问题 使用MySQL和Oracle生成Hibernate自动密钥

即使你用过 GenerationType.AUTO 如果没有任何SEQUENCE特定参数，您将无法保存指定的标识符。

如果您愿意做出一些妥协，有一些解决方法：

1. 一种方法是切换到指定的标识符。您可以使用 UUID 标识符，适用于MySQL和Oracle，您也可以手动分配值。

2. 另一种方法是使用自定义表生成器。

   首先定义一个可识别的接口：

   public interface Identifiable<T extends Serializable> {
       T getId();
   }
   

   然后扩展表生成器：

   public class AssignedTableGenerator extends TableGenerator {
   
       @Override
       public Serializable generate(SessionImplementor session, Object obj) {
           if(obj instanceof Identifiable) {
               Identifiable identifiable = (Identifiable) obj;
               Serializable id = identifiable.getId();
               if(id != null) {
                   return id;
               }
           }
           return super.generate(session, obj);
       }
   }
   

   此生成器能够将分配的标识符与合成生成的标识符混合：

   doInTransaction(session -> {
       for (int i = 0; i < 5; i++) {
           session.persist(new AssignTableSequenceIdentifier());
       }
       AssignTableSequenceIdentifier tableSequenceIdentifier = new AssignTableSequenceIdentifier();
       tableSequenceIdentifier.id = -1L;
       session.merge(tableSequenceIdentifier);
       session.flush();
   });
   

   生成以下语句：

   select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
   insert into sequence_table (sequence_name, next_val)  values (default,1)
   update sequence_table set next_val=2  where next_val=1 and sequence_name=default
   select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
   update sequence_table set next_val=3  where next_val=2 and sequence_name=default
   select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
   update sequence_table set next_val=4  where next_val=3 and sequence_name=default
   select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
   update sequence_table set next_val=5  where next_val=4 and sequence_name=default
   select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
   update sequence_table set next_val=6  where next_val=5 and sequence_name=default
   select identityvs0_.id as id1_0_0_ from assigneTableIdentifier identityvs0_ where identityvs0_.id=-1
   insert into assigneTableIdentifier (id) values (1, 2)
   insert into assigneTableIdentifier (id) values (2, 4)
   insert into assigneTableIdentifier (id) values (5, -1)
   

对于Oracle，您可以组合SEQUENCE和指定的生成器，如中所述 本文。

简而言之，考虑以下发电机：

public class AssignedSequenceStyleGenerator 
    extends SequenceStyleGenerator {

    @Override
    public Serializable generate(SessionImplementor session, 
        Object obj) {
        if(obj instanceof Identifiable) {
            Identifiable identifiable = (Identifiable) obj;
            Serializable id = identifiable.getId();
            if(id != null) {
                return id;
            }
        }
        return super.generate(session, obj);
    }
}

您可以将其映射到您的实体，如下所示：

@Id
@GenericGenerator(
    name = "assigned-sequence",
    strategy = "com.vladmihalcea.book.hpjp.hibernate.identifier.AssignedSequenceStyleGenerator",
    parameters = @org.hibernate.annotations.Parameter(
        name = "sequence_name", 
        value = "post_sequence"
    )
)
@GeneratedValue(
    generator = "assigned-sequence", 
    strategy = GenerationType.SEQUENCE
)
private Long id;