休眠未知服务异常：事务完成时请求未知服务

我有一个简单的类，它启动3个线程，并在每个线程中保存一个新对象。但是我得到了我无法理解的例外。任何人都可以帮我理解为什么例外？

package test;

import java.util.Date;

import org.hibernate.Session;

import domain.Event;

import util.HibernateUtil;

public class EventBeanTest {

    public static void main(String [] args) {

        Event e1 = new Event();
        e1.setTitle("111");
        e1.setDate(new Date());

        Event e2 = new Event();
        e2.setTitle("222");
        e2.setDate(new Date());

        Event e3 = new Event();
        e3.setTitle("333");
        e3.setDate(new Date());


        Thread t1 = new Thread(new EventRunnable(e1));
        Thread t2 = new Thread(new EventRunnable(e2));
        Thread t3 = new Thread(new EventRunnable(e3));

        t1.setName("event - 111");
        t2.setName("event - 222");
        t3.setName("event - 333");

        t1.start();
        t2.start();
        t3.start();

    }

}

class EventRunnable implements Runnable {
    private Event event;

    public EventRunnable(Event event) {
        this.event = event;
    }

    public void run() {

        System.out.println("Starting thread : " + Thread.currentThread().getName());

        Session session = HibernateUtil.getSessionFactory().getCurrentSession();

        session.beginTransaction();

        session.saveOrUpdate(event);
        session.getTransaction().commit();

        HibernateUtil.getSessionFactory().close();

        System.out.println("Finishing thread : " + Thread.currentThread().getName());

    }
}

这是日志文件的相关部分，显示异常：

Hibernate: select max(EVENT_ID) from test.EVENTS
Hibernate: insert into test.EVENTS (EVENT_DATE, TITLE, EVENT_ID) values (?, ?, ?)
Hibernate: insert into test.EVENTS (EVENT_DATE, TITLE, EVENT_ID) values (?, ?, ?)
Hibernate: insert into test.EVENTS (EVENT_DATE, TITLE, EVENT_ID) values (?, ?, ?)
Apr 22, 2012 2:46:55 PM org.hibernate.service.jdbc.connections.internal.DriverManagerConnectionProviderImpl stop
INFO: HHH000030: Cleaning up connection pool [jdbc:mysql://localhost:3306/test]
Finishing thread : event - 333
Apr 22, 2012 2:46:55 PM org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction afterAfterCompletion
INFO: HHH000425: Could not close session; swallowing exception[org.hibernate.service.UnknownServiceException: Unknown service requested [org.hibernate.stat.spi.StatisticsImplementor]] as transaction completed
Exception in thread "event - 222" org.hibernate.service.UnknownServiceException: Unknown service requested [org.hibernate.stat.spi.StatisticsImplementor]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:126)
    at org.hibernate.internal.SessionFactoryImpl.getStatisticsImplementor(SessionFactoryImpl.java:1708)
    at org.hibernate.internal.SessionFactoryImpl.getStatistics(SessionFactoryImpl.java:1704)
    at org.hibernate.engine.transaction.internal.TransactionCoordinatorImpl.afterTransaction(TransactionCoordinatorImpl.java:140)
    at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.afterTransactionCompletion(JdbcTransaction.java:138)
    at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:184)
    at test.EventRunnable.run(EventBeanTest.java:60)
    at java.lang.Thread.run(Thread.java:722)
Apr 22, 2012 2:46:55 PM org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction afterAfterCompletion
INFO: HHH000425: Could not close session; swallowing exception[org.hibernate.service.UnknownServiceException: Unknown service requested [org.hibernate.stat.spi.StatisticsImplementor]] as transaction completed
Exception in thread "event - 111" org.hibernate.service.UnknownServiceException: Unknown service requested [org.hibernate.stat.spi.StatisticsImplementor]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:126)
    at org.hibernate.internal.SessionFactoryImpl.getStatisticsImplementor(SessionFactoryImpl.java:1708)
    at org.hibernate.internal.SessionFactoryImpl.getStatistics(SessionFactoryImpl.java:1704)
    at org.hibernate.engine.transaction.internal.TransactionCoordinatorImpl.afterTransaction(TransactionCoordinatorImpl.java:140)
    at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.afterTransactionCompletion(JdbcTransaction.java:138)
    at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:184)
    at test.EventRunnable.run(EventBeanTest.java:60)
    at java.lang.Thread.run(Thread.java:722)

编辑 1

<?xml version='1.0' encoding='utf-8'?>

<session-factory>

    <!-- Database connection settings -->
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="connection.url">jdbc:mysql://localhost:3306/test</property>
    <property name="connection.username">vishnu</property>
    <property name="connection.password">con02305</property>

    <!-- JDBC connection pool (use the built-in) -->
    <property name="connection.pool_size">1</property>

    <!-- SQL dialect -->
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>

    <!-- Enable Hibernate's automatic session context management -->
    <property name="current_session_context_class">thread</property>

    <!-- Disable the second-level cache  -->
    <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

    <!-- Echo all executed SQL to stdout -->
    <property name="show_sql">true</property>

    <!-- Drop and re-create the database schema on startup 
    <property name="hbm2ddl.auto">update</property> -->

    <property name="default_schema">test</property>
    <property name="show_sql">true</property>

    <mapping resource="domain/Event.hbm.xml"/>

</session-factory>