用MySQL和Oracle实现Hibernate自动密钥生成

Question

带注释的类中的followig代码可用于两个数据库的自动键生成，但如果主键是自设置的，则会失败。

@GeneratedValue(strategy=GenerationType.AUTO, generator="sequence_generator")
@SequenceGenerator(name="sequence_generator", sequenceName="SEQUENCE1")
@Column(name = "id", unique = true, nullable = false)
public Integer getId() {
    return this.id;
}

如果没有@GeneratedValue和@SequenceGenerator注释，则可以手动设置主键，但自动生成无法工作。

Answer 1

即使使用了generationtype.auto而不使用任何特定于序列的参数，也无法保存分配的标识符。

如果你愿意做出一些妥协，有一些变通办法：

>

一种方法是切换到分配的标识符。您可以使用UUID标识符，这对MySQL和Oracle都适用，您还可以手动分配值。

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

    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);
        }
    }

    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，您可以将序列和分配的生成器组合在一起。简而言之，考虑到下面的生成器：

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;