Hibernate, being an Object->Relational DB Mapper for Java, needs access to JDBC connections. Out of the box, Hibernate is fairly self contained when it comes to connection control. By default, Hibernate ships with the ability to obtain a data source implementation ( javax.sql.DataSource ) from JNDI by setting the properties appropriately:

hibernate.connection.datasource = java:/comp/env/jdbc/test

Alternatively, if JNDI isn't an option, you can use a Hibernate-internal connection pool implementation (C3PO), and simply give driver/url information for Hibernate to create and pool its own connections:

hibernate.connection.driver_class = org.postgresql.Driver
hibernate.connection.url = jdbc: postgresql://localhost/mydatabase
hibernate.connection.username = myuser
hibernate.connection.password = secret
hibernate.c3p0.min_size=5
hibernate.c3p0.max_size=20
hibernate.c3p0.timeout=1800
hibernate.c3p0.max_statements=50

In both of these cases, it is the same process to get a Hibernate Session to work with:

Session session = sessionFactory.openSession();

Maybe neither of those are options will work for you. The data source implementation gives you control over the connections, but it requires JNDI availability. The connection pooling implementation doesn't require JNDI, but now Hibernate has it's own pool of connections, and you cannot gain access to them (well, you can, but it's a breach of the Hibernate API rules). Additionally, if you have a need to give Hibernate connections to use on a per-session basis, you can always ask the session factory for a session running on a particular connection:

Connection conn = // ... get connection from some outside source.
Session session = sessionFactory.openSession(conn);

This isn't always favorable, however, as you now have a lot of code interacting with your connection pool, and it may be potentially difficult (or even impossible) given your constraints to integrate your code to that level.

If you have legacy connection pooling or non-JNDI data sources that you need to get to, you can simply create an implementation of the Hibernate org.hibernate.connection.ConnectionProvider interface. The ConnectionProvider API is fairly straightforward, in consists of four methods:

• configure(Properties) - This method tells the connection provider to prepare itself using the Hibernate properties passed in.
• getConnection() throws SQLException - Get a method from your connection source for hibernate to use.
• closeConnection(Connection) throws SQLException - Release a connection Hibernate has been using (potentilly putting it back in to your legacy pool, or whatever else).
• close() - This method tells the connection provider to release any resources for shutdown.

Once implemented, it is simply a matter of telling Hibernate at configuration-time that you want it to use your connection provider:

hibernate.connection.provider_class=com.javalobby.tnt.MyConnectionProvider

The only problem you will have now is the fact that you don't have control over the instantiation of the connection provider implementation, and it needs to obtain access to your connection source at 'configure' time so that it can begin passing those connections to Hibernate. There are a couple of solutions, all of them based in static fields - straight static references, singletons, and thread-locals. It largely depends on your implementation what will work for you - I'll just address static references as they are the most simple, and most straightforward:

// configuring hibernate here.
Configuration configuration = new Configuration()
	.setProperty(Environment.CONNECTION_PROVIDER, "com.javalobby.tnt.MyConnectionProvider");
 
// providing reference to connection pool here.
LegacyConnectionPool legacyPool = getLegacyConnectionPool();
com.javalobby.tnt.MyConnectionProvider.connectionPool = legacyPool;
 
// Get the session factory 
// (this will invoke the construction of the connection provider, etc)
SessionFactory sessionFactory = configuration.buildSessionFactory();

Here is the implementation of the connection provider:

package com.javalobby.tnt;
 
import java.sql.*;
import java.util.*;
 
import org.hibernate.*;
import org.hibernate.connection.*;
 
public class MyConnectionProvider implements ConnectionProvider {
 public static LegacyConnectionPool connectionPool;
	
 public void configure(Properties props) throws HibernateException {
  if(connectionPool == null) {
   throw new HibernateException("Connection pool must be set.");
 }
 
 public Connection getConnection() throws SQLException {
  return connectionPool.getConnection();		
 }
 
 public void closeConnection(Connection conn) throws SQLException {
  connectionPool.freeConnection(conn);
 }
 
 public void close() {
 }
 
}