CREATE TABLE `conferenceinfo` (
  `conferenceId` bigint(20) NOT NULL auto_increment,

  。。。。
  `version` int(11) NOT NULL default '0',
  PRIMARY KEY  (`conferenceId`),
  UNIQUE KEY `conferenceId` (`conferenceId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `agenda` (
  `agendaid` bigint(20) NOT NULL auto_increment,
  `conferenceId` bigint(20) NOT NULL,

  。。。.

  `version` int(11) NOT NULL default '0',
  PRIMARY KEY  (`agendaid`),
  UNIQUE KEY `Agendaid` (`agendaid`),
  KEY `conferenceId` (`conferenceId`),
  CONSTRAINT `agenda_ibfk_1` FOREIGN KEY (`conferenceId`) REFERENCES `conferenceinfo` (`conferenceId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

<hibernate-mapping>
    <class name="com.avonaco.pojos.Conferenceinfo"
       table="conferenceinfo"
       catalog="cnc_conference"
       dynamic-update="true"
       dynamic-insert="true" >
        <id name="conferenceId" type="java.lang.Long">
            <column name="conferenceId" />
            <generator class="native" />
        </id>
        <version name="version" column="version" />

。。。.....

        <set name="agendas" inverse="true" cascade="all">
            <key>
                <column name="conferenceId" not-null="true" />
            </key>
            <one-to-many class="com.avonaco.pojos.Agenda" />
        </set>

    </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="com.avonaco.pojos.Agenda"
       table="agenda"
       catalog="cnc_conference"
       dynamic-update="false"
       dynamic-insert="false" >
        <id name="agendaid" type="java.lang.Long">
            <column name="agendaid" />
            <generator class="native" />
        </id>
        <version name="version" column="version" />

        <many-to-one name="conferenceinfo" class="com.avonaco.pojos.Conferenceinfo" fetch="select"
        cascade="none" outer-join="auto" update="true" insert="true" access="property">
            <column name="conferenceId" not-null="true" />
        </many-to-one>
       
    。。。。

    </class>
</hibernate-mapping>

Conferenceinfo con = new Conferenceinfo();
//test data
con.setTimeZone("1");
     。。。
Agenda ag1 = new Agenda(con,new Date(),"lqk1",new Date(),"lqk1",new Date());
Agenda ag2 = new Agenda(con,new Date(),"lqk2",new Date(),"lqk2",new Date());
Set<Agenda> ags = new HashSet<Agenda>();
ags.add(ag1);
ags.add(ag2);
con.getAgendas().add(ags);
      
this.conferenceinfoDAO.save(con);

Couldn't get property agendaid from bean [com.avonaco.pojos.Agenda]. Nested exception is java.lang.IllegalArgumentException: object is not an instance of declaring class
                at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke

// add the set of agendas as an Agenda
con.getAgendas().add(ags); // NOT WHAT HIB EXPECTS

// replace the set with this one
con.setAgendas(ags);

// add these new ones to the existing ones
con.getAgendas().addAll(ags);