基于按annotation的hibernate主键生成策略

这里讨论代理主键，业务主键（比如说复合键等）这里不讨论。 
一、JPA通用策略生成器 
通过annotation来映射hibernate实体的,基于annotation的hibernate主键标识为@Id, 
其生成规则由@GeneratedValue设定的.这里的@id和@GeneratedValue都是JPA的标准用法, 
JPA提供四种标准用法,由@GeneratedValue的源代码可以明显看出.

Java代码 
@Target({METHOD,FIELD})      
    @Retention(RUNTIME)      
    public @interface GeneratedValue{      
        GenerationType strategy() default AUTO;      
        String generator() default "";      
    }   

@Target({METHOD,FIELD})  
    @Retention(RUNTIME)  
    public @interface GeneratedValue{  
        GenerationType strategy() default AUTO;  
        String generator() default "";  
    }

其中GenerationType:

Java代码 
public enum GenerationType{      
    TABLE,      
    SEQUENCE,      
    IDENTITY,      
    AUTO     
}  

public enum GenerationType{  
    TABLE,  
    SEQUENCE,  
    IDENTITY,  
    AUTO 
}

JPA提供的四种标准用法为TABLE,SEQUENCE,IDENTITY,AUTO. 
TABLE：使用一个特定的数据库表格来保存主键。 
SEQUENCE：根据底层数据库的序列来生成主键，条件是数据库支持序列。 
IDENTITY：主键由数据库自动生成（主要是自动增长型） 
AUTO：主键由程序控制。

1、TABLE

Java代码 
@Id   
@GeneratedValue(strategy = GenerationType.TABLE, generator="payablemoney_gen")    
@TableGenerator(name = "pk_gen",    
    table="tb_generator",    
    pkColumnName="gen_name",    
    valueColumnName="gen_value",    
    pkColumnValue="PAYABLEMOENY_PK",    
    allocationSize=1   
)  

@Id
@GeneratedValue(strategy = GenerationType.TABLE, generator="payablemoney_gen")
@TableGenerator(name = "pk_gen",
    table="tb_generator",
    pkColumnName="gen_name",
    valueColumnName="gen_value",
    pkColumnValue="PAYABLEMOENY_PK",
    allocationSize=1
)

这里应用表tb_generator，定义为

Sql代码 
CREATE TABLE  tb_generator (    
  id NUMBER NOT NULL,    
  gen_name VARCHAR2(255) NOT NULL,    
  gen_value NUMBER NOT NULL,    
  PRIMARY KEY(id)    
)  

CREATE TABLE  tb_generator (
  id NUMBER NOT NULL,
  gen_name VARCHAR2(255) NOT NULL,
  gen_value NUMBER NOT NULL,
  PRIMARY KEY(id)
)

插入纪录，供生成主键使用，

Sql代码 
INSERT INTO tb_generator(id, gen_name, gen_value) VALUES (1,PAYABLEMOENY_PK', 1);  

INSERT INTO tb_generator(id, gen_name, gen_value) VALUES (1,PAYABLEMOENY_PK', 1);

在主键生成后，这条纪录的value值，按allocationSize递增。

@TableGenerator的定义：

Java代码 
@Target({TYPE, METHOD, FIELD})     
@Retention(RUNTIME)    
public @interface TableGenerator {    
  String name();    
  String table() default "";    
  String catalog() default "";    
  String schema() default "";    
  String pkColumnName() default "";    
  String valueColumnName() default "";    
  String pkColumnValue() default "";    
  int initialValue() default 0;    
  int allocationSize() default 50;    
  UniqueConstraint[] uniqueConstraints() default {};    
}  

@Target({TYPE, METHOD, FIELD}) 
@Retention(RUNTIME)
public @interface TableGenerator {
  String name();
  String table() default "";
  String catalog() default "";
  String schema() default "";
  String pkColumnName() default "";
  String valueColumnName() default "";
  String pkColumnValue() default "";
  int initialValue() default 0;
  int allocationSize() default 50;
  UniqueConstraint[] uniqueConstraints() default {};
}

其中属性说明： 
name属性表示该表主键生成策略的名称，它被引用在@GeneratedValue中设置的“generator”值中。 
table属性表示表生成策略所持久化的表名，例如，这里表使用的是数据库中的“tb_generator”。 
catalog属性和schema具体指定表所在的目录名或是数据库名。 
pkColumnName属性的值表示在持久化表中，该主键生成策略所对应键值的名称。例如在“tb_generator”中将“gen_name”作为主键的键值 
valueColumnName属性的值表示在持久化表中，该主键当前所生成的值，它的值将会随着每次创建累加。例如，在“tb_generator”中将“gen_value”作为主键的值 
pkColumnValue属性的值表示在持久化表中，该生成策略所对应的主键。例如在“tb_generator”表中，将“gen_name”的值为“CUSTOMER_PK”。 
initialValue表示主键初识值，默认为0。 
allocationSize表示每次主键值增加的大小，例如设置成1，则表示每次创建新记录后自动加1，默认为50。 
UniqueConstraint与@Table标记中的用法类似。

2、SEQUENCE

Java代码 
@Id   
@GeneratedValue(strategy = GenerationType.SEQUENCE,generator="payablemoney_seq")    
@SequenceGenerator(name="payablemoney_seq", sequenceName="seq_payment")  

@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE,generator="payablemoney_seq")
@SequenceGenerator(name="payablemoney_seq", sequenceName="seq_payment")

@SequenceGenerator定义

Java代码 
@Target({TYPE, METHOD, FIELD})     
@Retention(RUNTIME)    
public @interface SequenceGenerator {    
 String name();    
 String sequenceName() default "";    
 int initialValue() default 0;    
 int allocationSize() default 50;    
}  

@Target({TYPE, METHOD, FIELD}) 
@Retention(RUNTIME)
public @interface SequenceGenerator {
 String name();
 String sequenceName() default "";
 int initialValue() default 0;
 int allocationSize() default 50;
}

name属性表示该表主键生成策略的名称，它被引用在@GeneratedValue中设置的“generator”值中。 
sequenceName属性表示生成策略用到的数据库序列名称。 
initialValue表示主键初识值，默认为0。 
allocationSize表示每次主键值增加的大小，例如设置成1，则表示每次创建新记录后自动加1，默认为50。

3、IDENTITY

Java代码 
@Id   
@GeneratedValue(strategy = GenerationType.IDENTITY)  

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)

4、AUTO

Java代码 
@Id   
@GeneratedValue(strategy = GenerationType.AUTO)  

@Id
@GeneratedValue(strategy = GenerationType.AUTO)

在指定主键时，如果不指定主键生成策略，默认为AUTO。

Java代码 
@Id  

@Id

跟下面的定义是一样的。

Java代码 
@Id   
@GeneratedValue(strategy = GenerationType.AUTO)  

@Id
@GeneratedValue(strategy = GenerationType.AUTO)

二、hibernate主键策略生成器 
hibernate提供多种主键生成策略，有点是类似于JPA，有的是hibernate特有： 
native: 对于 oracle 采用 Sequence 方式，对于MySQL 和 SQL Server 采用identity（自增主键生成机制），native就是将主键的生成工作交由数据库完成，hibernate不管（很常用）。 
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）。 
hilo: 使用hilo生成策略，要在数据库中建立一张额外的表，默认表名为hibernate_unique_key,默认字段为integer类型，名称是next_hi（比较少用）。 
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。 
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INDENTITY。 
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）。 
sequence: 调用底层数据库的序列来生成主键，要设定序列名，不然hibernate无法找到。 
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 Oracle（比较少用） 
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。 
foreign: 使用另外一个相关联的对象的主键。通常和<one-to-one>联合起来使用。 
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，ORACLE的rawtohex(sys_guid())函数等。 
uuid.hex: 看uuid，建议用uuid替换。 
sequence-identity: sequence策略的扩展，采用立即检索策略来获取sequence值，需要JDBC3.0和JDK4以上（含1.4）版本

hibernate提供了多种生成器供选择,基于Annotation的方式通过@GenericGenerator实现. 
hibernate每种主键生成策略提供接口org.hibernate.id.IdentifierGenerator的实现类,如果要实现自定义的主键生成策略也必须实现此接口.

Java代码 
public interface IdentifierGenerator {    
       
    public static final String ENTITY_NAME = "entity_name";    
        
     
  public Serializable generate(SessionImplementor session, Object object)     
    throws HibernateException;    
}  

public interface IdentifierGenerator {
   
    public static final String ENTITY_NAME = "entity_name";
    
 
  public Serializable generate(SessionImplementor session, Object object) 
    throws HibernateException;
}

IdentifierGenerator提供一generate方法,generate方法返回产生的主键.

三、@GenericGenerator 
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)  

@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")  

@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 {};    
}  

@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);    
}  

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

Java代码 
@GeneratedValue(generator = "paymentableGenerator")      
@GenericGenerator(name = "paymentableGenerator", strategy = "native")   

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "native")

2、uuid

Java代码 
@GeneratedValue(generator = "paymentableGenerator")      
@GenericGenerator(name = "paymentableGenerator", strategy = "uuid")   

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "uuid")

3、hilo

Java代码 
@GeneratedValue(generator = "paymentableGenerator")      
@GenericGenerator(name = "paymentableGenerator", strategy = "hilo")   

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "hilo")

4、assigned

Java代码 
@GeneratedValue(generator = "paymentableGenerator")      
@GenericGenerator(name = "paymentableGenerator", strategy = "assigned")   

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "assigned")

5、identity

Java代码 
@GeneratedValue(generator = "paymentableGenerator")      
@GenericGenerator(name = "paymentableGenerator", strategy = "identity")   

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "identity")

6、select

Java代码 
@GeneratedValue(generator = "paymentableGenerator")    
@GenericGenerator(name="select", strategy="select",    
     parameters = { @Parameter(name = "key", value = "idstoerung") })  

@GeneratedValue(generator = "paymentableGenerator")
@GenericGenerator(name="select", strategy="select",
     parameters = { @Parameter(name = "key", value = "idstoerung") })

7、sequence

Java代码 
@GeneratedValue(generator = "paymentableGenerator")    
@GenericGenerator(name = "paymentableGenerator", strategy = "sequence",     
         parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") })  

@GeneratedValue(generator = "paymentableGenerator")
@GenericGenerator(name = "paymentableGenerator", strategy = "sequence", 
         parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") })

8、seqhilo

Java代码 
@GeneratedValue(generator = "paymentableGenerator")    
@GenericGenerator(name = "paymentableGenerator", strategy = "seqhilo",     
         parameters = { @Parameter(name = "max_lo", value = "5") })  

@GeneratedValue(generator = "paymentableGenerator")
@GenericGenerator(name = "paymentableGenerator", strategy = "seqhilo", 
         parameters = { @Parameter(name = "max_lo", value = "5") })

9、increment

Java代码 
@GeneratedValue(generator = "paymentableGenerator")      
@GenericGenerator(name = "paymentableGenerator", strategy = "increment")   

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "increment")

10、foreign

@GeneratedValue(generator = "idGenerator")
@GenericGenerator(name = "idGenerator", strategy = "foreign", 
         parameters = { @Parameter(name = "property", value = "employee") })

注意：直接使用@PrimaryKeyJoinColumn 报错（?）

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@OneToOne(cascade = CascadeType.ALL)    
@PrimaryKeyJoinColumn  

@OneToOne(cascade = CascadeType.ALL) 
@PrimaryKeyJoinColumn

例如

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@Entity  
public class Employee {   
  @Id Integer id;   
       
  @OneToOne @PrimaryKeyJoinColumn  
  EmployeeInfo info;   
  ...   
} 

@Entity
public class Employee {
  @Id Integer id;
    
  @OneToOne @PrimaryKeyJoinColumn
  EmployeeInfo info;
  ...
}

应该为

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@Entity  
public class Employee {   
  @Id    
  @GeneratedValue(generator = "idGenerator")   
  @GenericGenerator(name = "idGenerator", strategy = "foreign",    
         parameters = { @Parameter(name = "property", value = "info") })    
  Integer id;   
       
  @OneToOne  
  EmployeeInfo info;   
  ...   
} 

@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

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@GeneratedValue(generator = "paymentableGenerator")     
@GenericGenerator(name = "paymentableGenerator", strategy = "guid")  

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "guid")

12、uuid.hex

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@GeneratedValue(generator = "paymentableGenerator")     
@GenericGenerator(name = "paymentableGenerator", strategy = "uuid.hex")  

@GeneratedValue(generator = "paymentableGenerator")  
@GenericGenerator(name = "paymentableGenerator", strategy = "uuid.hex")

13、sequence-identity

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@GeneratedValue(generator = "paymentableGenerator")   
@GenericGenerator(name = "paymentableGenerator", strategy = "sequence-identity",    
         parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") }) 

@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。

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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;   
 }   
} 

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。

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@GeneratedValue(generator = "paymentableGenerator")   
@GenericGenerator(name = "paymentableGenerator", strategy = "AssignedSequenceGenerator",    
     parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") }) 

@GeneratedValue(generator = "paymentableGenerator")
@GenericGenerator(name = "paymentableGenerator", strategy = "AssignedSequenceGenerator", 
     parameters = { @Parameter(name = "sequence", value = "seq_payablemoney") })

值得注意的是，定义的这种策略，就像打开了潘多拉魔盒，非常不可控。正常情况下，不建议这么做。