Exception in thread "main" org.hibernate.MappingException: Unknown entity: $Proxy3
   at org.hibernate.impl.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:610)
   at org.hibernate.impl.SessionImpl.getEntityPersister(SessionImpl.java:1246)
   at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:180)
   at org.hibernate.event.def.AbstractSaveEventListener.getEntityState(AbstractSaveEventListener.java:459)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:84)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
   at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:501)
   at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:495)
   at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:491)
   at db.StudiengangMapper.update(StudiengangMapper.java:200)
   at server.LehrveranstaltungVerwaltungImpl.update(LehrveranstaltungVerwaltungImpl.java:262)
   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 sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:305)
   at sun.rmi.transport.Transport$1.run(Transport.java:159)
   at java.security.AccessController.doPrivileged(Native Method)
   at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
   at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
   at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
   at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
   at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
   at java.lang.Thread.run(Thread.java:619)
   at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(StreamRemoteCall.java:255)
   at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:233)
   at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:142)
   at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:178)
   at java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:132)
   at $Proxy0.update(Unknown Source)
   at clients.HELPClientStudiengang.main(HELPClientStudiengang.java:130)

   public Studiengang update(Studiengang a) throws HibernateException, RemoteException {
      
      Session session = HibernateUtil.getSession();
      Transaction tx = session.beginTransaction();
      session.update(a);
      tx.commit();
      session.close();
      session = null;   
      
      return a;
   }

<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>

<class
   name="data.StudiengangImpl"
   table="studiengang">
   
        <id name="ID" column="studiengang_id">

            <generator class="native"/>

        </id>
       
        <property name="name" column="name"></property>
      
      
</class>

</hibernate-mapping>

package data;

import java.rmi.RemoteException;

public class StudiengangImpl extends java.rmi.server.UnicastRemoteObject implements Studiengang {

   /**
    * ID
    */
   
   private int ID = 0;
   
   /**
    * Name
    */
   
   private String name = " ";

   
   public StudiengangImpl() throws RemoteException {
      super();
   }
   
   public int getID() throws RemoteException {
      return this.ID;
   }

   public void setID(int ID) throws RemoteException {
      this.ID = ID;
   }

   public String getName() throws RemoteException {
      return this.name;
   }

   public void setName(String name) throws RemoteException {
      this.name = name;
   }

}