Spring GeneratedValue批注用法 [英] Spring GeneratedValue annotation usage
问题描述
假设我有一个Car实体:
@Entity
public class Car {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Integer id;
当我向数据库添加新对象时，spring如何知道要自动增加什么值?
How does spring know what value to autoincrement when I add a new object to the database?
推荐答案
Here is a good explanation of primary keys generation strategies
有4个生成主键的选项
GenerationType.AUTO
GenerationType.AUTO是默认世代类型，并允许 持久性提供者选择生成策略.
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.
如果使用Hibernate作为持久性提供程序，它将选择一个 基于数据库特定方言的生成策略.对于大多数 流行的数据库，它选择GenerationType.SEQUENCE .
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是最容易的使用方式，但不是最好的一种 从性能的角度来看.它依靠自动递增 数据库列，并让数据库为每个数据库生成一个新值 插入操作.从数据库的角度来看，这是非常有效的，因为对自动增量列进行了高度优化，并且不需要任何其他语句.
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.
如果您使用Hibernate，则此方法存在明显的缺点. Hibernate要求每个受管实体都有一个主键值，并且 因此必须立即执行插入语句.这 阻止它使用其他优化技术(例如JDBC) 批处理.
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使用数据库序列来生成 独特的价值观.它需要附加的select语句才能获得 数据库序列中的下一个值.但这没有表现 对大多数应用程序的影响.
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.
如果您不提供任何其他信息，则Hibernate将
从其默认序列中请求下一个值.你可以改变
通过在生成器中引用@SequenceGenerator的名称
@GeneratedValue批注的属性. @SequenceGenerator
注释使您可以定义生成器的名称，名称和
数据库序列的模式和分配的大小
顺序.
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:通常首选Long作为ID，而不是Integer，因此用完的可能性较小)
(Side note: Usually prefer Long for ids instead of Integer so you're less likely to run out)
GenerationType.TABLE
GenerationType.TABLE仅如今很少使用.它模拟 通过在数据库中存储和更新其当前值来生成序列 需要使用悲观锁的表 交易按顺序进行.这会减慢您的速度 申请，因此，您应优先选择 GenerationType.SEQUENCE (如果您的数据库支持序列)， 最受欢迎的数据库.
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.
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