hibenrate(六)ID的生成策略
ID:
@GeneratedValue注解可以定义该标识符的生成策略:
AUTO - 可以是identity column类型,或者sequence类型或者table类型,取决于不同的底层数据库. （常用）
TABLE - 使用表保存id值 (不重要)
IDENTITY - identity column
SEQUENCE - sequence
例：
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)public Long getId() { ... }联合主键:
下面是定义组合主键的几种语法:
a)将组件类注解为@Embeddable,并将组件的属性注解为@Id
*StudentPK.java
@Embeddablepublic class StudentPK implements java.io.Serializable{private int id;public int getId() {return id;}…………*Student.java
Private StudentPK pk;@Idpublic StudentPK getPK() {return PK;}b)将组件的属性注解为@EmbeddedId
*StudentPK.java
public class StudentPK implements java.io.Serializable{private int id;public int getId() {return id;}…………*Student.java
Private StudentPK pk;@EmbeddedIdpublic StudentPK getPK() {return PK;}c)将类注解为@IdClass,并将该实体中所有属于主键的属性都注解为@Id(常用)[/b[b]]*StudentPK.java
public class StudentPK implements java.io.Serializable{private int id;public int getId() {return id;}…………*Student.java
@IdClass(StudentPK.class)public class Student {private int id;private String name;private int age;@Idpublic int getId() {return id;}@Idpublic String getName() {return name;}………………}