hibernate中创建表错误： org.hibernate.exception.SQLGrammarException: canot insert into ***** 2011-05-20

今天早早的就起来看视频做实验，看的是尚学堂的hibernate，马士兵老师讲的。讲的是一个数据表的多对一，一对多的映射。看完视频自己接着写，不幸的是，调试了一早上都没有调试出来。后来在传智博客上的QQ群交流后，差不多一个小时多以后终于把问题解决了。

1.Group类

package com.riverro.hibernate;

//一对多
//一个组可以对应多个组员

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

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="t_group")
public class Group {
private int id;
private String name;
private Set<User> users = new HashSet<User>();
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@OneToMany(mappedBy="group",
cascade={CascadeType.ALL}
)

public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}
}

2.User类

package com.riverro.hibernate;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name="t_user")
public class User {
private int id;
private String name;
private Group group;

@ManyToOne(cascade={CascadeType.ALL})
public Group getGroup() {
return group;
}
public void setGroup(Group group) {
this.group = group;
}

@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}

3.测试类

package com.riverro.hibernate;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class HibernateORMappingTest {
private static SessionFactory sessionFactory;

@BeforeClass
public static void beforeClass() {
//new SchemaExport(new AnnotationConfiguration().configure()).create(true, true);
sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
}
@AfterClass
public static void afterClass() {
sessionFactory.close();
}

@Test
public void testSchemaExport() {
new SchemaExport(new AnnotationConfiguration().configure()).create(true, true);
}

@Test
public void testSaveUser() {
User u = new User();
u.setName("u1");
Group g = new Group();
g.setName("g1");
u.setGroup(g);

Session s = sessionFactory.getCurrentSession();
s.beginTransaction();
s.save(g);
s.save(u);
s.getTransaction().commit();
}

@Test
public void testSaveGroup() {
User u1 = new User();
u1.setName("u1");
User u2 = new User();
u2.setName("u2");

Group g = new Group();
g.setName("g1");
g.getUsers().add(u1);
g.getUsers().add(u2);
u1.setGroup(g);
u2.setGroup(g);

Session s = sessionFactory.getCurrentSession();
s.beginTransaction();
s.save(g);

s.getTransaction().commit();
}
}

4.hibernat.cfg.xml配置文件

<?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/hibernate</property>
<property name="connection.username"
>root</property>
<property name="connection.password"
>root</property>

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

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

<!-- Enable Hibernate's automatic session context management -->
<property name="current_session_context_class"
>thread</property>

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

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

<property name="format_sql"
>true</property>

<!-- Drop and re-create the database schema on startup -->

<property name="hibernate.hbm2ddl.auto"
>update</property>

<mapping class="com.riverro.hibernate.Group"/>
<mapping class="com.riverro.hibernate.User"/>

</session-factory>

</hibernate-configuration
>

5.在调试的时候一直错误，说是sql语法错误，但是检查了很多遍都没有找到哪里有错误，实在弄不清楚。后面到QQ群上提问，有很多大虾都是很乐意的帮组我这个菜鸟。出了很多注意也和我检查了， 特别提到的是这里要写上<property name="hibernate.hbm2ddl.auto">update</property>
。试试create等等。但是都无果而终，我又用了另外的一个方法。先在数据库中建好表，然后再测试，这次测试通过了，数据也添加到了数据库表中。但是始终觉得还是不对，为什么hibernate不自己帮我创建表呢？后面QQ群上的一个大虾说是不是数据库方言不对，看看这个，要是这个再找不到，就实在没有办法了。我看了，我的数据库方言是：HSQLDialect。果真有错误，查了hibernate的文档，文档上说的是链接MySQL的时候用的方言是：org.hibernate.dialect.MySQL5Dialect。修改过来之后就正确了。

6.结论：

从这个实验中，可以看出，设计的时候要非常小心，检查Exception的时候要多方位的去查找，从熟悉的到不熟悉的都要进行查找。多看文档，多小心，则很容易把错误纠正的。