JPA 实体映射

一、实体基本映射

 1 /*
 2 * @Entity:将领域对象标注为一个实体,表示保存到数据库中
 3 * @@Table:保存到数据库中表名,默认表名为类名,可通过name属性命名
 4 *
 5 * */
 6 @Entity
 7 @Table(name="t_user")
 8 public class User {
 9
10
11     @Id //主键
12     @GeneratedValue // JPA自动选择合适的生成策略
13     @Column(name="id_") //列名,默认为属性名,可通过name属性指定列名
14     private Integer id;
15
16
17 /*    @Id
18     @GeneratedValue(generator="uuidGenerator")
19     @GenericGenerator(name="uuidGenerator",strategy="uuid") //UUID生成策略
20     @Column(name="id_",length=32)
21     private String id;*/
22
23
24 /*    @Id
25     @GeneratedValue(strategy = GenerationType.AUTO) //JPA自动选择合适的策略,是默认选项
26     @Column(name="id_")
27     private Integer id;*/
28
29
30     /*
31      *
32     @Id
33     @GeneratedValue(strategy=GenerationType.IDENTITY) //表示自增键字段,oracle不支持这种方式
34     @Column(name="id_")
35     private Integer id;*/
36
37
38     /*
39     @Id
40     @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "idGenerator") //通过序列产生主键,通过@SequenceGenerator注解指定序列列名,mysql不支持这种方式
41     @SequenceGenerator(name = "idGenerator",sequenceName="mySeq",allocationSize=1)
42     @Column(name="id_")
43     private Integer id;*/
44
45     /*
46     @Id
47     @GeneratedValue(strategy = GenerationType.TABLE, generator = "userGenerator")//通过表产生主键，框架借由表模拟序列产生主键，使用该策略可以使应用更易于数据库移植。
48     @TableGenerator(name = "userGenerator",table="pk_generator",
49                     pkColumnName="gen_name",
50                     valueColumnName="gen_value",
51                     pkColumnValue="user_pk",
52                     initialValue=0,
53                     allocationSize=1)
54     @Column(name="id_")
55     private Integer id;
56     */
57
58     //length:字段的长度;nullable:是否允许为空;unique:指定是否唯一性;insertable updatable:指定该字段是否插入或修改到数据库表中,对应表中需通过columnDefinition指定默认值;
59     @Column(name="name_", length=60, nullable=false,unique=true,insertable=false,updatable=false)
60     private String name;
61
62     @Column(name="address_", length=60, nullable=false)
63     private String address;
64
65     @Column(name="phone_", length=11, nullable=true,columnDefinition="CHAR(10) default ‘000‘") //columnDefinition:底层数据库的字段类型,根据不同数据库配置
66     private String phone;
67
68     @Column(name="inCome_", precision=12, scale=2) //precision:浮点型总长度;scale:小数的位数
69     private BigDecimal inCome;
70
71     @Temporal(TemporalType.DATE) //如果属性是时间类型，因为数据表对时间类型有更严格的划分，所以必须指定具体时间类型。
72     private Date birthday;
73     //Date 日期型，精确到年月日，例如“2008-08-08”
74     //Time 时间型，精确到时分秒，例如“20:00:00”
75     //Timestamp 时间戳，精确到纳秒，例如“2008-08-08 20:00:00.000000001”
76
77     @Lob
78     @Column(name="pic_")
79     @Basic(fetch=FetchType.LAZY)
80     private byte[] pic;
81
82     @Lob
83     @Column(name="note_")
84     @Basic(fetch=FetchType.LAZY)
85     private String note;
86
87     ...........
88 }    

二、实体表间映射

1、一对一实体映射：人与地址

 1 @Entity
 2 public class Person {
 3     @Id
 4     @GeneratedValue(strategy = GenerationType.AUTO)
 5     private Long id;
 6     private String name;
 7     private int age;
 8     //@OneToOne 一对一映射,级联操作，通过cascade属性设置；懒加载操作，通过fetch属性设置
 9     @OneToOne(cascade={CascadeType.REFRESH,CascadeType.REMOVE},fetch=FetchType.LAZY)
10     //@JoinColumn:Person中address字段对应的"address_id"表示Address中"aid"字段,即外键关系;如自动建表,@JoinColum注解可省略
11     @JoinColumn(name="address_id",referencedColumnName="aid")
12     //@JoinColumns(value={@JoinColumn(name="address_id",referencedColumnName="aid"),@JoinColumn(name="address_id2",referencedColumnName="aid2")})
13     private Address address;
14 ...........
15 }

 1 @Entity
 2 public class Address {
 3
 4     @Id
 5     @GeneratedValue(strategy = GenerationType.AUTO)
 6     private Long aid;
 7     private String street;
 8     private String city;
 9     private String country;
10
11
12     @Column(name="aid_")
13     public Long getAid() {
14         return aid;
15     }
16 .............
17 }

2、一对多实体映射：员工表与部门表

 1 @Entity
 2 public class Depart {
 3     @Id
 4     @GeneratedValue
 5     private Long did;
 6     private String name;
 7
 8     @OneToMany //一对多映射
 9     /*
10         通过中间表关联
11       @JoinTable 配置中间表信息,name:中间表名;
12        joinColumns:部门表与中间表间的关系,"depart_id"部门表中的主键字段作为中间表中外键(did)字段
13        inverseJoinColumns:被拥有者,即员工表与中间表的关系
14     */
15     @JoinTable(name = "depart_employee",
16         joinColumns = @JoinColumn(name = "depart_id",referencedColumnName="did"),
17         inverseJoinColumns = @JoinColumn(name = "employee_id",referencedColumnName="eid"))
18     /*
19         通过设置外键关联
20         设置"did"字段做为另一方的外键"depart_id"
21     @JoinColumn(name="depart_id",referencedColumnName="did")
22     */
23     private List<Employee> employees;
24 .....
25 }

1 @Entity
2 public class Employee {
3     @Id
4     @GeneratedValue(strategy = GenerationType.AUTO)
5     private Long eid;
6     private String  name;
7 .......
8 }

3、多对多实体映射：老师与学生

 1 @Entity
 2 public class Teacher {
 3
 4     @Id
 5     @GeneratedValue(strategy = GenerationType.AUTO)
 6     private Long tid;
 7     private String name;
 8     private Boolean gender;
 9     private int age;
10     private int height;
11     @ManyToMany
12     @JoinTable(name = "teacher_student",
13         joinColumns = @JoinColumn(name = "teacher_id",referencedColumnName="tid"),
14         inverseJoinColumns = @JoinColumn(name = "student_id",referencedColumnName="sid"))
15     private List<Student> students;
16 ..........
17 }

 1 @Entity
 2 public class Student {
 3
 4     @Id
 5     @GeneratedValue(strategy = GenerationType.AUTO)
 6     private Long sid;
 7     private String name;
 8     private Boolean gender;
 9     private int age;
10     private int height;
11
12     @ManyToMany(mappedBy = "students")
13     private List<Teacher> teachers;
14 ...............
15 }

原文地址：https://www.cnblogs.com/gdwkong/p/8608672.html