基于annotation的hibernate主键生成策略
自定义主键生成策略，由@GenericGenerator实现。?
hibernate在JPA的基础上进行了扩展，可以用一下方式引入hibernate独有的主键生成策略，就是通过@GenericGenerator加入的。?
比如说，JPA标准用法?
Java代码?
@Id?
@GeneratedValue(GenerationType.AUTO)?
就可以用hibernate特有以下用法来实现?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "assigned")?
@GenericGenerator的定义:?
Java代码?
@Target({PACKAGE, TYPE, METHOD, FIELD})??
@Retention(RUNTIME)??
public @interface GenericGenerator {??
String name();??
String strategy();??
Parameter[] parameters() default {};??
}?
name属性指定生成器名称。?
strategy属性指定具体生成器的类名。?
parameters得到strategy指定的具体生成器所用到的参数。?
对于这些hibernate主键生成策略和各自的具体生成器之间的关系,在org.hibernate.id.IdentifierGeneratorFactory中指定了,?
Java代码?
static {??
?? GENERATORS.put("uuid", UUIDHexGenerator.class);??
?? GENERATORS.put("hilo", TableHiLoGenerator.class);??
?? GENERATORS.put("assigned", Assigned.class);??
?? GENERATORS.put("identity", IdentityGenerator.class);??
?? GENERATORS.put("select", SelectGenerator.class);??
?? GENERATORS.put("sequence", SequenceGenerator.class);??
?? GENERATORS.put("seqhilo", SequenceHiLoGenerator.class);??
?? GENERATORS.put("increment", IncrementGenerator.class);??
?? GENERATORS.put("foreign", ForeignGenerator.class);??
?? GENERATORS.put("guid", GUIDGenerator.class);??
?? GENERATORS.put("uuid.hex", UUIDHexGenerator.class); //uuid.hex is deprecated??
?? GENERATORS.put("sequence-identity", SequenceIdentityGenerator.class);??
}?
上面十二种策略，加上native，hibernate一共默认支持十三种生成策略。?
1、native 对于orcale采用Sequence方式，对于MySQL和SQL Server采用identity(处境主键生成机制)，native就是将主键的生成工作交给数据库本身，hibernate不管。（很常用）?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "native")??
2、uuid? 采用128位的uuid算法生成主键，uuid被编码为一个32位16进制数字的字符串。占用空间大（字符串类型）。?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "uuid")??
3、hilo 要在数据库中建立一张额外的表，默认表名为hibernate_unque_key，默认字段为integer类型，名称是next_hi（比较少用）?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "hilo")??
4、assigned 在插入数据的时候主键由程序处理（很常用），这是<generator>元素没有指定时的默认生成策略。等同于JPA中的AUTO。?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "assigned")??
5、identity 使用SQL Server和MySQL的自增字段，这个方法不能放到Oracle中，Oracle不支持自增字段，要设定sequence(MySQL和SQL Server中很常用)。等同于JPA中的IDENTITY?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "identity")??
6、select 使用触发器生成主键（主要用于早期的数据库主键生成机制，少用）?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")??
@GenericGenerator(name="select", strategy="select",??
????? parameters = { @Parameter(name = "key", value = "idstoerung") })?
7、sequence 调用谨慎数据库的序列来生成主键，要设定序列名，不然hibernate无法找到。?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")??
@GenericGenerator(name = "paymentableGenerator", strategy = "sequence",??
????????? parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") })?
8、seqhilo 通过hilo算法实现，但是主键历史保存在Sequence中，适用于支持Sequence的数据库，如Orcale(比较少用）?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")??
@GenericGenerator(name = "paymentableGenerator", strategy = "seqhilo",??
????????? parameters = { @Parameter(name = "max_lo", value = "5") })?
9、increment 插入数据的时候hibernate会给主键添加一个自增的主键，但是一个hibernate实例就维护一个计数器，所以在多个实例运行的时候不能使用这个方法。?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "increment")??
10、foreign?
Java代码?
@GeneratedValue(generator = "idGenerator")??
@GenericGenerator(name = "idGenerator", strategy = "foreign",??
????????? parameters = { @Parameter(name = "property", value = "employee") })?
注意：直接使用@PrimaryKeyJoinColumn 报错（?）?
Java代码?
@OneToOne(cascade = CascadeType.ALL)??
@PrimaryKeyJoinColumn??
例如?
Java代码?
@Entity?
public class Employee {??
? @Id Integer id;??
??????
? @OneToOne @PrimaryKeyJoinColumn?
?? EmployeeInfo info;??
?????
}?
应该为?
Java代码?
@Entity?
public class Employee {??
? @Id??
? @GeneratedValue(generator = "idGenerator")??
? @GenericGenerator(name = "idGenerator", strategy = "foreign",??
????????? parameters = { @Parameter(name = "property", value = "info") })??
?? Integer id;??
??????
? @OneToOne?
?? EmployeeInfo info;??
?????
}?
11、guid 采用数据库底层的guid算法机制，对应MySQL的uuid()函数，SQL Server的newid()函数，ORCALE的rawtohex(sys_guid())函数等?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "guid")??
12、uuid.hex 看uudi,建议用uuid替换?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")????
@GenericGenerator(name = "paymentableGenerator", strategy = "uuid.hex")??
13、sequence-identity?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")??
@GenericGenerator(name = "paymentableGenerator", strategy = "sequence-identity",??
????????? parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") })?
四、通过@GenericGenerator自定义主键生成策略?
如果实际应用中，主键策略为程序指定了就用程序指定的主键（assigned），没有指定就从sequence中取。?
明显上面所讨论的策略都不满足，只好自己扩展了，集成assigned和sequence两种策略。?
Java代码?
public class AssignedSequenceGenerator extends SequenceGenerator implements??
PersistentIdentifierGenerator, Configurable {??
private String entityName;??
????
public void configure(Type type, Properties params, Dialect dialect) throws MappingException {??
?? entityName = params.getProperty(ENTITY_NAME);??
? if (entityName==null) {??
?? throw new MappingException("no entity name");??
?? }??
????
? super.configure(type, params, dialect);????
}??
public Serializable generate(SessionImplementor session, Object obj)??
? throws HibernateException {??
????
?? Serializable id = session.getEntityPersister( entityName, obj )??
???? .getIdentifier( obj, session.getEntityMode() );??
????
? if (id==null) {??
??? id = super.generate(session, obj);??
?? }??
????
? return id;??
}??
}?
实际应用中，定义同sequence。?
Java代码?
@GeneratedValue(generator = "paymentableGenerator")??
@GenericGenerator(name = "paymentableGenerator", strategy = "AssignedSequenceGenerator",??
????? parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") })?
