Transactions

This chapter covers the use of EJB module property sheets to program container-managed transactions. For bean-managed transactions, see Building Enterprise JavaBeans Components.

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Default Transaction Boundaries

Transaction boundaries are determined by the Transaction Attribute properties of the enterprise beans involved in the transaction.

When you create an enterprise bean with the IDE's EJB wizards and select container-managed transactions, the wizard creates the enterprise bean with the default value for the transaction attribute property. This section shows you how to view the default transaction attribute settings and interpret them.

To open the Transaction Settings property editor and review the default settings:

Right-click the EJB module node and choose Properties Transaction Settings property ellipsis (...) button.

The Transaction Setting property editor opens.

FIGURE 6-1 shows the Transaction Settings Property Editor for the CatalogData EJB module that was covered in Chapter 3. The values displayed in FIGURE 6-1 are the default transaction attribute settings.

 FIGURE 6-1 Default Transaction Attribute Settings

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The default transaction attribute settings are displayed in the EJB or Method - Trans-attribute field. To understand this display, notice the following features of the display:

• Every enterprise beans in the module appears in this display. Each enterprise bean is represented by a node.
• Each enterprise bean name is followed by the bean's transaction attribute. For example, the first node in FIGURE 6-1, CatalogManagerBean, is followed by the word Required. Required is the default setting for the transaction attribute property. In FIGURE 6-1, all of the enterprise beans have the default setting.
• The nodes that represent enterprise beans can be expanded to display nodes that represent the methods of each enterprise bean. In FIGURE 6-1, all of the enterprise bean nodes are expanded.
• Methods have their own transaction attribute values. The transaction attribute values appear after the method names. If the transaction attribute value is null, the method inherits its transaction attribute value from the enterprise beans.
• In FIGURE 6-1, none of the method nodes displays a transaction attribute value. All of the methods inherit the Required value. This is the default setting.

Required is one of the transaction values defined in the Java 2 Platform, Enterprise Edition Specification. The rules for methods that have the Required transaction attribute are listed below:

• If a method with the Required attribute is called without an active transaction, the application server starts a new transaction.
• If a method with the Required attribute is called with an active transaction in progress, the application server executes the method within the active transaction.

This is the default behavior for enterprise bean methods.

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Redefining the Transaction Boundaries

A business transaction in an EJB module often spans several enterprise beans. A common architecture for enterprise bean business logic is one session bean and several entity beans. Clients call the session bean, and then the session bean calls methods of the entity beans.

For example, the client has data for two new related database records. The session bean generates database inserts by calling create methods of the two entity beans. The two database inserts must be in the same transaction. FIGURE 6-2 shows a transaction of this type.

 FIGURE 6-2 Complex Transaction

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When you assemble these entity beans into an EJB module, you want the application server to recognize the boundaries of the business transaction. You want the application server to include all of the work that the EJB module performs after a client call in a single database transaction. You want the application server to do the following:

• You want the application server to open a transaction when a client calls the session bean.
• You want the application server to keep the transaction open while the session bean calls the entity beans.
• You want the application server to close the transaction when the last call from the session bean to an entity bean returns and the session bean method that was called by the client completes.

If the application server recognizes these transaction boundaries, all of the work performed by the EJB module will be committed or rolled back together.

To configure transaction boundaries, you open the Transaction Settings property editor and modify the transaction attributes of the enterprise beans involved in the transaction.

The procedure that follows demonstrates editing the transaction attribute settings. This procedure changes the default transaction settings that are shown in FIGURE 6-1 to transaction attribute settings that produce the transaction boundaries shown in FIGURE 6-2.

To modify transaction attributes:

1. Right-click the EJB module node and choose Properties Transaction Settings property ellipsis (...) button.

The Transaction Settings property editor opens.

2. Change the transaction attributes of the session bean methods to RequiresNew.

You want to change the behavior of the CatalogManagerBean methods so that a call to either of its business methods (getAllItems and getOneItemDetail) starts a new transaction. Changing the transaction attribute to RequiresNew accomplishes this.

a. Click a session bean method to select it.

b. Change the value of the Trans-attribute field to RequiresNew.

Notice that the new transaction attribute value appears after the method node.

3. Change the transaction attributes of the entity bean methods to Mandatory.

You want to change the behavior of the entity bean methods so that these methods execute within the boundaries of the transactions opened by the session bean methods. Changing the transaction attribute to Mandatory accomplishes this. It tells the application server that these methods can only be executed if a transaction is already in progress.

a. Click an entity bean method to select it.

b. Change the value of the Trans-attribute field to Mandatory.

Notice that the new transaction attribute value appears after the method node.

4. Click OK to close the editor and save your changes.

FIGURE 6-3 shows the Transaction Settings Property Editor with the new transaction attributes settings.

 FIGURE 6-3 Modified Transaction Settings

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The transaction boundaries now match the business transactions. The changes that are visible in the property editor are described in the following list:

• The transaction attributes of the session bean methods have been set to RequiresNew, because you want the container to open a new transaction whenever a client calls one of these methods.
• The transaction attributes of entity bean business methods have been set to Mandatory, which means that these methods must be called with a transaction in progress. You want these methods to be called after the session bean has opened a transaction, and you want these methods to execute entirely within the boundaries of the session bean's transaction.
• Notice that the transaction attributes were modified on the method level, and the method nodes now display the new transaction attribute values after the method names.

For each EJB module that you work with, you must analyze the business logic in the module and determine what transaction boundaries are implied by the logic. Then use the Transaction Attribute property editor to implement those transaction boundaries. Set the transaction attributes of the enterprise beans, or the individual methods, involved in those transactions to specify the transaction boundaries.