JPA 实体映射

一、实体基本映射

 /*
 * @Entity:将领域对象标注为一个实体,表示保存到数据库中
 * @@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
 *
 * */
 @Entity
 @Table(name="t_user")
 public class User {

     @Id //主键
     @GeneratedValue // JPA自动选择合适的生成策略
     @Column(name="id_") //列名,默认为属性名,可通过name属性指定列名
     private Integer id;    

 /*    @Id
     @GeneratedValue(generator="uuidGenerator")
     @GenericGenerator(name="uuidGenerator",strategy="uuid") //UUID生成策略
     @Column(name="id_",length=32)
     private String id;*/

 /*    @Id
     @GeneratedValue(strategy = GenerationType.AUTO) //JPA自动选择合适的策略,是默认选项
     @Column(name="id_")
     private Integer id;*/

     /*
      *
     @Id
     @GeneratedValue(strategy=GenerationType.IDENTITY) //表示自增键字段,oracle不支持这种方式
     @Column(name="id_")
     private Integer id;*/

     /*
     @Id
     @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
     @SequenceGenerator(name = "idGenerator",sequenceName="mySeq",allocationSize=1)
     @Column(name="id_")
     private Integer id;*/    

     /*
     @Id
     @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。
     @TableGenerator(name = "userGenerator",table="pk_generator",
                     pkColumnName="gen_name",
                     valueColumnName="gen_value",
                     pkColumnValue="user_pk",
                     initialValue=0,
                     allocationSize=1)
     @Column(name="id_")
     private Integer id;
     */

     //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;
     @Column(name="name_", length=60, nullable=false,unique=true,insertable=false,updatable=false)
     private String name;

     @Column(name="address_", length=60, nullable=false)
     private String address;

     @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
     private String phone;

     @Column(name="inCome_", precision=12, scale=2) //precision:浮点型总长度;scale:小数的位数
     private BigDecimal inCome;

     @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。
     private Date birthday;
     //Date 日期型，精确到年月日，例如“2008-08-08”
     //Time 时间型，精确到时分秒，例如“20:00:00”
     //Timestamp 时间戳，精确到纳秒，例如“2008-08-08 20:00:00.000000001”

     @Lob
     @Column(name="pic_")
     @Basic(fetch=FetchType.LAZY)
     private byte[] pic;

     @Lob
     @Column(name="note_")
     @Basic(fetch=FetchType.LAZY)
     private String note;

     ...........
 }    

二、实体表间映射

1、一对一实体映射：人与地址

 @Entity
 public class Person {
     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
     private String name;
     private int age;
     //@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
     @OneToOne(cascade={CascadeType.REFRESH,CascadeType.REMOVE},fetch=FetchType.LAZY)
     //@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
     @JoinColumn(name="address_id",referencedColumnName="aid")
     //@JoinColumns(value={@JoinColumn(name="address_id",referencedColumnName="aid"),@JoinColumn(name="address_id2",referencedColumnName="aid2")})
     private Address address;
 ...........
 }

 @Entity
 public class Address {

     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long aid;
     private String street;
     private String city;
     private String country;

     @Column(name="aid_")
     public Long getAid() {
         return aid;
     }
 .............
 }

2、一对多实体映射：员工表与部门表

 @Entity
 public class Depart {
     @Id
     @GeneratedValue
     private Long did;
     private String name;

     @OneToMany //一对多映射
     /*
         通过中间表关联
       @JoinTable 配置中间表信息,name:中间表名;
        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
        inverseJoinColumns:被拥有者,即员工表与中间表的关系
     */
     @JoinTable(name = "depart_employee",
         joinColumns = @JoinColumn(name = "depart_id",referencedColumnName="did"),
         inverseJoinColumns = @JoinColumn(name = "employee_id",referencedColumnName="eid"))
     /*
         通过设置外键关联
         设置"did"字段做为另一方的外键"depart_id"
     @JoinColumn(name="depart_id",referencedColumnName="did")
     */
     private List<Employee> employees;
 .....
 }

 @Entity
 public class Employee {
     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long eid;
     private String  name;
 .......
 }

3、多对多实体映射：老师与学生

 @Entity
 public class Teacher {

     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long tid;
     private String name;
     private Boolean gender;
     private int age;
     private int height;
     @ManyToMany
     @JoinTable(name = "teacher_student",
         joinColumns = @JoinColumn(name = "teacher_id",referencedColumnName="tid"),
         inverseJoinColumns = @JoinColumn(name = "student_id",referencedColumnName="sid"))
     private List<Student> students;
 ..........
 }

 @Entity
 public class Student {

     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long sid;
     private String name;
     private Boolean gender;
     private int age;
     private int height;

     @ManyToMany(mappedBy = "students")
     private List<Teacher> teachers;
 ...............
 }