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

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

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

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

5、identity
Java代码
@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") }) 

7、sequence
Java代码
@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") }) 

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

12、uuid.hex
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") }) 


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。