Hibernate和C3P0问题：MySQLNonTransientConnectionException，连接太多

Question

我在使用c3p0和hibernate时遇到异常。

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections
    at sun.reflect.GeneratedConstructorAccessor413.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.Util.getInstance(Util.java:386)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1015)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4237)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4169)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:928)
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1750)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1290)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2502)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2535)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2320)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:834)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:46)
    at sun.reflect.GeneratedConstructorAccessor407.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:416)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:347)
    at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:146)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:195)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:184)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:200)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1086)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquireAndDecrementPendingAcquiresWithinLockOnSuccess(BasicResourcePool.java:1073)
    at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:44)
    at com.mchange.v2.resourcepool.BasicResourcePool$ScatteredAcquireTask.run(BasicResourcePool.java:1810)
    at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:648) 

这是我的HibernateUtil类（我把它命名为 SessionFactoryBuilder ）

public class SessionFactoryBuilder
{
    private static SessionFactoryBuilder instance;
    private static SessionFactory sessionFactory;

    private SessionFactoryBuilder()
    {
        buildConfig();
        System.out.println("hehehehe");
    }

    private static void buildConfig()
    {
        Configuration configuration = new Configuration().configure();
            StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder()
                            .applySettings(configuration.getProperties());
            sessionFactory = configuration.buildSessionFactory(builder.build());
    }

    public static SessionFactoryBuilder getInstance()
    {
         if(instance == null) 
         {
            instance = new SessionFactoryBuilder();

         }
      return instance;
    }

    public SessionFactory getSessionFactory()
    {
        return sessionFactory;
    }  

}

最后，这是我的 hibernate.cfg.xml 文件，

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="show_sql">true</property>
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/*****</property>
    <property name="hibernate.connection.username">myglukose</property>  
    <property name="hibernate.c3p0.min_size">5</property>
    <property name="hibernate.c3p0.max_size">20</property>
    <property name="hibernate.c3p0.timeout">3000</property>
    <property name="hibernate.c3p0.max_statements">50</property>
    <property name="hibernate.c3p0.idle_test_period">300</property>
    <property name="hibernate.c3p0.testConnectionOnCheckout">true</property>
    <property name="hibernate.c3p0.preferredTestQuery">SELECT 1</property>
    <property name="hibernate.connection.password">********</property>
    <mapping resource="beans/Reminder.hbm.xml"/>
    <mapping resource="beans/StressType.hbm.xml"/>
    <mapping resource="beans/Medication.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

更新

下面是我的数据库实现类，Service类通过接口使用它。本课程使用 SessionFactoryBuilder

public class StressTypeDAOImpl implements StressTypeDAOInterface{

    private static final SessionFactoryBuilder sessionFactoryBuilder = SessionFactoryBuilder.getInstance();

    @Override
    public Session openCurrentSession() {
        Session session=sessionFactoryBuilder.getSessionFactory().openSession();
        return session;
    }

    @Override
    public Transaction openTransaction(Session session) {
        Transaction transaction=session.beginTransaction();
        return transaction;
    }

    @Override
    public List<StressType> getAllStressTypes(Session session) {
        List<StressType> list=(List<StressType>)session.createQuery("from StressType").list();
        return list;
    }

}

这是我的服务类，

public class StressTypeService {

    private static StressTypeDAOInterface stressTypeDAOInterface;

    public StressTypeService(){
        stressTypeDAOInterface=new StressTypeDAOImpl();
    }

    public Session getSession(){
        Session session=stressTypeDAOInterface.openCurrentSession();
        return session;
    }

    public Transaction getTransaction(Session session){
        Transaction transaction=stressTypeDAOInterface.openTransaction(session);
        return transaction;
    }    

    public List<StressType> getAllStressTypes(){
        Session session=stressTypeDAOInterface.openCurrentSession();
        Transaction transaction=null;

        List<StressType> list=new ArrayList<StressType>();
        try{
            transaction=stressTypeDAOInterface.openTransaction(session);
            list=stressTypeDAOInterface.getAllStressTypes(session);
            transaction.commit();
        }catch(Exception ex){
            ex.printStackTrace();
        }finally{
            session.close();
        }
        return list;
    }    

}

对此例外有任何想法吗？