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Propagating the Hibernate Session
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You ve tried the examples in previous chapters and stored and loaded objects inside transactions. Very likely you ve noticed that code examples of five lines are excellent to help you understand a particular issue and learn an API and how objects change their state. If you take the next step and try to apply what you ve learned in your own application, you ll probably soon realize that you re missing two important concepts. The first concept we ll show you in this chapter persistence context propagation is useful when you have to call several classes to complete a particular action in your application and they all need database access. So far, we had only a single method that opened and closed a persistence context (a Session or an EntityManager) internally. Instead of passing the persistence context between classes and methods manually, we ll show you the mechanisms in Hibernate and Java Persistence that can take care of propagation automatically. Hibernate can help you to create more complex units of work. The next design problem you ll run into is that of application flow when your application user has to be guided through several screens to complete a unit of work. You must create code that controls the navigation from screen to screen however, this is outside of the scope of persistence, and we won t have much to say about it in this chapter. What is partly the responsibility of the persistence mechanism is the atomicity and isolation of data access for a unit of work that spans possible user think-time. We call a unit of work that completes in several client/server request and response cycles a conversation. Hibernate and Java Persistence offer several strategies for the implementation of conversations, and in this chapter we show you how the pieces fit together with realistic examples. We start with Hibernate and then, in the second half of the chapter, discuss JPA conversations. Let s create more complex data access examples first, to see how several classes can reuse the same persistence context through automatic propagation.
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11.1 Propagating the Hibernate Session
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Recall the use case we introduced in the previous chapter: An event that triggers the end of an auction has to be processed (chapter 10, section 10.1, Transaction essentials ). For the following examples, it doesn t matter who triggered this event; probably an automatic timer ends auctions when their end date and time is reached. It could also be a human operator who triggers the event.
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Implementing conversations
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To process the event, you need to execute a sequence of operations: check the winning bid for the auction, charge the cost of the auction, notify the seller and winner, and so on. You could write a single class that has one big procedure. A better design is to move the responsibility for each of these steps into reusable smaller components and to separate them by concern. We ll have much more to say about this in chapter 16. For now, assume that you followed our advice and that several classes need to be called inside the same unit of work to process the closing of an auction.
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11.1.1 The use case for Session propagation
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Look at the code example in Listing 11.1, which controls the processing of the event.
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Listing 11.1 Controller code that closes and ends an auction
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public class ManageAuction { ItemDAO PaymentDAO itemDAO = new ItemDAO(); paymentDAO = new PaymentDAO();
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public void endAuction(Item item) { // Reattach item itemDAO.makePersistent(item); // Set winning bid Bid winningBid = itemDAO.getMaxBid( item.getId() ); item.setSuccessfulBid(winningBid); item.setBuyer( winningBid.getBidder() ); // Charge seller Payment payment = new Payment(item); paymentDAO.makePersistent(payment); // Notify seller and winner ... } ... }
The ManageAuction class is called a controller. Its responsibility is to coordinate all the steps necessary to process a particular event. The method endAuction() is called by the timer (or user interface) when the event is triggered. The controller doesn t contain all the code necessary to complete and close the auction; it delegates as much as possible to other classes. First, it needs two stateless service
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