for example, the user object with two attribute id, name. when we want to save this object to database, we simply create a session and call .save
(object_name), but if the object obtain a reference to other object, for example, id, name, address_ref(this is reference to Address object with class type Address), how to persist this object to database !!??

Cascading is off by default. If you turn on an appropriate cascade for that relation, hibernate will save the associated object automatically. If not, you must explicity perform a save on both objects.

You can find information on cascade in the hibernate doc Chp. 5.

the object i want to save refer to other object that has been persisted, i want to persist the current object into the database. so, i can't save the both objects.

i am trying to insert the value in databases but it is fired by hibernate...

Hibernate: select unwanted0_.ID as ID0_0_, unwanted0_.name as name0_0_ from UNWANTED unwanted0_ where unwanted0_.ID=?
org.hibernate.MappingException: Unknown entity: java.lang.Long
at org.hibernate.impl.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:548)
at org.hibernate.impl.SessionImpl.getEntityPersister(SessionImpl.java:1338)
at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:180)
at org.hibernate.event.def.AbstractSaveEventListener.getEntityState(AbstractSaveEventListener.java:487)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:70)
at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:38)
at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:618)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:592)
at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:596)
at org.apache.jsp.unwanted_jsp._jspService(unwanted_jsp.java:131)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:874)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Unknown Source)

org.apache.jasper.JasperException: Exception in JSP: /unwanted.jsp:4

1: <%@ include file="protected.jsp" %>
2: <%@ include file="header.jsp" %>
3: <%
4: Long id=Long.valueOf(request.getParameter("Id"));
5: Session sess = HibernateUtil.currentSession();
6: Unwanted u= new Unwanted();
7:


Stacktrace:
my mapping looks like it..
<?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.ats.custFlight.pojo.Unwanted" table="unwanted" lazy="false">
<id column="ID" name="id" type="java.lang.Long"/>
<property column="name" name="name" type="java.lang.String"/>
</class>
</hibernate-mapping>



my code to upload data is--

Long id=Long.valueOf(request.getParameter("Id"));
Session sess = HibernateUtil.currentSession();
Unwanted u= new Unwanted();

Unwanted u2 = (Unwanted) sess.get (Unwanted.class,(long)77);
u.setId(id);
u.setName("RAHUL");
try{
sess.save(u);
}catch( Exception e )
{
e.printStackTrace();
}
System.out.println(" size is " + u.getId());

my hibernate.util code is--
package com.ats.custFlight.util;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.hibernate.*;
import org.hibernate.cfg.*;

public class HibernateUtil {

private static Log log = LogFactory.getLog(HibernateUtil.class);

private static final SessionFactory sessionFactory;

static {
try {
// Create the SessionFactory
sessionFactory = new Configuration().configure().buildSessionFactory();

} catch (Throwable ex) {
// Make sure you log the exception, as it might be swallowed
log.error("Initial SessionFactory creation failed.", ex);
ex.printStackTrace();
throw new ExceptionInInitializerError(ex);
}
}

public static final ThreadLocal < Session > session = new ThreadLocal < Session > ();

public static Session currentSession() throws HibernateException {
Session s = (Session) session.get();
// Open a new Session, if this Thread has none yet
if (s == null) {
s = sessionFactory.openSession();
session.set(s);
}
return s;
}

public static void closeSession() throws HibernateException {
Session s = (Session) session.get();
session.set(null);
if (s != null)
s.close();
}
}
and JAVA CLASS IS..

package com.ats.custFlight.pojo;

public class Unwanted {

Long id;
String name;

public String getName() {
return name;
}

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

public Long getId() {
return id;
}

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