JdbcTransactionFactory does this:

public ConnectionReleaseMode getDefaultReleaseMode() { return ConnectionReleaseMode.ON_CLOSE; }

This seems to be a mistake because:

1) In section 13.5. Connection release modes of the 4.1.4.Final Core Reference Manual it says:

for JDBCTransactionFactory, [org.hibernate.transaction.TransactionFactory.getDefaultReleaseMode()] returns ConnectionReleaseMode.AFTER_TRANSACTION

2) In ConnectionReleaseMode.java it says:

/**

• Indicates that JDBC connections should be released after each transaction
• ends (works with both JTA-registered synch and HibernateTransaction API).
• This mode may not be used with an application server JTA datasource.
• <p/>
• This is the default mode starting in 3.1; was previously {@link #ON_CLOSE}.
  */
  AFTER_TRANSACTION("after_transaction"),

You can work around this by setting hibernate.connection.release_mode=after_transaction