<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="be.eschool.model.domain">
  <class name="Klas" table="klassen">
        <id name="id" column="id">
         <generator class="native"/>
      </id>
      <property name="naam"/>
      <set name="leerlingen" table="leerling_klassen" cascade="all" lazy="true">
         <key column="klas_id"/>
         <many-to-many column="lln_id" class="Leerling"/>
      </set>
  </class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="be.eschool.model.domain">
  <class name="Leerling" table="leerlingen">
        <id name="id" column="id">
         <generator class="native"/>
      </id>
      <property name="naam"/>
      <property name="voornaam"/>
      <property name="adres"/>
      <property name="gemeente"/>
      <property name="pcode"/>
      
      <set name="klassen" table="leerling_klassen" cascade="all" lazy="true">
         <key column="lln_id"/>
         <many-to-many column="klas_id" class="Klas" order-by="naam"/>
      </set>
  </class>
</hibernate-mapping>

package be.eschool.model.domain;


import java.util.Set;

public class Klas {
   
   private int id;
   private String naam;
   private boolean selected;
   
   private Set<Leerling> leerlingen;
   
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
   public String getNaam() {
      return naam;
   }
   public void setNaam(String naam) {
      this.naam = naam;
   }
   public boolean isSelected() {
      return selected;
   }
   public void setSelected(boolean selected) {
      this.selected = selected;
   }
   
   public Set<Leerling> getLeerlingen() {
      leerlingen.size();
      return leerlingen;
   }
   public void setLeerlingen(Set<Leerling> leerlingen) {
      this.leerlingen = leerlingen;
   }

   public String toString()
   {
      return this.getNaam();
   }
}

package be.eschool.model.domain;

import java.util.HashSet;
import java.util.Set;

public class Leerling
{
   private int id;
   private String voornaam,naam,adres,gemeente,pcode;
   private boolean selected = false;
   
   private Set<Klas> klassen = new HashSet<Klas>();
   
   public String getNaam() {
      return naam;
   }

   public void setNaam(String naam) {
      this.naam = naam;
   }

   public int getId() {
      return id;
   }

   public void setId(int id) {
      this.id = id;
   }
   
   public Leerling(){}

   public String getVoornaam() {
      return voornaam;
   }

   public void setVoornaam(String voornaam) {
      this.voornaam = voornaam;
   }
   
   public boolean isSelected()
   {
      return this.selected;
   }
   
   public void setSelected(boolean sel)
   {
      this.selected = sel;
   }

   public String getAdres() {
      return adres;
   }

   public void setAdres(String adres) {
      this.adres = adres;
   }

   public String getGemeente() {
      return gemeente;
   }

   public void setGemeente(String gemeente) {
      this.gemeente = gemeente;
   }

   public String getPcode() {
      return pcode;
   }

   public void setPcode(String pcode) {
      this.pcode = pcode;
   }

   public Set<Klas> getKlassen() {
      klassen.size();
      return klassen;
   }

   public void setKlassen(Set<Klas> klassen) {
      this.klassen = klassen;
   }
}

16:26:22,492 ERROR [LazyInitializationException:42] failed to lazily initialize a collection of role: be.eschool.model.domai
.leerlingen, no session or session was closed
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: be.eschool.model.domain.Klas.le
en, no session or session was closed
        at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollecti
a:380)
        at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPers
Collection.java:372)
        at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:119)
        at org.hibernate.collection.PersistentSet.size(PersistentSet.java:162)
        at be.eschool.model.domain.Klas.getLeerlingen(Klas.java:34)
        at sun.reflect.GeneratedMethodAccessor135.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:169)
        at org.hibernate.tuple.entity.AbstractEntityTuplizer.getPropertyValue(AbstractEntityTuplizer.java:293)
        at org.hibernate.persister.entity.AbstractEntityPersister.getPropertyValue(AbstractEntityPersister.java:3595)
        at org.hibernate.engine.Cascade.cascade(Cascade.java:154)
        at org.hibernate.event.def.DefaultMergeEventListener.cascadeOnMerge(DefaultMergeEventListener.java:563)
        at org.hibernate.event.def.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:423)
        at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:234)
        at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:715)
        at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:697)
        at org.hibernate.engine.CascadingAction$6.cascade(CascadingAction.java:268)
        at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:292)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:240)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:193)
        at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:320)
        at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:266)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:243)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:193)
        at org.hibernate.engine.Cascade.cascade(Cascade.java:154)
        at org.hibernate.event.def.DefaultMergeEventListener.cascadeOnMerge(DefaultMergeEventListener.java:563)
        at org.hibernate.event.def.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:423)
        at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:234)
        at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:715)
        at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:697)
        at org.hibernate.engine.CascadingAction$6.cascade(CascadingAction.java:268)
        at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:292)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:240)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:193)
        at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:320)
        at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:266)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:243)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:193)
        at org.hibernate.engine.Cascade.cascade(Cascade.java:154)
        at org.hibernate.event.def.DefaultMergeEventListener.cascadeOnMerge(DefaultMergeEventListener.java:563)
        at org.hibernate.event.def.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:423)
        at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:234)
        at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:715)
        at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:697)
        at org.hibernate.engine.CascadingAction$6.cascade(CascadingAction.java:268)
        at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:292)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:240)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:193)
        at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:320)
        at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:266)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:243)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:193)
        at org.hibernate.engine.Cascade.cascade(Cascade.java:154)
        at org.hibernate.event.def.DefaultMergeEventListener.cascadeOnMerge(DefaultMergeEventListener.java:563)
        at org.hibernate.event.def.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:423)
        at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:234)
        at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:84)
        at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:705)
        at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:689)
        at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:693)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at org.hibernate.context.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.jav

        at $Proxy223.merge(Unknown Source)
        at be.eschool.model.controller.LeerlingController.saveLeerling(LeerlingController.java:26)
        at be.eschool.beans.LeerlingEdit.submitListener(LeerlingEdit.java:257)
        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:585)
        at org.apache.el.parser.AstValue.invoke(AstValue.java:172)
        at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276)
        at com.sun.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:68)
        at javax.faces.event.MethodExpressionActionListener.processAction(MethodExpressionActionListener.java:88)
        at javax.faces.event.ActionEvent.processListener(ActionEvent.java:77)
        at javax.faces.component.UIComponentBase.broadcast(UIComponentBase.java:743)
        at javax.faces.component.UICommand.broadcast(UICommand.java:368)
        at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:447)
        at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:752)
        at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:97)
        at com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:251)
        at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:117)
        at com.icesoft.faces.webapp.http.core.JsfLifecycleExecutor.apply(JsfLifecycleExecutor.java:17)
        at com.icesoft.faces.webapp.http.core.ReceiveSendUpdates.renderCycle(ReceiveSendUpdates.java:54)
        at com.icesoft.faces.webapp.http.core.ReceiveSendUpdates.service(ReceiveSendUpdates.java:42)
        at com.icesoft.faces.webapp.http.core.ViewBoundServer.service(ViewBoundServer.java:65)
        at com.icesoft.faces.webapp.http.core.RequestVerifier.service(RequestVerifier.java:44)
        at com.icesoft.faces.webapp.http.common.standard.PathDispatcherServer$Matcher.serviceOnMatch(PathDispatcherServer.ja

        at com.icesoft.faces.webapp.http.common.standard.PathDispatcherServer.service(PathDispatcherServer.java:19)
        at com.icesoft.faces.webapp.http.servlet.ThreadBlockingAdaptingServlet.service(ThreadBlockingAdaptingServlet.java:19
        at com.icesoft.faces.webapp.http.servlet.EnvironmentAdaptingServlet.service(EnvironmentAdaptingServlet.java:63)
        at com.icesoft.faces.webapp.http.servlet.MainSessionBoundServlet.service(MainSessionBoundServlet.java:139)
        at com.icesoft.faces.webapp.http.servlet.SessionDispatcher.service(SessionDispatcher.java:53)
        at com.icesoft.faces.webapp.http.servlet.PathDispatcher$Matcher.serviceOnMatch(PathDispatcher.java:52)
        at com.icesoft.faces.webapp.http.servlet.PathDispatcher.service(PathDispatcher.java:29)
        at com.icesoft.faces.webapp.http.servlet.MainServlet.service(MainServlet.java:82)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        at com.icesoft.faces.webapp.xmlhttp.BlockingServlet.service(BlockingServlet.java:46)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:433)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
        at java.lang.Thread.run(Thread.java:595)
16:55:46,432 ERROR [SessionDispatcher:154] java.lang.IllegalStateException: getAttributeNames: Session already invalidated

<?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="hibernate.current_session_context_class">org.hibernate.context.ThreadLocalSessionContext</property>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.password"></property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost/eschool</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>

       
        <mapping resource="be/eschool/model/domain/Leerling.hbm.xml" />
        <mapping resource="be/eschool/model/domain/Klas.hbm.xml" />
        <mapping resource="be/eschool/model/domain/Gemeente.hbm.xml" />
    </session-factory>
</hibernate-configuration>

class HibernateSessionRequestFilter implements Filter
    {
        private SessionFactory sessionFactory;

        public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,
            ServletException
        {

            try
            {
                sessionFactory.getCurrentSession().beginTransaction();
                chain.doFilter(request, response);
                sessionFactory.getCurrentSession().getTransaction().commit();
            }
            catch (Throwable ex)
            {
                // Rollback only
                ex.printStackTrace();
                if (sessionFactory.getCurrentSession().getTransaction().isActive())
                    sessionFactory.getCurrentSession().getTransaction().rollback();
                // Let others handle it... maybe another interceptor for exceptions?
                throw new ServletException(ex);
            }
        }

        public void init(FilterConfig filterConfig) throws ServletException
        {
            sessionFactory = HibernateUtil.getSessionFactory();
        }

        public void destroy()
        {
        }
    }

<filter>
        <filter-name>HibernateFilter</filter-name>
        <filter-class>abc.xyz.HibernateSessionRequestFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>HibernateFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

Set classes = student.getKlassen();
classes.add(myKlas);
session.beginTransaction();
session.save(student);