective reassociation of detached instances. This means the application can efficiently reattach a subgraph of a graph of detached objects with the current ( second ) Hibernate Session. Once a detached object has been reattached to a new Hibernate persistence manager, it may be considered a persistent instance, and its state will be synchronized with the database at the end of the transaction (due to Hibernate s automatic dirty checking of persistent instances). Reattachment might result in the creation of new rows in the database when a reference is created from a detached instance to a new transient instance. For example, a new Bid might have been added to a detached Item while it was on the presentation tier. Hibernate can detect that the Bid is new and must be inserted in the database. For this to work, Hibernate must be able to distinguish between a new transient instance and an old detached instance. Transient instances (such as the Bid) might need to be saved; detached instances (such as the Item) might need to be reattached (and later updated in the database). There are several ways to distinguish between transient and detached instances, but the nicest approach is to look at the value of the identifier property. Hibernate can examine the identifier of a transient or detached object on reattachment and treat the object (and the
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