报错：org.hibernate.AssertionFailure: null id in com.tt.hibernate.entities.News entry (don't flush the Session after an exception occurs)

在使用hibernate创建数据库的表格时，出现了如下报错：

十二月 28, 2016 10:17:02 上午 org.hibernate.tool.hbm2ddl.SchemaExport perform
ERROR: HHH000389: Unsuccessful: create table NEWS (ID integer not null auto_increment, TITLE varchar(255), AUTHOR varchar(255), DATE datetime, DESC varchar(255), CONTENT varchar(255), IMAGE longblob, primary key (ID)) ENGINE=InnoDB
十二月 28, 2016 10:17:02 上午 org.hibernate.tool.hbm2ddl.SchemaExport perform
ERROR: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DESC varchar(255),
CONTENT varchar(255),
IMAGE longblob,
' at line 6
十二月 28, 2016 10:17:02 上午 org.hibernate.tool.hbm2ddl.SchemaExport execute
INFO: HHH000230: Schema export complete
Hibernate: 
create table NEWS (
ID integer not null auto_increment,
TITLE varchar(255),
AUTHOR varchar(255),
DATE datetime,
DESC varchar(255),
CONTENT varchar(255),
IMAGE longblob,
primary key (ID)
) ENGINE=InnoDB
Hibernate: 
insert 
into
NEWS
(TITLE, AUTHOR, DATE, DESC, CONTENT, IMAGE) 
values
(?, ?, ?, ?, ?, ?)
十二月 28, 2016 10:17:03 上午 org.hibernate.engine.jdbc.spi.SqlExceptionHelper logExceptions
WARN: SQL Error: 1064, SQLState: 42000
十二月 28, 2016 10:17:03 上午 org.hibernate.engine.jdbc.spi.SqlExceptionHelper logExceptions
ERROR: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DESC, CONTENT, IMAGE) values ('CC', 'cc', '2016-12-28 10:17:02.785', 'DESC', 'CO' at line 1
destroy...
十二月 28, 2016 10:17:03 上午 org.hibernate.AssertionFailure <init>
ERROR: HHH000099: an assertion failure occured (this may indicate a bug in Hibernate, but is more likely due to unsafe use of the session): org.hibernate.AssertionFailure: null id in com.tt.hibernate.entities.News entry (don't flush the Session after an exception occurs)

各部分代码如下：

HibernateTest.java：

 package com.tt.hibernate.entities;

 import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.sql.Blob;
 import java.sql.Connection;
 import java.sql.SQLException;
 import java.util.Date;

 import org.hibernate.Hibernate;
 import org.hibernate.Session;
 import org.hibernate.SessionFactory;
 import org.hibernate.Transaction;
 import org.hibernate.cfg.Configuration;
 import org.hibernate.jdbc.Work;
 import org.hibernate.service.ServiceRegistry;
 import org.hibernate.service.ServiceRegistryBuilder;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;

 public class HibernateTest {

     private SessionFactory sessionFactory;
     private Session session;
     private Transaction transaction;

     @Before
     public void init(){
         System.out.println("init...");

         Configuration configuration = new Configuration().configure();
         ServiceRegistry serviceRegistry =
                 new ServiceRegistryBuilder().applySettings(configuration.getProperties())
                                             .buildServiceRegistry();
         sessionFactory = configuration.buildSessionFactory(serviceRegistry);

         session = sessionFactory.openSession();

         transaction = session.beginTransaction();
     }

     @After
     public void destroy() {
         System.out.println("destroy...");

         transaction.commit();
         session.close();
         sessionFactory.close();
     }

     @Test
     public void testBlob() throws IOException, SQLException{
         News news = new News();

         news.setAuthor("cc");
         news.setTitle("CC");
         news.setDesc("DESC");
         news.setContent("CONTENT");
         news.setDate(new Date());

         InputStream stream = new FileInputStream("SHQ.jpg");
         Blob image = Hibernate.getLobCreator(session)
                               .createBlob(stream, stream.available());

         news.setImage(image);

         session.save(news);
     }

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>

         <!-- Hibernate连接数据库的基本信息 -->
         <property name="conncection.username">root</property>
         <property name="connection.password">1234</property>
         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
         <property name="connection.url">jdbc:mysql://localhost:3306/Hibernate</property>

         <!-- Hibernate的基本配置 -->

         <!-- Hibernate使用的数据库方言 -->
         <property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>

         <!-- 运行时是否打印SQL -->
         <property name="show_sql">true</property>

         <!-- 运行时是否格式化SQL -->
         <property name="format_sql">true</property>

         <!-- 生成数据表的策略 -->
         <property name="hbm2ddl.auto">create</property>

         <!-- 设置Hibernate的事务隔离级别 ：2代表读已提交-->
         <property name="connection.isolation">2</property>

         <!-- 删除对象后，使其OID设置为null -->
         <property name="use_identifier_rollback">true</property>      

         <!-- 配置C3P0数据源 -->
         <property name="hibernate.c3p0.max_size">10</property>
         <property name="hibernate.c3p0.min_size">5</property>
         <property name="hibernate.c3p0.acquire_increment">2</property>

         <property name="hibernate.c3p0.idle_test_period">2000</property>
         <property name="hibernate.c3p0.timeout">2000</property>

         <property name="hibernate.c3p0.max_statements">10</property>

         <!-- 设定JDBC的statement读取数据的时候每次从数据库中取出的记录条数 -->
         <property name="hibernate.jdbc.fetch_size">100</property>

         <!-- 设定对数据库进行批量删除，批量更新和批量插入的时候的批次大小 -->
         <property name="jdbc.batch_size">30</property>

         <!-- 需要关联的hibernate映射文件 .jbm.xml -->
         <mapping resource="com/tt/hibernate/entities/News.hbm.xml"/>
         <!--  <mapping resource="com/tt/hibernate/entities/Worker.hbm.xml"/>-->

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

News.hbm.xml

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated 2016-12-25 12:12:49 by Hibernate Tools 3.5.0.Final -->
 <hibernate-mapping package="com.tt.hibernate.entities">
     <class name="News" table="NEWS">
         <id name="id" type="java.lang.Integer">
             <column name="ID" />
             <generator class="native" />
         </id>
         <property name="title" type="java.lang.String">
             <column name="TITLE" />
         </property>
         <property name="author" type="java.lang.String">
             <column name="AUTHOR" />
         </property>
         <property name="date" type="java.util.Date">
             <column name="DATE" />
         </property>
         <property name="desc" type="java.lang.String">
             <column name="DESC" />
         </property>
         <property name="content" type="java.lang.String">
             <column name="CONTENT" />
         </property>
         <property name="image" type="java.sql.Blob">
             <column name="IMAGE" />
         </property>
     </class>
 </hibernate-mapping>

根据报错信息，定位到SQL语句的第6行：Content varchar(255)

症状分析：

如果将News.hbm.xml文件里的desc属性设置改为下面的映射派生属性，运行正常而且可以看到news表格里是没有DESC这一列。

那么为什么会出现这样的现象？为什么desc只能作为派生属性存在呢？它和title、author和date存在什么区别？唯一的区别 后三者作为参数参与了new News对象的创建。具体原因还需要深究。