org.hibernate.HibernateException: Javassist Enhancement failed

[main] INFO org.hibernate.event.def.DefaultLoadEventListener - Error performing load command
org.hibernate.HibernateException: Javassist Enhancement failed: entities.Employee
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:143)
    at org.hibernate.proxy.pojo.javassist.JavassistProxyFactory.getProxy(JavassistProxyFactory.java:72)
    at org.hibernate.tuple.entity.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:634)
    at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:3713)
    at org.hibernate.event.def.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:360)
    at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:281)
    at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152)
    at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1080)
    at org.hibernate.impl.SessionImpl.load(SessionImpl.java:977)
    at org.hibernate.impl.SessionImpl.load(SessionImpl.java:970)
    at Operations.read(Operations.java:23)
    at Operations.main(Operations.java:50)
Caused by: java.lang.InstantiationException: entities.Employee_$$_javassist_0
    at java.lang.Class.newInstance0(Unknown Source)
    at java.lang.Class.newInstance(Unknown Source)
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:140)
    ... 11 more
Exception in thread "main" org.hibernate.HibernateException: Javassist Enhancement failed: entities.Employee
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:143)
    at org.hibernate.proxy.pojo.javassist.JavassistProxyFactory.getProxy(JavassistProxyFactory.java:72)
    at org.hibernate.tuple.entity.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:634)
    at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:3713)
    at org.hibernate.event.def.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:360)
    at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:281)
    at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152)
    at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1080)
    at org.hibernate.impl.SessionImpl.load(SessionImpl.java:977)
    at org.hibernate.impl.SessionImpl.load(SessionImpl.java:970)
    at Operations.read(Operations.java:23)
    at Operations.main(Operations.java:50)
Caused by: java.lang.InstantiationException: entities.Employee_$$_javassist_0
    at java.lang.Class.newInstance0(Unknown Source)
    at java.lang.Class.newInstance(Unknown Source)
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:140)
    ... 11 more

This Exception is generated by Hibernate if no default constructor is avaible in entity class. here in entities.Employee class, the constructor Employee() must avaible in class.

Using HQL in Hibernate

HQL stands for Hibernate Query Language that works on persistent objects and their properties. It is similar to SQL but we use the class and Properties in syntax of query instead of table and columns names. It is case sensitive while we write the name of java classes and properties. You have to user Class name to be mapped to table instead of database table name.
SQL Table

package domain;
import javax.persistence.*;
@Entity
public class Employee{
    private int empId;
    private String name;
    private String email;
    private String designation;
    private long salary;
    private String department;
    @Id @GeneratedValue
    public int getEmpId() {
        return empId;
    }
    public void setEmpId(int empId) {
        this.empId = empId;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public String getDesignation() {
        return designation;
    }
    public void setDesignation(String designation) {
        this.designation = designation;
    }
    public long getSalary() {
        return salary;
    }
    public void setSalary(long salary) {
        this.salary = salary;
    }
    public String getDepartment() {
        return department;
    }
    public void setDepartment(String department) {
        this.department = department;
    }
    @Override
    public String toString() {
        return "Employee [empId=" + empId + ", name=" + name + ", email="
                + email + ", designation=" + designation + ", salary=" + salary
                + ", department=" + department + "]";
    }
    
}


Simple select Query
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.cfg.AnnotationConfiguration;
import domain.*;
public class SelectHQL {
    public static void main(String[] args) {
        System.out.println("############## Read ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        Query query = session.createQuery("from Employee where department = 'SEO'");
        List list = query.list();
        for(Object obj:list){
            System.out.println((Employee)obj);
        }
        session.getTransaction().commit();
    }
}


Retrieving Specific Column(s)
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.cfg.AnnotationConfiguration;
import domain.*;
public class SelectHQL2 {
    public static void main(String[] args) {
        System.out.println("############## Read ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        Query query = session.createQuery("select e.name from Employee e where department = 'SEO'");
        //The result set for this query will contain a List of Java String objects
        List list = query.list();
        for(Object obj:list){
            System.out.println((String)obj);
        }
        session.getTransaction().commit();
    }
}


Parametrized HQL
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.cfg.AnnotationConfiguration;
import domain.*;
public class SelectHQLParams {
    public static void main(String[] args) {
        System.out.println("############## Read ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        Query query = session.createQuery("from Employee where department = :x");
        query.setString("x""SEO");
        List list = query.list();
        for(Object obj:list){
            System.out.println((Employeeobj);
        }
        session.getTransaction().commit();
    }
}

Getting data from selected columns using HQL
 Sometimes it is required that you need to fetch the data from the table using Hobernate from the selected columns you use select query by specifying the properties to which data need to be fetch. If you specify the name of field instead of Java class, hibernate returns the single valued list * if you specify more then one fields in select query , hibernate returns List of array of object to be retrieve using Iterator.

import java.util.Iterator;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.cfg.AnnotationConfiguration;
public class SelectHQL3 {
    public static void main(String[] args) {
        System.out.println("############## Read ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        Query query = session.createQuery("select e.name,e.email,e.department from Employee e where department = 'SEO'");
        //This result set contains a List of Object arrays—each array represents one set of properties
        Iterator it=query.iterate();
        while (it.hasNext()) {
            Object[] val = (Object[]) it.next();
            System.out.println(val[0]+"\t"+val[1]);
        }
        session.getTransaction().commit();
    }
}


Inheritance mapping in hibernate :Table per class

This mapping is not known as good mapping for projects. In this approach all of the fields of each type in the inheritance hierarchy are stored in distinct tables.
It has many drawbacks while dealing with polymorphic and association. This strategy supports bidirectional one-to-many association.

package domain;
import javax.persistence.*;
@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Employee{
    private int empId;
    private String name;
    private String email;
    @Id 
    public int getEmpId() {
        return empId;
    }
    public void setEmpId(int empId) {
        this.empId = empId;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    
}

package domain;
import javax.persistence.*;
@Entity
public class FullTimeEmployee extends Employee{
    private String designation;
    private String empCode;
    public String getDesignation() {
        return designation;
    }
    public void setDesignation(String designation) {
        this.designation = designation;
    }
    public String getEmpCode() {
        return empCode;
    }
    public void setEmpCode(String empCode) {
        this.empCode = empCode;
    }
    @Override
    public String toString() {
        return "FullTimeEmployee [designation=" + designation + ", empCode="
                + empCode + ", getEmpId()=" + getEmpId() + ", getName()="
                + getName() + ", getEmail()=" + getEmail() + "]";
    }
    
}

package domain;
import javax.persistence.*;
@Entity
public class PartTimeEmployee extends Employee{
    private int duration;
    private String reportTo;
    public int getDuration() {
        return duration;
    }
    public void setDuration(int duration) {
        this.duration = duration;
    }
    public String getReportTo() {
        return reportTo;
    }
    public void setReportTo(String reportTo) {
        this.reportTo = reportTo;
    }
    @Override
    public String toString() {
        return "PartTimeEmployee [duration=" + duration + ", reportTo="
                + reportTo + ", getEmpId()=" + getEmpId() + ", getName()="
                + getName() + ", getEmail()=" + getEmail() + "]";
    }
     
}

import org.hibernate.Session;
import org.hibernate.cfg.AnnotationConfiguration;

import domain.FullTimeEmployee;
import domain.PartTimeEmployee;

public class Operations {
    public static void create() {
        System.out.println("############## Create ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        PartTimeEmployee pe = new PartTimeEmployee();
        pe.setEmpId(1);
        pe.setName("abc");
        pe.setEmail("abc@rmail.com");
        pe.setDuration(4);
        pe.setReportTo("PM");
        session.save(pe);

        FullTimeEmployee fe = new FullTimeEmployee();
        fe.setEmpId(2);
        fe.setName("ssd");
        fe.setDesignation("manager");
        fe.setEmail("mail@email.com");
        fe.setEmpCode("86789");
        session.save(fe);

        session.getTransaction().commit();
    }

    public static void read() {
        System.out.println("############## Read ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        System.out.println("########### load contact and person");
        PartTimeEmployee pe = (PartTimeEmployee) session.load(
                PartTimeEmployee.class1);
        FullTimeEmployee fe = (FullTimeEmployee) session.load(
                FullTimeEmployee.class2);
        System.out.println(pe);
        System.out.println(fe);
        session.getTransaction().commit();
    }

    public static void update() {
        System.out.println("############## Update ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        // TODO
        session.getTransaction().commit();
    }

    public static void delete() {
        System.out.println("############## Delete ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        // TODO
        session.getTransaction().commit();
    }

    public static void main(String[] args) {
        create();
        read();
        // update();
        // delete();
    }
}


<?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>
        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/test</property>
        <property name="connection.username">root</property>
        <property name="connection.password" />
        <property name="connection.pool_size">1</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="current_session_context_class">thread</property>
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto" >update</property>
        <mapping class="domain.Employee"/>
        <mapping class="domain.PartTimeEmployee"/>
        <mapping class="domain.FullTimeEmployee"/>
    </session-factory>
</hibernate-configuration>


TABLES  AFTER EXECUTION




Inheritance mapping in hibernate :JOINED strategy

We can map each class in individual table in database using JOINED strategy.
using @Inheritance(strategy=InheritanceType.JOINED) annotation, we can define this strategy. In this approach, primary key of the super class works as foreign key in sub class.So at the time of dealing with data, Hibernate Joins the tables for getting consolidated values for Subclass Object.
you can use @PrimaryKeyJoinColumn and @PrimaryKeyJoinColumns annotations to define the primary key(s) of the joined subclass table.

This approach is better than single table strategy.


 package domain;

import javax.persistence.*;
@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Employee{
    private int empId;
    private String name;
    private String email;
    @Id @GeneratedValue
    public int getEmpId() {
        return empId;
    }
    public void setEmpId(int empId) {
        this.empId = empId;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    
}


package domain;
import javax.persistence.*;
@Entity
public class FullTimeEmployee extends Employee{
    private String designation;
    private String empCode;
    public String getDesignation() {
        return designation;
    }
    public void setDesignation(String designation) {
        this.designation = designation;
    }
    public String getEmpCode() {
        return empCode;
    }
    public void setEmpCode(String empCode) {
        this.empCode = empCode;
    }
    @Override
    public String toString() {
        return "FullTimeEmployee [designation=" + designation + ", empCode="
                + empCode + ", getEmpId()=" + getEmpId() + ", getName()="
                + getName() + ", getEmail()=" + getEmail() + "]";
    }
    
}


package domain;
import javax.persistence.*;
@Entity
public class PartTimeEmployee extends Employee{
    private int duration;
    private String reportTo;
    public int getDuration() {
        return duration;
    }
    public void setDuration(int duration) {
        this.duration = duration;
    }
    public String getReportTo() {
        return reportTo;
    }
    public void setReportTo(String reportTo) {
        this.reportTo = reportTo;
    }
    @Override
    public String toString() {
        return "PartTimeEmployee [duration=" + duration + ", reportTo="
                + reportTo + ", getEmpId()=" + getEmpId() + ", getName()="
                + getName() + ", getEmail()=" + getEmail() + "]";
    }
     
}



Tables : 




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