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Transactions and security
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ArrayIndexOutOfBounds or NullPointerException that you didn t plan for, it will still roll back the CMT. However, in such cases the container will also assume
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that the bean is in inconsistent state and will destroy the instance. Because unnecessarily destroying bean instances is costly, you should never deliberately use system exceptions. Although the simplified code is very tempting, we recommend that you use application exceptions for CMT rollback carefully. Using the setRollbackOnly method, however verbose, removes the guesswork from automated transaction management, especially for junior developers who might have a hard time understanding the intricacies of exception handling in EJB. However, don t interpret this to mean you should avoid using custom application exceptions in general. In fact, we encourage the use of this powerful and intuitive errror-handling mechanism widely used in the Java realm. As you can clearly see, CMT relieves you from all but the most unavoidable details of EJB transaction management. However, for certain circumstances, CMT may not give you the level of control you need. BMT gives you this additional control while still providing a powerful, high-level API, as you ll see next.
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6.3 Bean-managed transactions
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The greatest strength of CMT is also its greatest weakness. Using CMT, you are limited to having the transaction boundaries set at the beginning and end of business methods and relying on the container to determine when a transaction starts, commits, or rolls back. BMT, on the other hand, allows you to specify exactly these details programmatically, using semantics similar to the JDBC transaction model with which you might already be familiar. However, even in this case, the container helps you by actually creating the physical transaction as well as taking care of a few low-level details. With BMT, you must be much more aware of the underlying JTA transaction API, primarily the javax.transaction.UserTransaction interface, which we ll introduce shortly. But first, we ll redevelop the Snag-It ordering code in BMT so that we can use it in the next few sections. You ll learn more about the javax.transaction.UserTransaction interface and how to use it. We ll also discuss the pros and cons of using BMT over CMT.
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6.3.1 Snag-It ordering using BMT
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Listing 6.3 reimplements the code in listing 6.1 using BMT. It checks if there are any bids on the item ordered, validates the user s credit card, charges the customer,
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Bean-managed transactions
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and removes the item from bidding. Note that the import statements are omitted and error handling trivialized to keep the code sample short.
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Listing 6.3 Implementing Snag-It using BMT
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@Stateless) @TransactionManagement(TransactionManagementType.BEAN) Uses BMT public class OrderManagerBean { Injects UserTransaction @Resource private UserTransaction userTransaction;
public void placeSnagItOrder(Item item, Customer customer){ try { userTransaction.begin(); Starts transaction if (!bidsExisting(item)){ validateCredit(customer); chargeCustomer(customer, item); removeItemFromBidding(item); } Commits transaction userTransaction.commit(); } catch (CreditValidationException cve) { userTransaction.rollback(); Rolls back } catch (CreditProcessingException cpe){ transaction on userTransaction.rollback(); exception } catch (DatabaseException de) { userTransaction.rollback(); } catch (Exception e) { e.printStackTrace(); } }
Briefly scanning the code, you ll note that the @TransactionManagement annotation specifies the value TransactionManagementType.BEAN as opposed to TransactionManagementType.CONTAINER, indicating that we are using BMT this time b. The TransactionAttribute annotation is missing altogether since it is applicable only for CMT. A UserTransaction, the JTA representation of a BMT, is injected C and used explicitly to begin D, commit E, or roll back F a transaction. The transaction boundary is much smaller than the entire method and includes only calls that really need to be atomic. The sections that follow discuss the code in greater detail, starting with getting a reference to the javax.transaction. UserTransaction.
Transactions and security
6.3.2 Getting a UserTransaction
The UserTransaction interface encapsulates the basic functionality provided by a Java EE transaction manager. JTA has a few other interfaces used under different circumstances. We won t cover them, as most of the time you ll be dealing with UserTransaction. (For full coverage of JTA, check out http://java.sun.com/ products/jta/.) As you might expect, the UserTransaction interface is too intricate under the hood to be instantiated directly and must be obtained from the container. In listing 6.3, we used the simplest way of getting a UserTransaction: injecting it through the @Resource annotation. There are a couple of other ways to do this: using JNDI lookup or through the EJBContext. JNDI lookup The application server binds the UserTransaction to the JNDI name java:comp/ UserTransaction. You can look it up directly using JNDI with this code:
Context context = new InitialContext(); UserTransaction userTransaction = (UserTransaction) context.lookup("java:comp/UserTransaction"); userTransaction.begin(); // Perform transacted tasks. userTransaction.commit();
This method is typically used outside of EJBs for example, if you need to use a transaction in a helper or a nonmanaged class in the EJB or web tier where dependency injection is not supported. If you find yourself in this situation, you might want to think long and hard about moving the transactional code to an EJB where you have access to greater abstractions. EJBContext You can also get a UserTransaction by invoking the getUserTransaction method of the EJBContext. This approach is useful if you re using a SessionContext or MessageDrivenContext for some other purpose anyway, and a separate injection just to get a transaction instance would be redundant. Note that you can only use the getUserTransaction method if you re using BMT. Calling this in a CMT environment will cause the context to throw an IllegalStateException. The following code shows the getUserTransaction method in action:
@Resource private SessionContext context; ... UserTransaction userTransaction = context.getUserTransaction(); userTransaction.begin();
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