Learn Together: 2008-01-06

Bean-Managed Transactions

In a bean-managed transaction, the code in the session or message-driven bean explicitly marks the boundaries of the transaction. An entity bean cannot have bean-managed transactions; it must use container-managed transactions instead. Although beans with container-managed transactions require less coding, they have one limitation: When a method is executing, it can be associated with either a single transaction or no transaction at all. If this limitation will make coding your bean difficult, you should consider using bean-managed transactions.

The following pseudocode illustrates the kind of fine-grained control you can obtain with bean-managed transactions. By checking various conditions, the pseudocode decides whether to start or stop different transactions within the business method.

begin transaction
...
update table-a
...
if (condition-x)
  commit transaction
else if (condition-y)
  update table-b
  commit transaction
else
  rollback transaction
  begin transaction
  update table-c
  commit transaction

When coding a bean-managed transaction for session or message-driven beans, you must decide whether to use JDBC or JTA transactions. The sections that follow discuss both types of transactions.

JDBC Transactions

A JDBC transaction is controlled by the transaction manager of the DBMS. You may want to use JDBC transactions when wrapping legacy code inside a session bean. To code a JDBC transaction, you invoke the commit and rollback methods of the java.sql.Connection interface. The beginning of a transaction is implicit. A transaction begins with the first SQL statement that follows the most recent commit, rollback, or connect statement. (This rule is generally true, but may vary with DBMS vendor.)

Source Code

The source code for the following example is in the j2eetutorial/examples/src/ejb/warehouse directory. To compile the code, go to the j2eetutorial/examples directory and type ant bank. To create the database tables, type ant create-warehouse-table. A sample WarehouseApp.ear file is in the j2eetutorial/examples/ears directory.

The following code is from the WarehouseEJB example, a session bean that uses the Connection interface's methods to delimit bean-managed transactions. The ship method starts by invoking setAutoCommit on the Connection object named con. This invocation tells the DBMS not to automatically commit every SQL statement. Next, the ship method calls routines that update the order_item and inventory database tables. If the updates succeed, the transaction is committed. If an exception is thrown, however, the transaction is rolled back.

public void ship (String productId, String orderId, int
quantity) {

  try {
     con.setAutoCommit(false);
     updateOrderItem(productId, orderId);
     updateInventory(productId, quantity);
     con.commit();
  } catch (Exception ex) {
      try {
         con.rollback();
         throw new EJBException("Transaction failed: " +
            ex.getMessage());
      } catch (SQLException sqx) {
          throw new EJBException("Rollback failed: " +
             sqx.getMessage());
      }
  }
}

JTA Transactions

JTA is the abbreviation for the Java Transaction API. This API allows you to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK implements the transaction manager with the Java Transaction Service ("JTS"). But your code doesn't call the JTS methods directly. Instead, it invokes the JTA methods, which then call the lower-level JTS routines.

A JTA transaction is controlled by the J2EE transaction manager. You may want to use a JTA transaction because it can span updates to multiple databases from different vendors. A particular DBMS's transaction manager may not work with heterogeneous databases. However, the J2EE transaction manager does have one limitation--it does not support nested transactions. In other words, it cannot start a transaction for an instance until the previous transaction has ended.

The source code for the following example is in the j2eetutorial/examples/src/ejb/teller directory. To compile the code, go to the j2eetutorial/examples directory and type ant teller. To create the database tables, type ant create-bank-teller. A sample TellerApp.ear file is in the j2eetutorial/examples/ears directory.

To demarcate a JTA transaction, you invoke the begin, commit, and rollback methods of the javax.transaction.UserTransaction interface. The following code, taken from the TellerBean class, demonstrates the UserTransaction methods. The begin and commit invocations delimit the updates to the database. If the updates fail, the code invokes the rollback method and throws an EJBException.

public void withdrawCash(double amount) {

  UserTransaction ut = context.getUserTransaction();

  try {
     ut.begin();
     updateChecking(amount);
     machineBalance -= amount;
     insertMachine(machineBalance);
     ut.commit();
  } catch (Exception ex) {
      try {
         ut.rollback();
      } catch (SystemException syex) {
          throw new EJBException
             ("Rollback failed: " + syex.getMessage());
      }
      throw new EJBException
         ("Transaction failed: " + ex.getMessage());
   }
}

Returning without Committing

In a stateless session bean with bean-managed transactions, a business method must commit or roll back a transaction before returning. However, a stateful session bean does not have this restriction.

In a stateful session bean with a JTA transaction, the association between the bean instance and the transaction is retained across multiple client calls. Even if each business method called by the client opens and closes the database connection, the association is retained until the instance completes the transaction.

In a stateful session bean with a JDBC transaction, the JDBC connection retains the association between the bean instance and the transaction across multiple calls. If the connection is closed, the association is not retained.

Methods Not Allowed in Bean-Managed Transactions

Do not invoke the getRollbackOnly and setRollbackOnly methods of the EJBContext interface in bean-managed transactions. These methods should be used only in container-managed transactions. For bean-managed transactions, invoke the getStatus and rollback methods of the UserTransaction interface.

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