To store, organize, and retrieve data, most applications use a relational database. Java EE 5 components may access relational databases through the JDBC API. For information on this API, see http://java.sun.com/products/jdbc.
In the JDBC API, databases are accessed by using DataSource objects. A DataSource has a set of properties that identify and describe the real world data source that it represents. These properties include information such as the location of the database server, the name of the database, the network protocol to use to communicate with the server, and so on. In the Application Server, a data source is called a JDBC resource.
Applications access a data source using a connection, and a DataSource object can be thought of as a factory for connections to the particular data source that the DataSource instance represents. In a basic DataSource implementation, a call to the getConnection method returns a connection object that is a physical connection to the data source.
If a DataSource object is registered with a JNDI naming service, an application can use the JNDI API to access that DataSource object, which can then be used to connect to the data source it represents.
DataSource objects that implement connection pooling also produce a connection to the particular data source that the DataSource class represents. The connection object that the getConnection method returns is a handle to a PooledConnection object rather than being a physical connection. An application uses the connection object in the same way that it uses a connection. Connection pooling has no effect on application code except that a pooled connection, like all connections, should always be explicitly closed. When an application closes a connection that is pooled, the connection is returned to a pool of reusable connections. The next time getConnection is called, a handle to one of these pooled connections will be returned if one is available. Because connection pooling avoids creating a new physical connection every time one is requested, applications can run significantly faster.
A JDBC connection pool is a group of reusable connections for a particular database. Because creating each new physical connection is time consuming, the server maintains a pool of available connections to increase performance. When an application requests a connection, it obtains one from the pool. When an application closes a connection, the connection is returned to the pool.
Applications that use the Persistence API specify the DataSource object they are using in the jta-data-source element of the persistence.xml file.
This is typically the only reference to a JDBC object for a persistence unit. The application code does not refer to any JDBC objects. For more details, see Persistence Units.