Package org.apache.commons.dbcp2

Database Connection Pool API.

Q: How do I use the DBCP package?

A: There are two primary ways to access the DBCP pool, as a Driver, or as a DataSource. You'll want to create an instance of PoolingDriver or PoolingDataSource. When using one of these interfaces, you can just use your JDBC objects the way you normally would. Closing a Connection will simply return it to its pool.

Q: But PoolingDriver and PoolingDataSource both expect an ObjectPool as an input. Where do I get one of those?

A: The ObjectPool interface is defined in Commons Pool. You can use one of the provided implementations such as GenericObjectPool, ProxiedObjectPool or SoftReferenceObjectPool or you can create your own.

Q: Ok, I've found an ObjectPool implementation that I think suits my connection pooling needs. But it wants a PooledObjectFactory. What should I use for that?

A: The DBCP package provides a class for this purpose. It's called PoolableConnectionFactory. It implements the factory and lifecycle methods of PooledObjectFactory for Connections. But it doesn't create the actual database Connections itself, it uses a ConnectionFactory for that. The PoolableConnectionFactory will take Connections created by the ConnectionFactory and wrap them with classes that implement the pooling behaviour.

Several implementations of ConnectionFactory are provided--one that uses DriverManager to create connections (DriverManagerConnectionFactory), one that uses a Driver to create connections (DriverConnectionFactory), one that uses a DataSource to create connections (DataSourceConnectionFactory).

Q: I think I'm starting to get it, but can you walk me though it again?

A: Sure. Let's assume you want to create a DataSource that pools Connections. Let's also assume that those pooled Connections should be obtained from the DriverManager. You'll want to create a PoolingDataSource.

The PoolingDataSource uses an underlying ObjectPool to create and store its Connection.

To create a ObjectPool, you'll need a PooledObjectFactory that creates the actual Connections. That's what PoolableConnectionFactory is for.

To create the PoolableConnectionFactory, you'll need at least two things:

1. A ConnectionFactory from which the actual database Connections will be obtained.
2. An empty and factory-less ObjectPool in which the Connections will be stored.
   
   When you pass an ObjectPool into the PoolableConnectionFactory, it will automatically register itself as the PooledObjectFactory for that pool.

In code, that might look like this:

GenericObjectPool connectionPool = new GenericObjectPool(null);
 ConnectionFactory connectionFactory = new DriverManagerConnectionFactory("jdbc:some:connect:string", "username", "password");
 PoolableConnectionFactory poolableConnectionFactory = new PoolableConnectionFactory(connectionFactory,connectionPool,null,null,false,true);
 PoolingDataSource dataSource = new PoolingDataSource(connectionPool);

To create a PoolingDriver, we do the same thing, except that instead of creating a DataSource on the last line, we create a PoolingDriver, and register the connectionPool with it. E.g.,:

GenericObjectPool connectionPool = new GenericObjectPool(null);
 ConnectionFactory connectionFactory = new DriverManagerConnectionFactory("jdbc:some:connect:string", "username", "password");
 PoolableConnectionFactory poolableConnectionFactory = new PoolableConnectionFactory(connectionFactory,connectionPool,null,null,false,true);
 PoolingDriver driver = new PoolingDriver();
 driver.registerPool("example",connectionPool);

Since the PoolingDriver registers itself with the DriverManager when it is created, now you can just go to the DriverManager to create your Connections, like you normally would:

Connection conn = DriverManager.getConnection("jdbc:apache:commons:dbcp:example");