java如何设置自增属性

Question

In my entity I have this code:

@Entity
@Table(name = "product")
class Product{   
...
    @GeneratedValue(strategy=GenerationType.AUTO)
    int rank;
}

When I try to save an object of type product, in DB, the rank value remains always 0

Can I set an attribute other than the id auto-increment?

Answer 1

The solution proposed by @Alain Cruz, is the way! But it is the half of the answer... You will need:

1) To Modify your rank attribute like this:

@Generated(GenerationTime.INSERT)
@Column(name = "column_name", insertable = false)
Long rank;

2) Create a before insertion trigger that monitors Products entities, checking that if rank comes null , you will change that value for a new value returned by the desired sequence...

I have done this approach to generate special codes using SQL Functions stored in my database... You can find more info here

Answer 2

There are different ways to auto generate a value, but they must always be done on the @Id field. If it is not declared, then you won't be able to auto increment your value.

There are different types of strategies to increment your Id. In this blog , you can learn more about them. For your example, IDENTITY should do the trick.

@Entity
@Table(name = "product")
class Product{   

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    int rank;
}

Update

After doing some research, it seems that Hibernate now allows to auto increment other non-id fields, but it uses another annotation for the purpose. I haven't tried it, but maybe it could work. This in case your rank is not your id.

@Generated(GenerationTime.INSERT)
@Column(name = "column_name", insertable = false)
int rank;

Answer 3

@Entity
@Table(name = "product")
class Product{   
...
    @GeneratedValue(strategy=GenerationType.IDENTITY )
    int rank;
}

Answer 4

The following should work and if it does not, then try what comes after:

@Entity
@Table(name = "product")
class Product{   
...
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int rank;
}

public int getId() {
    return id;
}



public void setId(int id) {
    this.id = id;
}

If that does not work, then go to your database and set rank variable to be auto incremented.