`尽管使用c3p0,但无法打开连接问题

Could not open connection 错误然后我会重新启动我的应用程序,以消除这个问题。我无法找出问题的根源。

hibernate-configuration>
<session-factory>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/mydb?autoReconnect=true</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">abc123</property>
    <property name="hibernate.dialect">config.CustomDialect</property>

    <property name="connection.provider_class">org.hibernate.connection.C3P0ConnectionProvider</property>
    <property name="hibernate.c3p0.initialPoolSize">5</property>
    <property name="hibernate.c3p0.minPoolSize">5</property>
    <property name="hibernate.c3p0.maxPoolSize">100</property>
    <property name="hibernate.c3p0.checkoutTimeout">3000</property>
    <property name="hibernate.c3p0.maxStatementsPerConnection">30</property>
    <property name="hibernate.c3p0.unreturnedConnectionTimeout">3000</property>
    <property name="hibernate.c3p0.debugUnreturnedConnectionStackTraces">true</property>

    <property name="show_sql">true</property>
    <property name="format_sql">true</property>
    <property name="hbm2ddl.auto">update</property>

...
POJO mappings
</session-factory>
</hibernate-configuration>

public class HibernateUtil {

private static final SessionFactory sessionFactory;
static {
    try {
        Configuration configuration = new Configuration().configure();
        StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder()
                .applySettings(configuration.getProperties());
        sessionFactory = configuration.buildSessionFactory(builder.build());

    } catch (Exception ex) {
        throw new ExceptionInInitializerError(ex);
    }
}

public Session openSession() {
    return sessionFactory.openSession();
}
}

我在应用程序中添加了c3p0调试配置,以剔除未返回的连接(在内存泄漏的情况下),并为其生成堆栈跟踪,但日志中没有显示任何内容。

https://pastebin.com/MGb4Miau

这里有人能帮我找出问题所在吗?

编辑 : CustomDialect 类别:

public class CustomDialect extends MySQL5InnoDBDialect {
public String getTableTypeString() {
    return " ENGINE=InnoDB DEFAULT CHARSET=utf8";
    }
}