Database Connections for Enterprise Beans
The persistence type of an enterprise bean determines whether or not you code the connection routine. You must code the connection for enterprise beans that access a database and do not have container-managed persistence. Such beans include entity beans with bean-managed persistence and session beans. For entity beans with container-managed persistence,
deploytoolgenerates the connection routines for you.
How to Connect
The code examples in this section are from the
SavingsAccountBeanclass, which connects to the database via the following steps.
- Specify the database name.private String dbName = "java:comp/env/jdbc/SavingsAccountDB";
- Obtain the
DataSourceassociated with the logical name.InitialContext ic = new InitialContext(); DataSource ds = (DataSource) ic.lookup(dbName);
- Get the
DataSource.Connection con = ds.getConnection();
When to Connect
When coding an enterprise bean, you must decide how long it will retain the connection. Generally you have two choices: either hold the connection for the lifetime of the bean, or hold it only during each database call. Your choice determines the method (or methods) in which your bean connects to a database.
You can design an enterprise bean that holds a database connection for its entire lifetime. Because the bean connects and disconnects just once, its code is slightly easier to write. But there's a trade-off--other components cannot acquire the connection. Session and entity beans issue the lifelong connections in different methods.
The EJB container invokes the
ejbCreatemethod at the beginning of a session bean's life cycle and invokes the
ejbRemovemethod at the end. To retain a connection for the lifetime of a session bean, you connect to the database in
ejbCreateand disconnect in
ejbRemove. If the session bean is stateful, you must also connect in
ejbActivateand disconnect in
ejbPassivate. A stateful session bean requires these additional calls because the EJB container may passivate the bean during its lifetime. During passivation, a stateful session bean is saved in secondary storage, but a database connection cannot be saved in this manner. Because a stateless session bean cannot be passivated, it does not require the additional calls in
ejbPassivate. For more information on activation and passivation, see The Life Cycle of a Stateful Session Bean. For an example of a stateful session bean with a long-term connection, see the
TellerBean.javacode in the
Entity Beans with Container-Managed Persistence
After instantiating an entity bean and moving it to the pooled stage, the EJB container invokes the
setEntityContextmethod. Conversely, the EJB container invokes the
unsetEntityContextmethod when the entity bean leaves the pooled stage and becomes eligible for garbage collection. To retain a database connection for its entire life span, an entity bean connects in the
setEntityContextmethod and disconnects in the
unsetEntityContextmethod. To see a diagram of the life cycle, see Figure 3-5. For an example of an entity bean with a long-term connection, see the
SavingsAccountBean.javacode in the
Briefly held connections allow many components to share the same connection. Because the EJB container manages a pool of database connections, enterprise beans can quickly obtain and release the connections. For example, a business method might connect to a database, insert a row, and then disconnect.
In a session bean, a business method that connects to a database should be transactional. The transaction will help maintain data integrity.
deploytool Tips for Specifying Database Users and Passwords
The instructions in this section do not apply to entity beans with container-managed persistence. For those entity beans, see the instructions in Specifying the Database JNDI Name, User Name, and Password.
To connect to the Cloudscape database bundled with this release, you do not specify a database user name and password; authentication is performed by a separate service. For more information about authentication, see Chapter 15.
However, some types of databases do require a user name and password during connection. For these databases, if the
getConnectioncall has no parameters, you must specify the database user name and password with
deploytool. To specify these values, perform these steps:
- Select the enterprise bean in the tree view.
- Select the Resource Refs tab.
- Select the appropriate row in the table labeled, Resource Factories Referenced in Code, and enter the database user name and password in the fields at the bottom.
If you wish to obtain the database user name and password programmatically, you do not need to specify them with
deploytool. In this case, you include the database user name and password in the arguments of the
getConnectionmethod:con = dataSource.getConnection(dbUser, dbPassword);
The EJB container maintains the pool of database connections. This pool is transparent to the enterprise beans. When an enterprise bean requests a connection, the container fetches one from the pool and assigns it to the bean. Because the time-consuming connection has already been made, the bean quickly gets a connection. The bean may release the connection after each database call, since it can rapidly get another connection. Because such a bean holds the connection for a short time, the same connection can be shared sequentially by many beans.