package com.HibernateLearn;

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

public class Client {
    public static void main(String[] args) {

        // creating configuration object
        Configuration cfg = new Configuration();
        cfg.configure("hibernate.cfg.xml");// populates the data of the
                                        // configuration file

        // creating seession factory object
        SessionFactory factory = cfg.buildSessionFactory();

        // creating session object
        Session session = factory.openSession();

        // creating transaction object
        Transaction t = session.beginTransaction();

        Employee e1 = new Employee();
        e1.setId(103);
        e1.setF_name("Ahammed");
        e1.setL_name("Naseer");
        //e1.getId();
        //e1.getF_name();
        e1.getL_name();
        session.persist(e1);// persisting the object

        t.commit();// transaction is committed
        session.close();

        System.out.println("successfully saved");

    }
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate
 Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-
configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="show_sql">true</property>
        <!--  <property name="hbm2ddl.auto">create-update</property> -->
        <!--Below are other values for hbm2ddl.auto validate: validate the schema,
        makes no changes to the database. update: update the schema. create: creates
        the schema, destroying previous data. create-drop: drop the schema at the
        end of the session. -->
        <!--property name="hibernate.cache.use_query_cache">true</property -->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/joctopusdb</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">abc123</property>
        <mapping resource="hibernate.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

<?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.HibernateLearn.Employee" table="emp_table">
        <id name="Id">
            <generator class="increment"></generator>
        </id>

        <property name="F_name"></property>
        <property name="L_name"></property>
    </class>
</hibernate-mapping>

Exception in thread "main" org.hibernate.PersistentObjectException: detached entity passed to persist: com.HibernateLearn.Employee

<id name="Id">
    <generator class="increment"></generator>
</id>