JPA通用策略生成器
查看JPA的源码可知：
package javax.persistence;
/**
* Defines the types of primary key generation strategies.
*
* @see GeneratedValue
*
* @since Java Persistence 1.0
*/
public enum GenerationType {
/**
* Indicates that the persistence provider must assign
* primary keys for the entity using an underlying
* database table to ensure uniqueness.
*/
TABLE,
/**
* Indicates that the persistence provider must assign
* primary keys for the entity using a database sequence.
*/
SEQUENCE,
/**
* Indicates that the persistence provider must assign
* primary keys for the entity using a database identity column.
*/
IDENTITY,
/**
* Indicates that the persistence provider should pick an
* appropriate strategy for the particular database. The
* <code>AUTO</code> generation strategy may expect a database
* resource to exist, or it may attempt to create one. A vendor
* may provide documentation on how to create such resources
* in the event that it does not support schema generation
* or cannot create the schema resource at runtime.
*/
AUTO
}查看源码知道默认类型为AUTO类型，这是由程序控制的
package javax.persistence;
import java.lang.annotation.Target;
import java.lang.annotation.Retention;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static javax.persistence.GenerationType.AUTO;
/**
* Provides for the specification of generation strategies for the
* values of primary keys.
*
* <p> The <code>GeneratedValue</code> annotation
* may be applied to a primary key property or field of an entity or
* mapped superclass in conjunction with the {@link Id} annotation.
* The use of the <code>GeneratedValue</code> annotation is only
* required to be supported for simple primary keys. Use of the
* <code>GeneratedValue</code> annotation is not supported for derived
* primary keys.
*
* @see Id
* @see TableGenerator
* @see SequenceGenerator
*
* @since Java Persistence 1.0
*/
@Target({METHOD, FIELD})
@Retention(RUNTIME)
public @interface GeneratedValue {
/**
* (Optional) The primary key generation strategy
* that the persistence provider must use to
* generate the annotated entity primary key.
*/
GenerationType strategy() default AUTO;
/**
* (Optional) The name of the primary key generator
* to use as specified in the {@link SequenceGenerator}
* or {@link TableGenerator} annotation.
* <p> Defaults to the id generator supplied by persistence provider.
*/
String generator() default "";
}JPA提供的四种标准用法为TABLE, SEQUENCE, IDENTITY, AUTO.。
TABLE：使用一个特定的数据库表格来保存主键。
SEQUENCE：根据底层数据库的序列来生成主键，条件是数据库支持序列。
IDENTITY：主键由数据库自动生成（主要是自动增长型） 。
AUTO：主键由程序控制。
详情请查看另外一篇，感觉写的很全，如有兴趣的话，可以看看：https://blog.csdn.net/tree_java/article/details/71158122