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; 
...............
}