即使不使用任何特定于 SEQUENCE 的参数，也无法保存分配的标识符。GenerationType.AUTO
如果您愿意做出一些妥协，有一些解决方法：
-
一种方法是切换到分配的标识符。您可以使用UUID标识符，它们适用于MySQL和Oracle，您也可以手动分配值。
-
另一种方法是使用自定义表生成器。
首先定义一个可识别接口：
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在一起：
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 和分配的生成器。简而言之，请考虑以下生成器：
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);
}
}
您可以按如下方式将其映射到实体：
@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;
所有代码都可以在GitHub上找到，就像一个魅力。
