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JDBC 2 introduced standard connection pooling features in an add-on API known as the JDBC 2.0 Optional Package (also known as the JDBC 2.0 Standard Extension). These features have since been included in the core JDBC 3 API.
The JDBC API provides a client
and a server interface for connection pooling. The client
interface is javax.sql.DataSource,
which is what application code will typically use to
acquire a pooled database connection. The server interface
is javax.sql.ConnectionPoolDataSource,
which is how most application servers will interface with
the PostgreSQL™ JDBC
driver.
In an application server environment, the
application server configuration will typically refer to
the PostgreSQL™
ConnectionPoolDataSource implementation,
while the application component code will typically acquire a
DataSource implementation provided by
the application server (not by
PostgreSQL™).
For an environment without an application server,
PostgreSQL™ provides two implementations
of DataSource which an application can use
directly. One implementation performs connection pooling,
while the other simply provides access to database connections
through the DataSource interface without
any pooling. Again, these implementations should not be used
in an application server environment unless the application
server does not support the
ConnectionPoolDataSource interface.