Hibernate4主键生成策略（注解方式）

IDENTITY: supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.

SEQUENCE (called seqhilo in Hibernate): uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.

TABLE (called MultipleHiLoPerTableGenerator in Hibernate) : uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column as a source of hi values.

The hi/lo algorithm generates identifiers that are unique only for a particular database.

AUTO: selects IDENTITY, SEQUENCE or TABLE depending upon the capabilities of the underlying database.

如果想使用hibernate增强的主键生成注解，需要设置

hibernate.id.new_generator_mappings=true

1. IDENTITY： 自动增长，支持数据库有DB2、MySQL、MS SQL Server、Sybase、HypersonicSQL

@Entity
public class User {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
......getter & setter....
}

2 SEQUENCE： 序列，常用于oracle数据

@Entity
public class User {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
private int id;
private String name;
......getter & setter....
}

@GeneratedValue中如果不指定generator具体名字，则由hibernate自动生成名为“hibernate_sequence”的序列.以下是指定数据库中序列名字为my_sequence.

@Entity
@SequenceGenerator(name="SEQ_GEN", sequenceName="my_sequence")
public class User {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE,generator="SEQ_GEN")
private int id;
private String name;
......getter & setter....
}

也可以为序列指定开始值，增长值：

initialValue: the value from which the id is to start generating

allocationSize: the amount to increment by when allocating id numbers from the generator

3. AUTO: 根据底层数据库，选择IDENTITY或SEQUENCE4. TABLE: 生成一张表来存储生成的主键信息

pkColumnName: the column name containing the entity identifier

valueColumnName: the column name containing the identifier value

pkColumnValue: the entity identifier

uniqueConstraints: any potential column constraint on the table containing the ids

其中: name指定生成策略的别名，table是存放主键生成的表名，pkColumnName:为字段名字，valueColumnName为字段值，pkColumnValue为具体字段对应的值pkColumnName可以理解为Map中的key, valueColumnName为Map中的value,
pkColumnValue具体的Key内容。

@Entity
@Table(name = "t_user")
@TableGenerator(name = "USER_GEN",   //别名
table = "GENERATOR_TABLE",    	 //生成的表名
pkColumnName = "t_key", 	  	 //key列名
valueColumnName = "t_value",  	 //value列名
pkColumnValue = "user", 		 //具体key内容
initialValue=10,				 //初始值
allocationSize = 20)			 //增长值
public class User {
@Id
@GeneratedValue(strategy = GenerationType.TABLE, generator="USER_GEN")
private int id;
private String name;
......getter & setter....
}

测试示例：
@Test
public void testUser1(){
Configuration configuration = new Configuration().configure();
ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties()).buildServiceRegistry();
SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);
Session session = sessionFactory.getCurrentSession();
session.beginTransaction();
User user = new User();
user.setName("zhangsan");
session.save(user);
session.getTransaction().commit();
}

具体生成的表如下：

t_key	t_value
user	1

PS: 如果是Mysql，发现一个问题，即使设置了initialValue值也是无效的，allocationSize这个值有设置的话，user表主键为1、20、40这样，但GENERATOR_TABLE

的增加方式还是1、2、3，有兴趣的可以试试，oracle数据库未测试。

5. 还有一种uuid的生成策略:

@Entity
public class Person {
@Id
@GeneratedValue(generator = "system-uuid")
@GenericGenerator(name = "system-uuid", strategy = "uuid")
private String id;
private String name;
.....getter & setter....
}

6. 支持自定义生成策略