GenerationType.AUTO vs GenerationType.IDENTITY in hibernate

问题

Currently we are using mysql as a database and we use

@Generated Value(strategy = GenerationType.IDENTITY)

It's working perfectly at certain situation we need to migrate our database to Oracle at that time it's not working properly.If any one knows what's actual difference present behind this and how it's working?

回答1:

How could it "work properly" (you don't define basic info like what you mean by that) with Oracle ? I don't see the relevance of AUTO to your question - that simply lets an implementation choose what it wants to use.

"IDENTITY" (as per JPA javadocs and spec - what you should be referring to) means autoincrement. There is no such concept in Oracle, yet there is in MySQL, SQLServer and a few others. I would expect any decent JPA implementation to flag an error when even trying such a thing.

Oracle would allow "SEQUENCE", or "TABLE" strategies to be used however

回答2:

Quoting Java Persistence/Identity and Sequencing:

Identity sequencing uses special IDENTITY columns in the database to allow the database to automatically assign an id to the object when its row is inserted. Identity columns are supported in many databases, such as MySQL, DB2, SQL Server, Sybase and Postgres. Oracle does not support IDENTITY columns but they can be simulated through using sequence objects and triggers.

so I prefer to use SEQUENCE instead

Sequence objects use special database objects to generate ids. Sequence objects are only supported in some databases, such as Oracle, DB2, and Postgres. Usually, a SEQUENCE object has a name, an INCREMENT, and other database object settings. Each time the .NEXTVAL is selected the sequence is incremented by the INCREMENT.

Example :

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="EMP_SEQ")
    @SequenceGenerator(name="EMP_SEQ", sequenceName="EMP_SEQ", allocationSize=100)
    private long id;
    ...
}

回答3:

Im using JPA and Oracle 11g, the solution that worked for me is the following

package com.example.springsocial.model;

import java.util.Date;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity
@Table(name = "rol", uniqueConstraints = {
        @UniqueConstraint(columnNames = "name")
})
public class Rol {
    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="rol_sequence")
    @SequenceGenerator(name="rol_sequence", sequenceName="rol_sequence", allocationSize=100)
    private Long id;

    @Column(nullable = false)
    private String name;

    private Date createdAt;
    @Column(nullable = true)
    private Date updatedAt;
    @Column(nullable = true)
    private Integer createdBy;
    @Column(nullable = true)
    private Integer updatedBy;

    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Date getCreatedAt() {
        return createdAt;
    }

    public void setCreatedAt(Date createdAt) {
        this.createdAt = createdAt;
    }

    public Date getUpdatedAt() {
        return updatedAt;
    }

    public void setUpdatedAt(Date updatedAt) {
        this.updatedAt = updatedAt;
    }

    public Integer getCreatedBy() {
        return createdBy;
    }

    public void setCreatedBy(Integer createdBy) {
        this.createdBy = createdBy;
    }

    public Integer getUpdatedBy() {
        return updatedBy;
    }

    public void setUpdatedBy(Integer updatedBy) {
        this.updatedBy = updatedBy;
    }
}

来源：https://stackoverflow.com/questions/33096466/generationtype-auto-vs-generationtype-identity-in-hibernate