The Java EE 5 Tutorial

Session Beans

The Duke’s Bank application has three session beans: AccountControllerBean, CustomerControllerBean, and TxControllerBean. (Tx stands for a business transaction, such as transferring funds.) These session beans provide a client’s view of the application’s business logic. Hidden from the clients are the server-side routines that implement the business logic, access databases, manage relationships, and perform error checking.

The AccountControllerBean Session Bean

The business methods of the AccountControllerBean session bean perform tasks that fall into the following categories: creating and removing entities, managing the account-customer relationship, and getting the account information.

The following methods create and remove entities:

These methods of the AccountControllerBean session bean call the create and remove methods of the Account entity. The createAccount and removeAccount methods throw application exceptions to indicate invalid method arguments. The createAccount method throws an IllegalAccountTypeException if the type argument is neither Checking, Savings, Credit, nor Money Market. The createAccount method also looks up the specified customer exists by invoking the EntityManager.find method. If the result of this verification is null, the createAccount method throws a CustomerNotFoundException.

The following methods manage the account-customer relationship:

The Account and Customer entities have a many-to-many relationship. A bank account can be jointly held by more than one customer, and a customer can have multiple accounts.

In the Duke’s Bank application, the addCustomerToAccount and removeCustomerFromAccount methods of the AccountControllerBean session bean manage the account-customer relationship. The addCustomerToAccount method, for example, starts by verifying that the customer exists. To create the relationship, the addCustomerToAccount method first looks up the Customer and Account entities using the EntityManager.find method, then it calls the Account.addCustomer method to associate the customer with the account.

The following methods get the account information:

The AccountControllerBean session bean has two get methods. The getAccountsOfCustomer method returns all of the accounts of a given customer by invoking the getAccounts method of the Account entity. Instead of implementing a get method for every instance variable, the AccountControllerBean has a getDetails method that returns an object (AccountDetails) that encapsulates the entire state of an Account entity. Because it can invoke a single method to retrieve the entire state, the client avoids the overhead associated with multiple remote calls.

The CustomerControllerBean Session Bean

A client creates a Customer entity by invoking the createCustomer method of the CustomerControllerBean session bean. To remove a customer, the client calls the removeCustomer method, which invokes the EntityManager.remove method on the Customer instance.

The CustomerControllerBean session bean has two methods that return multiple customers: getCustomersOfAccount and getCustomersOfLastName. getCustomersOfAccount calls the getCustomers method of the Account entity. getCustomersOfLastName uses the Customer.FindByLastName named query to search the database for customers with a matching last name, which is a named parameter to the query.

The TxControllerBean Session Bean

The TxControllerBean session bean handles bank transactions. In addition to its get methods, getTxsOfAccount and getDetails, the TxControllerBean bean has several methods that change the balances of the bank accounts:

These methods access an Account entity to verify the account type and to set the new balance. The withdraw and deposit methods are for standard accounts, whereas the makeCharge and makePayment methods are for accounts that include a line of credit. If the type method argument does not match the account, these methods throw an IllegalAccountTypeException. If a withdrawal were to result in a negative balance, the withdraw method throws an InsufficientFundsException. If a credit charge attempts to exceed the account’s credit line, the makeCharge method throws an InsufficientCreditException.

The transferFunds method also checks the account type and new balance; if necessary, it throws the same exceptions as the withdraw and makeCharge methods. The transferFunds method subtracts from the balance of one Account instance and adds the same amount to another instance. Both of these steps must complete to ensure data integrity. If either step fails, the entire operation is rolled back and the balances remain unchanged. The transferFunds method, like all methods in session beans that use container-managed transaction demarcation, has an implicit Required transaction attribute. That is, you don’t need to explicitly decorate the method with a @TransactionAttribute annotation.