Java @GeneratedValue 和 @GenericGenerator 的区别

Difference between @GeneratedValue and @GenericGenerator

提问by user1883212

Sometimes I find them together, sometimes alone... other times they seem to do the same.

有时我发现他们在一起，有时单独……有时他们似乎也做同样的事情。

What's the difference?

有什么不同？

Here are three examples. What do they do of different? Why can't I use just @GeneratedValue for all of them?

下面是三个例子。他们有什么不同？为什么我不能对所有这些只使用 @GeneratedValue？

Example 1

示例 1

@Id
@GeneratedValue(generator="increment")
@GenericGenerator(name="increment", strategy = "increment") 
Long id;

Example 2

示例 2

@Id @GeneratedValue(strategy=GenerationType.SEQUENCE)
private int userId;

Example 3

示例 3

@ElementCollection
@JoinTable(name="Address",
   joinColumns=@JoinColumn(name="user_id")
)
@GenericGenerator(name="hilo-gen", strategy="hilo")
@CollectionId(columns = @Column(name="Address_id"), generator = "hilo-gen", type = @Type(type="long"))
Collection<Addr> listOfAddresses = new ArrayList<Addr>();

回答by Kevin Bowersox

When using an ORMit is often necessary to generate a primary key value.

使用ORM 时，通常需要生成主键值。

The @GeneratedValueannotation denotes that a value for a column, which must be annotated with @Idis generated. The elements strategyand generatoron the annotation describe how the generated value is obtained.

的@GeneratedValue注释表示一个列中的值，必须以进行注释@Id被生成。注释上的元素strategy和generator描述了如何获得生成的值。

There are four possible values for the strategyelement on the @GeneratedValueannotation: IDENTITY, AUTO, TABLEand SEQUENCE. See more.

有四个可能的值strategy的元素@GeneratedValue注释：IDENTITY，AUTO，TABLE和SEQUENCE。 查看更多。

So to answer Part 2of your question, the code snippet is indicating that the value of userIdwill be obtained through a sequence in the database.

因此，要回答问题的第 2 部分，代码片段表明userId将通过数据库中的序列获取的值。

The generatorelement of the @GeneratedValueannotation denotes the name of the primary key generator. In Part1of your question, the code snippet indicates that a generatornamed incrementwill be used to obtain the primary key value. incrementis then defined in the next annotation @GenericGenerator. @GenericGeneratoris a hibernate annotation used to denote a custom generator, which can be a class or shortcut to a generator supplied by Hibernate. incrementis a shortcut to a Hibernate generator that:

注释的generator元素@GeneratedValue表示主键生成器的名称。在您问题的第 1部分中，代码片段表明将使用generator命名increment来获取主键值。 increment然后在下一个注释中定义@GenericGenerator。 @GenericGenerator是用于表示自定义生成器的 hibernate 注释，它可以是 Hibernate 提供的生成器的类或快捷方式。 increment是 Hibernate 生成器的快捷方式：

generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster.

生成 long、short 或 int 类型的标识符，这些标识符仅在没有其他进程将数据插入同一个表时才是唯一的。不要在集群中使用。

In the Third Partof your question, the code uses a hiloHibernate generator that:

在问题的第三部分中，代码使用了一个hiloHibernate 生成器：

uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.

给定一个表和列（默认分别为 hibernate_unique_key 和 next_hi）作为 hi 值的来源，使用 hi/lo 算法有效地生成 long、short 或 int 类型的标识符。hi/lo 算法生成仅对特定数据库唯一的标识符。

回答by Mohd Faizan Khan

@Entity
@Table(name="Honey")
public class Honey implements Serializable{
    private static final long serialVersionUID = 42L;
    @Id
    //@SequenceGenerator(name="honeySequence",sequenceName="HONEY_SEQ")
    @org.hibernate.annotations.GenericGenerator(name="honeySequence", strategy = "sequence", 
    parameters = { 
            @Parameter(name="sequence", value="HONEY_SEQ") } 
    )
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="honeySequence")
    private int Id;
    private String name;
    private String taste;

• @GeneratedValue is used only to get the generated value. The two arguments strategyand generatorare used to define how the value is obtained.
• @GenericGenerator is used to map a user defined sequence generator with your hibernate session.
• You can also use @SequenceGenerator which i have commented in my code. This is not a simple sequence generator but a generator which work on HILOalgorithm. Due to which you will find a lot of gaps in your sequence, like your first value will start from 50 because the default allocation size is 50.

• @GeneratedValue 仅用于获取生成的值。两个参数strategy和generator用于定义如何获取值。
• @GenericGenerator 用于将用户定义的序列生成器映射到您的休眠会话。
• 您还可以使用我在代码中注释的 @SequenceGenerator。这不是一个简单的序列生成器，而是一个适用于HILO算法的生成器。因此，您会发现序列中有很多差距，例如您的第一个值将从 50 开始，因为默认分配大小为 50。

So its better to use @GenericGenerator for your own architecture. But if you are bound to use @SequenceGenerator you have to manually edit your sequence to have two more attributes allocationSize=1and initialValue=1. And to work with these attributes you need to add apropert in your hibernate.cfg.xmlfile

因此，最好将 @GenericGenerator 用于您自己的架构。但是，如果您必须使用 @SequenceGenerator，则必须手动编辑序列以拥有另外两个属性allocationSize=1和initialValue=1。要使用这些属性，您需要在hibernate.cfg.xml文件中添加 apropert

<property name="hibernate.id.new_generator_mappings">true</property>

回答by naXa

To extend the @kevin-bowersox's answer.
Relationships between the Hibernate primary key generation strategies and specific generator respectively, as specified in org.hibernate.id.IdentifierGeneratorFactory

扩展@kevin-bowersox 的回答。
Hibernate 主键生成策略和具体生成器之间的关系，分别在org.hibernate.id.IdentifierGeneratorFactory

static {
    GENERATORS.put("uuid", UUIDHexGenerator.class);     // "deprecated" for new use
    GENERATORS.put("hilo", TableHiLoGenerator.class);   // removed in Hibernate 5
    GENERATORS.put("assigned", Assigned.class);
    GENERATORS.put("identity", IdentityGenerator.class);
    GENERATORS.put("select", SelectGenerator.class);
    GENERATORS.put("sequence", SequenceGenerator.class);
    GENERATORS.put("seqhilo", SequenceHiLoGenerator.class);
    GENERATORS.put("increment", IncrementGenerator.class);
    GENERATORS.put("foreign", ForeignGenerator.class);
    GENERATORS.put("guid", GUIDGenerator.class);
    GENERATORS.put("uuid.hex", UUIDHexGenerator.class); // uuid.hex is deprecated
    GENERATORS.put("sequence-identity", SequenceIdentityGenerator.class);
}

In Hibernate 4.3 I've found org.hibernate.id.factory.internal.DefaultIdentifierGeneratorFactoryclass with 3 more strategies:

在 Hibernate 4.3 中，我发现org.hibernate.id.factory.internal.DefaultIdentifierGeneratorFactory类还有 3 个策略：

    register("uuid2", UUIDGenerator.class);
    register("enhanced-sequence", SequenceStyleGenerator.class);
    register("enhanced-table", TableGenerator.class);

The above fifteen strategies, plus native, are sixteen generation strategies supported in Hibernate by default.

以上十五种策略，加上native，是Hibernate默认支持的十六种生成策略。

Example with native:

示例native：

@GeneratedValue(generator = "nativeGenerator")
@GenericGenerator(name = "nativeGenerator", strategy = "native")