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Enterprise Beans

Figure 18-2 takes a closer look at the access paths between the clients, enterprise beans, and database tables. As you can see, the end-user clients (Web and J2EE application clients) access only the session beans. Within the enterprise bean tier, the session beans are clients of the entity beans. On the back end of the application, the entity beans access the database tables that store the entity states.

Figure 18-2 Enterprise Beans in the Duke's Bank Application

The source code for these enterprise beans is in the j2eetutorial/bank/src/com/sun/ebank/ejb subdirectory.

Session Beans

The Duke's Bank application has three session beans: AccountControllerEJB, CustomerControllerEJB, and TxControllerEJB. (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.

AccountControllerEJB

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

The following methods create and remove entity beans:

• createAccount
• removeAccount

These methods of the AccountControllerEJB session bean call the create and remove methods of the AccountEJB entity bean. 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 verifies that the specified customer exists by invoking the findByPrimaryKey method of the CustomerEJB entity bean. If the result of this verification is false, the createAccount method throws a CustomerNotFoundException.

The following methods manage the account-customer relationship:

• addCustomerToAccount
• removeCustomerFromAccount

The AccountEJB and CustomerEJB entity beans have a many-to-many relationship. A bank account may be jointly held by more than one customer, and a customer may have multiple accounts. Because the entity beans use bean-managed persistence, there are several ways to manage this relationship. For more information, see Mapping Table Relationships for Bean-Managed Persistence.

In the Duke's Bank application, the addCustomerToAccount and removeCustomerFromAccount methods of the AccountControllerEJB 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 inserts a row into the customer_account_xref database table. In this cross-reference table, each row contains the customerId and accountId of the related entities. To remove a relationship, the removeCustomerFromAccount method deletes a row from the customer_account_xref table. If a client calls the removeAccount method, then all rows for the specified accountId are removed from the customer_account_xref table.

The following methods get the account information:

• getAccountsOfCustomer
• getDetails

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

CustomerControllerEJB

Because it is the AccountControllerEJB bean that manages the customer-account relationship, CustomerControllerEJB is the simpler of these two session beans. A client creates a CustomerEJB entity bean by invoking the createCustomer method of the CustomerControllerEJB session bean. To remove a customer, the client calls the removeCustomer method, which not only invokes the remove method of CustomerEJB, but also deletes from the customer_account_xref table all rows that identify the customer.

The CustomerControllerEJB session bean has two methods that return multiple customers: getCustomersOfAccount and getCustomersOfLastName. These methods call the corresponding finder methods--findbyAccountId and findByLastName--of CustomerEJB.

TxControllerEJB

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

• withdraw
• deposit
• makeCharge
• makePayment
• transferFunds

These methods access an AccountEJB entity bean to verify the account type and to set the new balance. The withdraw and deposit methods are for non-credit accounts, whereas the makeCharge and makePayment methods are for credit accounts. 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, then 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 AccountEJB instance and adds the same amount to another instance. Because both of these steps must complete, the transferFunds method has a Required transaction attribute. If either step fails, the entire operation is rolled back and the balances remain unchanged.

Entity Beans

For each business entity represented in our simple bank, the Duke's Bank application has a matching entity bean:

• AccountEJB
• CustomerEJB
• TxEJB

The purpose of these beans is to provide an object view of these database tables: account, customer, and tx. For each column in a table, the corresponding entity bean has an instance variable. Because they use bean-managed persistence, the entity beans contain the SQL statements that access the tables. For example, the create method of the CustomerEJB entity bean calls the SQL INSERT command.

Unlike the session beans, the entity beans do not validate method parameters (except for the primary key parameter of ejbCreate). During the design phase, we decided that the session beans would check the parameters and throw the application exceptions, such as CustomerNotInAccountException and IllegalAccountTypeException. Consequently, if some other application were to include these entity beans, its session beans would also have to validate the method parameters.

Helper Classes

The EJB JAR files include several helper classes that are used by the enterprise beans. The source code for these classes is in the j2eetutorial/bank/src/com/sun/ebank/util subdirectory. Table 18-1 briefly describes the helper classes.

Table 18-1 Helper Classes for the Application's Enterprise Beans 

Class Name	Description
AccountDetails	Encapsulates the state of an AccountEJB instance. Returned by the getDetails methods of AccountControllerEJB and AccountEJB.
CodedNames	Defines the strings that are the logical names in the calls of the lookup method. (For example: java:comp/env/ejb/account). The EJBGetter class references these strings.
CustomerDetails	Encapsulates the state of a CustomerEJB instance. Returned by the getDetails methods of CustomerControllerEJB and CustomerEJB.
DBHelper	Provides methods that generate the next primary keys (for example, getNextAccountId).
Debug	Has simple methods for printing a debugging message from an enterprise bean. These messages appear on the stdout of the J2EE server if it's run with the -verbose option.
DomainUtil	Contains validation methods: getAccountTypes, checkAccountType, and isCreditAccount.
EJBGetter	Has methods that locate (by invoking lookup) and return home interfaces (for example, getAccountControllerHome).
TxDetails	Encapsulates the state of a TxEJB instance. Returned by the getDetails methods of TxControllerEJB and TxEJB.

Database Tables

A database table of the Duke's Bank application may be categorized by its purpose: representing business entities and holding the next primary key.

Tables Representing Business Entities

Figure 18-3 shows relationships between the database tables. The customer and account tables have a many-to-many relationship: A customer may have several bank accounts, and each account may be owned by more than one customer. This many-to-many relationship is implemented by the cross reference table named customer_account_xref. The account and tx tables have a one-to-many relationship: A bank account may have many transactions, but each transaction refers to a single account.

Figure 18-3 makes use of several abbreviations. PK stands for primary key, the value that uniquely identifies a row in a table. FK is an abbreviation for foreign key, which is the primary key of the related table. Tx is short for transaction, such as a deposit or withdrawal.

Figure 18-3 Database Tables in the Duke's Bank Application

Tables That Hold the Next Primary Key

These tables have the following names:

• next_account_id
• next_customer_id
• next_tx_id

Each of these tables has a single column named id. The value of id is the next primary key that is passed to the create method of an entity bean. For example, before it creates a new AccountEJB entity bean, the AccountControllerEJB session bean must obtain a unique key by invoking the getNextAccountId method of the DBHelper class. The getNextAccountId method reads the id from the next_account_id table, increments the id value in the table, and then returns the id.

Protecting the Enterprise Beans

In the J2EE platform, you can protect an enterprise bean by specifying the security roles that can access its methods (see EJB-Tier Security). In the Duke's Bank application, two roles are defined--BankCustomer and BankAdmin--because two categories of operations are defined by the enterprise beans.

A user in the BankAdmin role is allowed to perform administrative functions: creating or removing an account, adding a customer to or removing a customer from an account, setting a credit line, and setting an initial balance. A user in the BankCustomer role is allowed to deposit, withdraw, transfer funds, make charges and payments, and list the account's transactions. Notice that there is no overlap in functions that users in either role can perform.

Access to these functions was restricted to the appropriate role by setting method permissions on selected methods of the CustomerControllerEJB, AccountControllerEJB, and TxControllerEJB enterprise beans. For example, by allowing only users in the BankAdmin role to access the createAccount method in the AccountControllerEJB enterprise bean, you have denied users in the BankCustomer role or any other role permission to create bank accounts. To see the method permissions that have been set, in deploytool locate the CustomerControllerEJB, AccountControllerEJB, and TxControllerEJB enterprise beans in the tree view. For each bean, select the Security tab and examine the method permissions.

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