JPA注解学习总结
核心提示：JPA 注解的几个要点 1.设置Pojo为实体 @Entity //标识这个pojo是一个jpa实体 public class Users implements Serializable{ } 2.设置表名 @Entity @Table (name= users ) //指定表名为users public class Users implements Serializable{ } 3.设置主键 public
JPA 注解的几个要点
1.设置Pojo为实体
@Entity //标识这个pojo是一个jpa实体
public class Users implements Serializable {
}
2.设置表名
@Entity
@Table(name = "users") //指定表名为users
public class Users implements Serializable {
}
3.设置主键
public class Users implements Serializable {
@Id
private String userCode;
4. 设置字段类型
通过@Column注解设置，包含的设置如下
.name：字段名
.unique：是否唯一
.nullable：是否可以为空
.inserttable：是否可以插入
.updateable：是否可以更新
.columnDefinition: 定义建表时创建此列的DDL
.secondaryTable: 从表名。如果此列不建在主表上（默认建在主表），该属性定义该列所在从表的名字。
@Column(name = "user_code", nullable = false, length=32)//设置属性userCode对应的字段为user_code，长度为32，非空
private String userCode;
@Column(name = "user_wages", nullable = true, precision=12, scale=2)//设置属性wages对应的字段为user_wages，12位数字可保留两位小数，可以为空
private double wages;
@Temporal(TemporalType.DATE)//设置为时间类型
private Date joinDate;
5.字段排序
在加载数据的时候可以为其指定顺序，使用@OrderBy注解实现
@Table(name = "USERS")
public class User {
@OrderBy(name = "group_name ASC, name DESC")
private List books = new ArrayList();
}
6.主键生成策略
public class Users implements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)//主键自增，注意，这种方式依赖于具体的数据库，如果数据库不支持自增主键，那么这个类型是没法用的
@Column(name = "user_id", nullable = false)
private int userId;
public class Users implements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.TABLE)//通过一个表来实现主键id的自增，这种方式不依赖于具体的数据库，可以解决数据迁移的问题
@Column(name = "user_code", nullable = false)
private String userCode;
public class Users implements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)//通过Sequence来实现表主键自增，这种方式依赖于数据库是否有SEQUENCE，如果没有就不能用
@SequenceGenerator(name="seq_user")
@Column(name = "user_id", nullable = false)
private int userId;
7.一对多映射关系
有T_One和T_Many两个表，他们是一对多的关系，注解范例如下
主Pojo
@Entity
@Table(name = "T_ONE")
public class One implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "ONE_ID", nullable = false)
private String oneId;
@Column(name = "DESCRIPTION")
private String description;
@OneToMany(cascade = CascadeType.ALL, mappedBy = "oneId")//指向多的那方的pojo的关联外键字段
private Collection<Many> manyCollection;
子Pojo
@Entity
@Table(name = "T_MANY")
public class Many implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "MANY_ID", nullable = false)
private String manyId;
@Column(name = "DESCRIPTION")
private String description;
@JoinColumn(name = "ONE_ID", referencedColumnName = "ONE_ID")//设置对应数据表的列名和引用的数据表的列名
@ManyToOne//设置在“一方”pojo的外键字段上
private One oneId;
8.多对多映射关系
貌似多对多关系不需要设置级联，以前用hibernate的时候着实为多对多的级联头疼了一阵子，JPA的多对多还需要实际的尝试一下才能有所体会。
估计JPA的多对多也是可以转换成两个一对多的。
第一个Pojo
@Entity
@Table(name = "T_MANYA")
public class ManyA implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "MANYA_ID", nullable = false)
private String manyaId;
@Column(name = "DESCRIPTION")
private String description;
@ManyToMany
@JoinTable(name = "TMANY1_TMANY2", joinColumns = {@JoinColumn(name = "MANYA_ID", referencedColumnName = "MANYA_ID")}, inverseJoinColumns = {@JoinColumn(name = "MANYB_ID", referencedColumnName = "MANYB_ID")})
private Collection<ManyB> manybIdCollection;
第二个Pojo
@Entity
@Table(name = "T_MANYB")
public class ManyB implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "MANYB_ID", nullable = false)
private String manybId;
@Column(name = "DESCRIPTION")
private String description;
@ManyToMany(mappedBy = "manybIdCollection")
private Collection<ManyA> manyaIdCollection;
9.一对一映射关系
主Pojo
@Entity
@Table(name = "T_ONEA")
public class OneA implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "ONEA_ID", nullable = false)
private String oneaId;
@Column(name = "DESCRIPTION")
private String description;
@OneToOne(cascade = CascadeType.ALL, mappedBy = "oneA")//主Pojo这方的设置比较简单，只要设置好级联和映射到从Pojo的外键就可以了。
private OneB oneB;
从Pojo
@Entity
@Table(name = "T_ONEB")
public class OneB implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "ONEA_ID", nullable = false)
private String oneaId;
@Column(name = "DESCRIPTION")
private String description;
@JoinColumn(name = "ONEA_ID", referencedColumnName = "ONEA_ID", insertable = false, updatable = false)//设置从方指向主方的关联外键，这个ONEA_ID其实是表T_ONEA的主键
@OneToOne
private OneA oneA;
10 大字段
@Lob //对应Blob字段类型
@Column(name = "PHOTO")
private Serializable photo;
@Lob //对应Clob字段类型
@Column(name = "DESCRIPTION")
private String description;
11.瞬时字段
不需要与数据库映射的字段，在保存的时候不需要保存倒数据库
@Transient
private int tempValue;
public int getTempValue(){
get tempValue;
}
public void setTempValue(int value){
this.tempValue = value;
}
@Entity --声明为一个实体bean @Table (name= "promotion_info" ) --为实体bean映射指定表(表名="promotion_info) @Id --声明了该实体bean的标识属性 @GeneratedValue --可以定义标识字段的生成策略. @Transient --将忽略这些字段和属性,不用持久化到数据库 @Column (name= "promotion_remark" )--声明列(字段名= "promotion_total" ) 属性还包括(length= 200 等) @Temporal (TemporalType.TIMESTAMP)--声明时间格式 @Enumerated --声明枚举 @Version --声明添加对乐观锁定的支持 @OneToOne --可以建立实体bean之间的一对一的关联 @OneToMany --可以建立实体bean之间的一对多的关联 @ManyToOne --可以建立实体bean之间的多对一的关联 @ManyToMany --可以建立实体bean之间的多对多的关联 @Formula --一个SQL表达式，这种属性是只读的,不在数据库生成属性(可以使用sum、average、max等) @Entity @Table (name= "promotion_info" ) public class Promotion implements Serializable { //AUTO--可以是identity类型的字段,或者sequence类型或者table类型,取决于不同的底层数据库 @Id @GeneratedValue (strategy = GenerationType.AUTO) private Long id; @Column (name= "group_start_amount" ) private Integer groupStartAmount= 0 ; @Column (name= "promotion_remark" ,length= 200 ) //@Lob 如果是文章内容可以使用 只需要把length=200去掉就可以了 private String remark; //DATE - java.sql.Date //TIME - java.sql.Time //TIMESTAMP - java.sql.Timestamp @Temporal (TemporalType.TIMESTAMP) @Column (name= "start_time" ) private Date startTime; //显示0 隐藏1 public static enum DisplayType { 显示,隐藏 } @Enumerated (value = EnumType.ORDINAL) //ORDINAL序数 private DisplayType displayType = DisplayType.显示; @Version private Integer version; //CascadeType.PERSIST -- 触发级联创建(create) //CascadeType.MERGE -- 触发级联合并(update) //FetchType.LAZY -- 延迟加载 @ManyToOne (cascade = {CascadeType.PERSIST,CascadeType.MERGE},fetch = FetchType.LAZY) private PromotionGroup promotionGroup; //单向ManyToMany //@JoinTable(关联的表名) //joinColumns -- promotion关联的列的外键 //inverseJoinColumns -- largess 关联列的外键 @ManyToMany (cascade = {CascadeType.PERSIST,CascadeType.MERGE}) @JoinTable (name= "promotion_largess" ,joinColumns={ @JoinColumn (name= "promotion_id" )},inverseJoinColumns={ @JoinColumn (name= "largess_id" )}) private Set<Largess> largess; //get set 省略.... } @Entity @Table (name= "promotion_group" ) public class PromotionGroup implements Serializable { @Id @GeneratedValue (strategy = GenerationType.AUTO) private Long id; //mappedBy的值"promotionGroup"指向owner(Promotion)端的关联属性,并且是双向关系 @OneToMany (mappedBy= "promotionGroup" ,cascade=CascadeType.ALL) private List<Promotion> promotion; //get set 省略.... } @Entity @Table (name= "largess" ) public class Largess implements Serializable { @Id @GeneratedValue (strategy = GenerationType.AUTO) private Long id; //1.sql语句中的字段和表名都应该和数据库相应,而不是类中的字段, //若带有参数如la.id= id,这个=id才是类中属性 //2.操作字段一定要用别名 @Formula (select max(la.id) from largess as la) private int maxId; @Formula (select COUNT(la.id) from largess la) private int count; @Transient private String img //get set 省略.... }