JPA通用策略生成器（@GeneratedValue 四种标准用法为TABLE, SEQUENCE, IDENTITY, AUTO）

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。