Hibernate 'unsaved transient instance' error on stress

Hello Experts,

I have a problem with my application (which uses hibernate, spring MVC and struts) which only happens when I stress the application.

When I test the application with one user it works flawlessly, however I have started stress testing using JMeter which can simulate as many simultaneous visits as you want, while trying this with 10+ simultaneous visits at the time it gives an hibernate error every now and then.
The error given on the page is the following:

javax.servlet.ServletException: org.springframework.dao.InvalidDataAccessApiUsageException: object references an unsaved transient instance - save the transient instance before flushing: com.GolfOnline.Beans.Course; nested exception is org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.GolfOnline.Beans.Course

The stack gives this as error:
object references an unsaved transient instance - save the transient instance before flushing: com.GolfOnline.Beans.Course

I am using the getHibernateTemplate helper class for the hibernate requests

Firstly I am not exactly sure why it gives me this error, I think it's because in one request I retrieve a 'Course' object and in this same request I use this to retrieve a 'GolferType' object using an association to a 'Course' object. With many simultaneous requests at the same time the first retrieved 'Course' object becomes detached for some reason and can't be used again in a query then??

Could you advise me on which way to go in order to solve the problem?
All relevant code is below and I have prepared a setup to reproduce the problem at:
http://www.maxvandenboom.nl/testprogram.html, this holds the example source (Netbeans project), database SQL and JMeter (and it's test plan to reproduce the error)

Any help solving this problem would be greatly appreciated!!!
Max

------------------------------------------------------- applicationContext.xml
 
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
       http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
       http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
    
    <!-- Hibernate (persistence) -->
    
    <bean id="golfOnlineDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
        <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
         --censored--
</bean>
 
    <bean id="golfOnlineSessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
        <property name="dataSource" ref="golfOnlineDataSource"/>
        <property name="mappingResources">
            <list>
                <value>com/GolfOnline/Hibernate/Course.hbm.xml</value>
                <value>com/GolfOnline/Hibernate/GolferType.hbm.xml</value>
            </list>
        </property>
        <property name="hibernateProperties">
            <props>
                <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
                <prop key="hibernate.show_sql">true</prop>
                <prop key="hibernate.current_session_context_class">thread</prop>
                <prop key="hibernate.connection.pool_size">0</prop>
                <prop key="hibernate.dbcp.maxActive">15</prop>
                <prop key="hibernate.dbcp.maxIdle">5</prop>
                <prop key="hibernate.dbcp.maxWait">120000</prop>
                <prop key="hibernate.dbcp.whenExhaustedAction">1</prop>
                <prop key="hibernate.dbcp.testOnBorrow">true</prop>
                <prop key="hibernate.dbcp.testOnReturn">true</prop>
                <prop key="hibernate.dbcp.validationQuery">--censored--</prop>
                <prop key="hibernate.dbcp.ps.maxActive">0</prop>
                <prop key="hibernate.dbcp.ps.maxIdle">0</prop>
                <prop key="hibernate.dbcp.ps.maxWait">-1</prop>
                <prop key="hibernate.dbcp.ps.whenExhaustedAction">2</prop>
            </props>
        </property>
    </bean>
    
    <bean id="golfOnlineTransactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
        <property name="sessionFactory">
            <ref local="golfOnlineSessionFactory"/>
        </property>
    </bean>
    
    <!-- Service objects -->
    <bean id="courseService" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">	
        <property name="transactionManager"><ref local="golfOnlineTransactionManager"/></property>
        <property name="target"><ref local="courseServiceTarget"/></property>
        <property name="transactionAttributes">
            <props>
                <prop key="get*">PROPAGATION_REQUIRED,readOnly</prop>
                <prop key="search*">PROPAGATION_REQUIRED,readOnly</prop>
                <prop key="check*">PROPAGATION_REQUIRED,readOnly</prop>
                <prop key="add*">PROPAGATION_REQUIRED</prop>
                <prop key="update*">PROPAGATION_REQUIRED</prop>
                <prop key="delete*">PROPAGATION_REQUIRED</prop>
            </props>
        </property>
    </bean>
    
    <!-- Service target objects -->
    <bean id="courseServiceTarget" class="com.GolfOnline.Services.CourseServiceImpl">
        <property name="courseDAO"><ref local="courseDAO"/></property>
        <property name="golferTypeDAO"><ref local="golferTypeDAO"/></property>
    </bean>
    
    <!-- DAO objects -->
    <bean id="courseDAO" class="com.GolfOnline.DAOs.CourseDAOImpl">
        <property name="sessionFactory" ref="golfOnlineSessionFactory"/>
    </bean>
    <bean id="golferTypeDAO" class="com.GolfOnline.DAOs.GolferTypeDAOImpl">
        <property name="sessionFactory" ref="golfOnlineSessionFactory"/>
    </bean>
    
</beans>
 
------------------------------------------------------- course.hbm
 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
  <class name="com.GolfOnline.Beans.Course" table="tbl_Course">
    <id column="cr_ID" name="id" type="short">
      <generator class="native"/>
    </id>
    <property column="cr_Name" name="name"/>
  </class>
</hibernate-mapping>
 
------------------------------------------------------- golfertype.hbm
 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
  <class name="com.GolfOnline.Beans.GolferType" table="tbl_GolferType">
    <id column="gt_ID" name="id" type="short">
      <generator class="native"/>
    </id>
    <many-to-one class="com.GolfOnline.Beans.Course" column="gt_Course_cr_ID" name="course"/>
    <property column="gt_Title" name="title"/>
  </class>
</hibernate-mapping>

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