Hibernate is generating auto increment alternating id for tables

Question

My environment : Hibernate 5, Java 8, Phpmyadmin in WAMP

Problem: Hibernate creates auto increment id within a table, but the next sequence is given to a different table.

Expected

Table 1        Table 2
1. Hello       1. Foo
2. World       2. Bar 

Instead it is creating

Table 1        Table 2
1. Hello       2. Foo
3. World       4. Bar 

Project Structure

hibernate.cfg.xml

  <?xml version='1.0' encoding='utf-8'?>
  <!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

  <hibernate-configuration>

<session-factory>

    <!-- Database connection settings -->
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="connection.url">jdbc:mysql://localhost:3306/ansarihibernate</property>
    <property name="connection.username">localuser</property>
    <property name="connection.password"></property>

    <!-- JDBC connection pool (use the built-in) -->
    <property name="connection.pool_size">1</property>

    <!-- SQL dialect -->
    <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

    <!-- Disable the second-level cache  -->
    <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

    <!-- Echo all executed SQL to stdout -->
    <property name="show_sql">true</property>

    <property name="hbm2ddl.auto">create</property>   

    <!-- Many To Many classes -->
    <mapping class="org.ansari.hibernate.manytomany.StudentM2M"/>       
    <mapping class="org.ansari.hibernate.manytomany.StudentM2MCertificates"/>

</session-factory>

MainM2M.java

package org.ansari.hibernate.manytomany;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class MainM2M {

public static void main(String[] args) {

    StudentM2MCertificates st1 = new StudentM2MCertificates();
    st1.setM2m_cert_det("Oracle Cert");
    StudentM2MCertificates st2 = new StudentM2MCertificates();
    st2.setM2m_cert_det("Big Data Cert");

    StudentM2M s1 = new StudentM2M();
    s1.setM2m_stu_name("Ansari");
    s1.getSetM2MCert().add(st1);

    StudentM2M s2 = new StudentM2M();
    s2.setM2m_stu_name("Mohammed");
    s2.getSetM2MCert().add(st2);                

    SessionFactory sessFac = new  Configuration().configure().buildSessionFactory();
    Session session = sessFac.openSession();
    session.beginTransaction();

    session.save(s1);
    session.save(s2);

    session.getTransaction().commit();
    session.close();
    sessFac.close();        
}
}

StudentM2M.java

package org.ansari.hibernate.manytomany;

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

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="STUDENTM2M")
public class StudentM2M {

@Id
@GeneratedValue
private int m2m_id;

private String  m2m_stu_name;

@ManyToMany(cascade=CascadeType.ALL)
private Set<StudentM2MCertificates> setM2MCert = new HashSet<StudentM2MCertificates>(0);

public Set<StudentM2MCertificates> getSetM2MCert() {
    return setM2MCert;
}

public void setSetM2MCert(Set<StudentM2MCertificates> setM2MCert) {
    this.setM2MCert = setM2MCert;
}

public int getM2m_id() {
    return m2m_id;
}

public void setM2m_id(int m2m_id) {
    this.m2m_id = m2m_id;
}

public String getM2m_stu_name() {
    return m2m_stu_name;
}

public void setM2m_stu_name(String m2m_stu_name) {
    this.m2m_stu_name = m2m_stu_name;
}
}

StudentM2MCertificates

package org.ansari.hibernate.manytomany;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="CERTM2M")
public class StudentM2MCertificates {

@Id
@GeneratedValue
private int m2m_cert_id;

private String m2m_cert_det;

public int getM2m_cert_id() {
    return m2m_cert_id;
}

public void setM2m_cert_id(int m2m_cert_id) {
    this.m2m_cert_id = m2m_cert_id;
}

public String getM2m_cert_det() {
    return m2m_cert_det;
}

public void setM2m_cert_det(String m2m_cert_det) {
    this.m2m_cert_det = m2m_cert_det;
}
}

Table Structure

certm2m

hibernate_sequence

studentm2m

studentm2m_certm2m

Answer 1

You haven't specified any generator for any of your entities. So Hibernate uses the default one for your database, which consists in using single sequence: hibernate_sequence, to generate the IDs of the entities.

Annotate your Student id field with something like

@SequenceGenerator(name = "studentGenerator", sequenceName = "STUDENT_SEQUENCE", allocationSize = 1)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "studentGenerator")

and it will use a dedicated sequence, STUDENT_SEQUENCE, for the Student entity. Do the same (with a different generator and sequence name) for the other entities.