如何使用 JPA GenerationType.AUTO 提供初始值或增量 ID

Question

I am using following code to define MyEntity ,

@Entity
@Table(name = "MY_TABLE")
public class MyEntity {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name = "MY_TABLE_ID")
private Integer myTableId;

@Column(name = "MY_TABLE_NM")
private String myTableName;

//Getters Setters
}

For the first POST after my application starts, I create MyEntity everything works fine, MY_TABLE_ID starts with 1 and works as expected.

My issue is, If somebody inserts data manually before I do my POST then I get duplicate key exception as myTableId is entered as 1 which is already present.

My main problem is I can't create database sequence for using GenerationType.SEQUENCE now to resolve this as database can't be altered now. I have tried various combinations of GenerationType , TableGenerator but I am unable to successfully tackle it.

Setting initialValue to some larger number to avoid duplicate values can temporarily resolve my problem but I am unable to do it too.

If someone can help me with initialValue with AUTO or give me some other better solution without database changes will be great :)

Answer 1

As MY_TABLE_ID is an identity column, following annotations will work.

@Entity
@Table(name = "MY_TABLE")
public class MyEntity {

@Id
@GeneratedValue(strategy=GenerationType.IDENTITY) // <-- IDENTITY instead of AUTO
@Column(name = "MY_TABLE_ID")
private Integer myTableId;

@Column(name = "MY_TABLE_NM")
private String myTableName;

//Getters Setters
}

The identity column will automatically assign an value as soon as the transaction is committed. You are not to set any values for an identity column, as its the job of the database to assign the values. Therefore you also don't need to think about any initial values (forget them completely for identity columns)

Answer 2

I tried various options in answers provided here and for similar questions on stackoverflow and other forums,

I had few limitations,

1. I couldn't create database sequence as my database changes were freezed.
2. I didn't want to introduce new Custom IdGenerator class because it would add confusion to other people working with me.

It was resolved using following change:

Adding GenericGenerator with increment strategy helped me, I made following changes to my code.

@Entity
@Table(name = "MY_TABLE")
public class MyEntity {

@Id
@GeneratedValue(strategy = GenerationType.AUTO, generator="seq")
@GenericGenerator(name = "seq", strategy="increment")
@Column(name = "MY_TABLE_ID")
private Integer myTableId;

@Column(name = "MY_TABLE_NM")
private String myTableName;

//Getters Setters
}

It helped me because, From Hiberbate DOCs

increment

An IdentifierGenerator that returns a long, constructed by counting from the maximum primary key value at startup. Not safe for use in a cluster!

Since, it was incrementing already existing myTableId even if it was manually inserted, this resolved my issue.

Answer 3

You can also implement your own generator if you need more control. See this interface IdentifierGenerator . So you can get the count of records, for example through a @NamedQuery . Then you can generate an identifier yourself.

public class MyEntityKeyGenerator implements IdentifierGenerator {
@Override
public Serializable generate(SessionImplementor session, Object object) {
    // SELECT count(ent) from MyEntity ent;
    Long count = (Long) session.getNamedQuery("count-query").uniqueResult();
    // calc and return id value
   }
}

Entity:

class MyEntity {
@Id
@GenericGenerator(name = "my_generator",
        strategy = "org.common.MyEntityKeyGenerator")
@GeneratedValue(generator = "my_generator")
private Long id;...

Just do not forget about the lock.

Answer 4

I use the generation type Identity, which basically means that the db, takes care of Id generation.

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@MappedSuperclass
@EntityListeners(EntityListener.class)
@EqualsAndHashCode(of = {"id", "createdAt"})
public abstract  class AbstractEntity<ID extends Serializable> implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private ID id;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "CREATED_AT", updatable = false)
    private Date createdAt;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "UPDATED_AT")
    private Date updatedAt;

}

You can also use, Sequence generation:

@Entity
@SequenceGenerator(name="seq", initialValue=1, allocationSize=100)
public class EntityWithSequenceId {
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq")
    @Id long id;
}