Hibernate 5: org.hibernate.MappingException: Unknown entity

Question

The following code works just fine with Hibernate 4.3, but when I try the same code using Hibernate 5.0 it results in the following error:

Exception in thread "main" org.hibernate.MappingException: Unknown entity: entity.Message
    at org.hibernate.internal.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:776)
    at org.hibernate.internal.SessionImpl.getEntityPersister(SessionImpl.java:1451)
    at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:100)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:192)
    at org.hibernate.event.internal.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:38)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:177)
    at org.hibernate.event.internal.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:32)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
    at org.hibernate.internal.SessionImpl.fireSave(SessionImpl.java:678)
    at org.hibernate.internal.SessionImpl.save(SessionImpl.java:670)
    at org.hibernate.internal.SessionImpl.save(SessionImpl.java:665)
    at client.Main.main(Main.java:14)

Could somebody help me understand why is it happening?

---

Following is the code being used to test the example:

Main.java

package client;
import org.hibernate.Session;
import util.HibernateUtil;
import entity.Message;

public class Main {
    public static void main(String[] args) {        
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();

        Message message = new Message( "Hello Hibernate 5" );

        session.save(message);    

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

HibernateUtil.java

package util;
import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {    
    private static final SessionFactory sessionFactory = buildSessionFactory();    
    private static SessionFactory buildSessionFactory() {
        try {           
            Configuration configuration = new Configuration().configure("hibernate.cfg.xml");     
            return configuration.buildSessionFactory( new StandardServiceRegistryBuilder().applySettings( configuration.getProperties() ).build() );
        }
        catch (Throwable ex) {                
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }   
}

Message.java

package entity;

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

@Entity
@Table(name="message")
public class Message {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="ID")  
    private Long id;

    @Column(name="TEXT")    
    private String text;

    public Message() {}
    public Message(String text) {
        this.text = text;
    }   
}

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>

        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/hello</property>
        <property name="connection.username">root</property>
        <property name="connection.password">pass</property>

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

        <mapping class="entity.Message"/>

    </session-factory>
</hibernate-configuration>

SQL Database Schema (RDBMS: MYSQL)

CREATE DATABASE `hello`;
USE `hello`;
CREATE TABLE `message` (
`ID` BIGINT(20) NOT NULL AUTO_INCREMENT,
`TEXT` VARCHAR(255) NULL DEFAULT NULL,
PRIMARY KEY (`ID`)
);

Answer 1

UPDATED: In Hibernate 5.0.x, configuration with standard service registry is deprecated

Instead you should bootstrap it with Metadata:

In your HibernateUtil class, you should add

   private static SessionFactory buildSessionFactory() {
    try {           
        StandardServiceRegistry standardRegistry = new StandardServiceRegistryBuilder()
        .configure( "hibernate.cfg.xml" )
        .build();

        Metadata metadata = new MetadataSources( standardRegistry )
        .getMetadataBuilder()
        .build();

        return metadata.getSessionFactoryBuilder().build();

    }

And as a side note, change the ID declaration of your Message Class to

@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name="ID")  
private Long id;

So hibernate can properly use mySql's auto increment, otherwise it will throw an exception.

Answer 2

Alternatively, you could do the following in Hibernate 5+ and have your SessionFactory/Session creating class extend the HibernateHelper class below:

package com.company.hibernatehelper;

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

public class HibernateHelper {
    private static SessionFactory sessionFactory = null;

    public static SessionFactory getSessionFactory() {
        if(sessionFactory == null) {
        sessionFactory = new  Configuration().configure().buildSessionFactory();
        }
        return sessionFactory;
    }
}