Spring GeneratedValue 注释用法
Spring GeneratedValue annotation usage
假设我有一个 Car 实体：
@Entity
public class Car {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Integer id;
当我将新对象添加到数据库时，spring 如何知道要自动增加什么值？
这是一个good explanation of primary keys generation strategies
There are 4 options to generate primary keys
GenerationType.AUTO
The GenerationType.AUTO is the default generation type and lets the
persistence provider choose the generation strategy.
If you use Hibernate as your persistence provider, it selects a
generation strategy based on the database specific dialect. For most
popular databases, it selects GenerationType.SEQUENCE.
GenerationType.IDENTITY
The GenerationType.IDENTITY is the easiest to use but not the best one
from a performance point of view. It relies on an auto-incremented
database column and lets the database generate a new value with each
insert operation. From a database point of view, this is very efficient because the auto-increment columns are highly optimized, and it doesn’t require any additional statements.
This approach has a significant drawback if you use Hibernate.
Hibernate requires a primary key value for each managed entity and
therefore has to perform the insert statement immediately. This
prevents it from using different optimization techniques like JDBC
batching.
GenerationType.SEQUENCE
The GenerationType.SEQUENCE uses a database sequence to generate
unique values. It requires additional select statements to get the
next value from a database sequence. But this has no performance
impact for most applications.
If you don’t provide any additional information, Hibernate will
request the next value from its default sequence. You can change that
by referencing the name of a @SequenceGenerator in the generator
attribute of the @GeneratedValue annotation. The @SequenceGenerator
annotation lets you define the name of the generator, the name, and
schema of the database sequence and the allocation size of the
sequence.
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@SequenceGenerator(name="car_generator", sequenceName = "car_seq", allocationSize=50)
private Long id;
（旁注：通常更喜欢 Long 作为 id 而不是 Integer 所以你不太可能 运行 出局）
GenerationType.TABLE
The GenerationType.TABLE gets only rarely used nowadays. It simulates
a sequence by storing and updating its current value in a database
table which requires the use of pessimistic locks which put all
transactions into a sequential order. This slows down your
application, and you should, therefore, prefer the
GenerationType.SEQUENCE, if your database supports sequences, which
most popular databases do.
假设我有一个 Car 实体：
@Entity
public class Car {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Integer id;
当我将新对象添加到数据库时，spring 如何知道要自动增加什么值？
这是一个good explanation of primary keys generation strategies
There are 4 options to generate primary keys
GenerationType.AUTO
The GenerationType.AUTO is the default generation type and lets the persistence provider choose the generation strategy.
If you use Hibernate as your persistence provider, it selects a generation strategy based on the database specific dialect. For most popular databases, it selects GenerationType.SEQUENCE.
GenerationType.IDENTITY
The GenerationType.IDENTITY is the easiest to use but not the best one from a performance point of view. It relies on an auto-incremented database column and lets the database generate a new value with each insert operation. From a database point of view, this is very efficient because the auto-increment columns are highly optimized, and it doesn’t require any additional statements.
This approach has a significant drawback if you use Hibernate. Hibernate requires a primary key value for each managed entity and therefore has to perform the insert statement immediately. This prevents it from using different optimization techniques like JDBC batching.
GenerationType.SEQUENCE
The GenerationType.SEQUENCE uses a database sequence to generate unique values. It requires additional select statements to get the next value from a database sequence. But this has no performance impact for most applications.
If you don’t provide any additional information, Hibernate will request the next value from its default sequence. You can change that by referencing the name of a
@SequenceGeneratorin the generator attribute of the@GeneratedValueannotation. The@SequenceGeneratorannotation lets you define the name of the generator, the name, and schema of the database sequence and the allocation size of the sequence.
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@SequenceGenerator(name="car_generator", sequenceName = "car_seq", allocationSize=50)
private Long id;
（旁注：通常更喜欢 Long 作为 id 而不是 Integer 所以你不太可能 运行 出局）
GenerationType.TABLE
The GenerationType.TABLE gets only rarely used nowadays. It simulates a sequence by storing and updating its current value in a database table which requires the use of pessimistic locks which put all transactions into a sequential order. This slows down your application, and you should, therefore, prefer the GenerationType.SEQUENCE, if your database supports sequences, which most popular databases do.