Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'katTest' defined in class path resource [applicationContext.xml]: Invocation of init method failed; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: kat.beans.Kategorie.kategories, no session or session was closed
Caused by: org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: kat.beans.Kategorie.kategories, no session or session was closed
   at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:358)
   at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:350)
   at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:97)
   at org.hibernate.collection.PersistentSet.size(PersistentSet.java:114)
   at kat.KatTest.init(KatTest.java:21)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.lang.reflect.Method.invoke(Method.java:597)
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeCustomInitMethod(AbstractAutowireCapableBeanFactory.java:1241)
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1206)
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1172)
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:428)
   at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:251)
   at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:156)
   at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:248)
   at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:160)
   at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:287)
   at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:352)
   at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:91)
   at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:75)
   at kat.Launcher.main(Launcher.java:9)

Kategorie root = kategorieDAO.findById(1);
System.out.println(root.getName()); // Works fine...

Collection<Kategorie> childs = root.getKategories(); // Throws the BeanCreationException

System.out.println("Size: " + childs.size());

Iterator<Kategorie> it = unterKategorien.iterator();

while (it.hasNext()) {
   Kategorie current = it.next();
   System.out.println(" - " + current.getName());
}

<bean id="hibernateDataSource"
   class="org.apache.commons.dbcp.BasicDataSource">
   <property name="driverClassName"
      value="com.mysql.jdbc.Driver">
   </property>
   <property name="url"
      value="jdbc:mysql://server/db">
   </property>
   <property name="username" value="user"></property>
   <property name="password" value="pass"></property>
</bean>

<bean id="sessionFactory"
   class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
   <property name="dataSource">
      <ref bean="hibernateDataSource" />
   </property>
   <property name="hibernateProperties">
      <props>
         <prop key="hibernate.dialect">
            org.hibernate.dialect.MySQLDialect
         </prop>
      </props>
   </property>
   <property name="mappingResources">
      <list>
         <value>kat/beans/Kategorie.hbm.xml</value>
      </list>
   </property>
</bean>

<bean id="kategorieDAO" class="kat.beans.KategorieDAO">
   <property name="sessionFactory">
      <ref bean="sessionFactory" />
   </property>
</bean>

<bean id="katTest" class="kat.KatTest" init-method="init">
   <property name="kategorieDAO" ref="kategorieDAO" />
</bean>

<hibernate-mapping>
    <class name="kat.beans.Kategorie" table="kategorie" catalog="db">
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="increment"></generator>
        </id>
        <many-to-one name="kategorie" class="kat.beans.Kategorie" fetch="select">
            <column name="parentId" />
        </many-to-one>
        <property name="name" type="java.lang.String">
            <column name="name" length="50" not-null="true" />
        </property>
        <set name="kategories" inverse="true">
            <key>
                <column name="parentId" />
            </key>
            <one-to-many class="kat.beans.Kategorie"/>
        </set>
    </class>
</hibernate-mapping>