NonUniqueObjectException

[sura watthana]

Hi

I am new to Hibernate. I am trying to implement a many-to-many association.
I have a simple scenario where I have two entities three tables.
one is Teacher.java with teacher table containg id and name.
the other is Student.java with student table containing id and name as well.

let me show you my java files, my tables and the hibernate mapping files.

package com.oxman.entity;
 
import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;
 

public class Teacher implements Serializable{
private int id;
private String name;
private Set<Student> students;

public Teacher(){

}

public Teacher(String name){
setName(name);
}

public Teacher(String name, Student student){
this(name);
addStudent(student);
}

public Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public void addStudent(Student student) {
    if (students == null) {
    students = new HashSet();
    }
    students.add(student);
}
}

package com.oxman.entity;
 
import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;
 
public class Student implements Serializable{
private int id;
private String name;
private Set<Teacher> teachers;

public Student(){

}

public Student(String name){
setName(name);
}

public Student(String name, Teacher teacher){
this(name);
addTeacher(teacher);
}

public Set<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}

public void addTeacher(Teacher teacher) {
    if (teachers == null) {
    teachers = new HashSet();
    }
    teachers.add(teacher);
}
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="com.oxman.entity.Teacher" table="teachers">
<id name="id" column="uid" type="int" unsaved-value="null">
<generator class="native"/>
</id>
<property name="name" type="string" length="255"/>
<set name="students" table="teachers_students" cascade="all">
<key column="teacher_id"/>
<many-to-many class="com.oxman.entity.Student"/>
</set>
</class>
</hibernate-mapping>


<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="com.oxman.entity.Student" table="students">
<id name="id" column="uid" type="int" unsaved-value="null">
<generator class="native"/>
</id>
<property name="name" type="string" length="255"/>
<set name="teachers" table="teachers_students" cascade="all">
<key column="student_id"/>
<many-to-many class="com.oxman.entity.Teacher"/>
</set>
</class>
</hibernate-mapping>

DROP TABLE IF EXISTS `teachers`;
CREATE TABLE `teachers` (
`uid` int(11) NOT NULL auto_increment,
`name` varchar(255) default NULL,
PRIMARY KEY (`uid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

DROP TABLE IF EXISTS `students`;
CREATE TABLE `students` (
`uid` int(11) NOT NULL auto_increment,
`name` varchar(255) default NULL,
PRIMARY KEY (`uid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

DROP TABLE IF EXISTS `teachers_students`;
CREATE TABLE `teachers_students` (
`teacher_id` int(11) NOT NULL default '0',
`student_id` int(11) NOT NULL default '0',
PRIMARY KEY (`teacher_id`,`student_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

I have tried to run this code

package com.oxman.test;
 
import java.util.HashSet;
 
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.StaleStateException;
import org.hibernate.Transaction;
 
import com.oxman.app.HibernateFactory;
import com.oxman.entity.Student;
import com.oxman.entity.Teacher;
 
public class StudentTeacherTest {
public static void main(String[] args){

SessionFactory factory = HibernateFactory.buildSessionFactory();
Session session = HibernateFactory.openSession();
Transaction tx= session.beginTransaction();

try{    
    
Teacher teacher = new Teacher();
teacher.setName("Tom");
teacher.setStudents(new HashSet<Student>());
teacher.addStudent(new Student("Foo",teacher));        [B]teacher.addStudent(new Student("Bar",teacher));[/B]
 
        tx.commit();
        session.close();
}catch(StaleStateException SSExc){
 SSExc.printStackTrace();
}catch(Exception Exc){
Exc.printStackTrace();
}finally{
    if(session != null){
    try{
    HibernateFactory.close(session);
    HibernateFactory.closeFactory();
    }catch(HibernateException ignore){
    //igonre    
    }
    }
    }
}
}

Here is the Exception stack trace

Hibernate: insert into teachers (name) values (?)
org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [com.oxman.entity.Student#0]
at org.hibernate.engine.StatefulPersistenceContext.checkUniqueness(StatefulPersistenceContext.java:590)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performUpdate(DefaultSaveOrUpdateEventListener.java:284)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsDetached(DefaultSaveOrUpdateEventListener.java:223)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:89)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:507)
at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:499)
at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:218)
at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:268)
at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:216)
at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:296)
at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:242)
at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:219)
at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
at org.hibernate.engine.Cascade.cascade(Cascade.java:130)
at org.hibernate.event.def.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:456)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:334)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:181)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:107)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:187)
at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:172)
at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:535)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:523)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:519)
at com.oxman.test.StudentTeacherTest.main(StudentTeacherTest.java:36)



Does anybody have any idea what went wrong?

thank you

Sura

[Bear Bibeault]

Please do not post questions pertaining to this same code more than once.