When you create a PersistenceManagerFactory / EntityManagerFactory you define the connection URL, driver name, and the username/password to use. This works perfectly well but does not "pool" the connections so that they are efficiently opened/closed when needed to utilise datastore resources in an optimum way. DataNucleus allows you to utilise a connection pool using C3P0 to efficiently manage the connections to the datastore.

C3P0 is a third-party library providing connection pooling. This is accessed by specifying the persistence property datanucleus.connectionPoolingType . To utilise C3P0-based connection pooling we do this

// Specify our persistence properties used for creating our PMF/EMF
Properties props = new Properties();
properties.setProperty("javax.jdo.PersistenceManagerFactoryClass",
                "org.datanucleus.jdo.JDOPersistenceManagerFactory");
properties.setProperty("datanucleus.ConnectionDriverName","com.mysql.jdbc.Driver");
properties.setProperty("datanucleus.ConnectionURL","jdbc:mysql://localhost/myDB");
properties.setProperty("datanucleus.ConnectionUserName","login");
properties.setProperty("datanucleus.ConnectionPassword","password");
properties.setProperty("datanucleus.connectionPoolingType", "C3P0");

So the PMF / EMF will use connection pooling using C3P0. To do this you will need the C3P0 JAR to be in the CLASSPATH. If you want to configure C3P0 further you can include a "c3p0.properties" in your CLASSPATH - see the C3P0 documentation for details.

You can also specify persistence properties to control the actual pooling. The currently supported properties for C3P0 are shown below

# Pooling of Connections
datanucleus.connectionPool.maxPoolSize=5
datanucleus.connectionPool.minPoolSize=3
datanucleus.connectionPool.initialPoolSize=3

# Pooling of PreparedStatements
datanucleus.connectionPool.maxStatements=20