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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, the deploytool generates the connect routines for you.
Coded Connections
How to Connect
The code examples in this section are from the AccountEJB class, which connects to the database with the following steps:
- Specify the database name.
private String dbName = "java:comp/env/jdbc/AccountDB";- Obtain the DataSource associated with the logical name.
InitialContext ic = new InitialContext(); DataSource ds = (DataSource) ic.lookup(dbName);- Get the Connection from the DataSource.
Connection con = ds.getConnection();When To Connect
When coding a component, you must decide how long the component will retain the connection. Generally you have two choices: either hold the connection for the lifetime of the component, or only during each database call. Your choice determines the method (or methods) in which your component connects to a database.
Longterm Connections
You can design a component that holds a database connection for its entire lifetime. Because the component connects and disconnects just once, its code is slightly easier to write. But there's a tradeoff-other components may not acquire the connection. Session and entity beans issue the lifelong connections in different methods.
The EJB container invokes the ejbCreate method at the beginning of a session bean's life cycle, and invokes the ejbRemove method at the end. To retain a connection for the lifetime of a session bean, you connect to the database in ejbCreate and disconnect in ejbRemove. If the session bean is stateful, you must also connect in ejbActivate and 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 may not be saved in this manner. Because a stateless session bean cannot be passivated, it does not require the additional calls in ejbActivate and ejbPassivate. For more information on activation and passivation, see The Life Cycle of a Session Bean. For an example of a stateful session bean with a longterm connection, see the TellerEJB.java code.
After instantiating an entity bean and moving it to the pooled stage, the EJB container invokes the setEntityContext method. Conversely, the EJB container invokes the unsetEntityContext method when the entity bean leaves the pooled stage and becomes eligible for garbage collection. To retain a database connection for its entire lifespan, an entity bean connects in the setEntityContext method and disconnects in the unsetEntityContext method. To see a diagram of the life cycle, refer to The Life Cycle of an Entity Bean. For an example of an entity bean with a longterm connection, see the AccountEJB.java code.
Shortterm Connections
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.
Specifying Database Users and Passwords
To connect to the Cloudscape database bundled with this release, you do not specify a database user and password. Authentication is performed by a separate service. For more information about authentication, see the chapter on Security.
However, some types of databases do require a user and password during connection. For these databases, if the getConnection call has no parameters, you must specify the database user and password with the deploytool. To specify these values, click on the Resource Ref's tabbed pane of the enterprise bean, select the appropriate row in the table labelled, "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 and password programmatically, you do not need to specify them with the deploytool. In this case, you include the database user and password in the arguments of the getConnection method:
con = dataSource.getConnection(dbUser, dbPassword);Container-Managed Connections
Connection Pooling
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. And because such a bean holds the connection for a short time, the same connection may be shared sequentially by many beans.
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