JPA 實體映射

一、實體基本映射

/*
* @Entity:將領域對象標注為一個實體,表示保存到數據庫中
* @@Table:保存到數據庫中表名,默認表名為類名,可通過name屬性命名
*
* */
@Entity
@Table(name="t_user")
public class User {


    @Id //主鍵
    @GeneratedValue // JPA自動選擇合適的生成策略
    @Column(name="id_") //列名,默認為屬性名,可通過name屬性指定列名
    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自動選擇合適的策略,是默認選項
    @Column(name="id_")
    private Integer id;*/
    
        
    /*
     *     
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY) //表示自增鍵字段,oracle不支持這種方式
    @Column(name="id_")
    private Integer id;*/
    
    
    /*    
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "idGenerator") //通過序列產生主鍵,通過@SequenceGenerator注解指定序列列名,mysql不支持這種方式
    @SequenceGenerator(name = "idGenerator",sequenceName="mySeq",allocationSize=1)
    @Column(name="id_")
    private Integer id;*/    
    
    /*    
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "userGenerator")//通過表產生主鍵，框架借由表模擬序列產生主鍵，使用該策略可以使應用更易於數據庫移植。
    @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:字段的長度;nullable:是否允許為空;unique:指定是否唯一性;insertable updatable:指定該字段是否插入或修改到數據庫表中,對應表中需通過columnDefinition指定默認值;
    @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:底層數據庫的字段類型,根據不同數據庫配置
    private String phone;
    
    @Column(name="inCome_", precision=12, scale=2) //precision:浮點型總長度;scale:小數的位數
    private BigDecimal inCome;
    
    @Temporal(TemporalType.DATE) //如果屬性是時間類型，因為數據表對時間類型有更嚴格的划分，所以必須指定具體時間類型。
    private Date birthday;
    //Date 日期型，精確到年月日，例如“2008-08-08”
    //Time 時間型，精確到時分秒，例如“20:00:00”
    //Timestamp 時間戳，精確到納秒，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、實體表間映射

1、一對一實體映射：人與地址

@Entity
public class Person {
    @Id 
    @GeneratedValue(strategy = GenerationType.AUTO) 
    private Long id; 
    private String name;
    private int age;
    //@OneToOne 一對一映射,級聯操作，通過cascade屬性設置；懶加載操作，通過fetch屬性設置
    @OneToOne(cascade={CascadeType.REFRESH,CascadeType.REMOVE},fetch=FetchType.LAZY)
    //@JoinColumn:Person中address字段對應的"address_id"表示Address中"aid"字段,即外鍵關系;如自動建表,@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、一對多實體映射：員工表與部門表

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

    @OneToMany //一對多映射
    /*
        通過中間表關聯
      @JoinTable 配置中間表信息,name:中間表名;
       joinColumns:部門表與中間表間的關系,"depart_id"部門表中的主鍵字段作為中間表中外鍵(did)字段
       inverseJoinColumns:被擁有者,即員工表與中間表的關系
    */
    @JoinTable(name = "depart_employee", 
        joinColumns = @JoinColumn(name = "depart_id",referencedColumnName="did"), 
        inverseJoinColumns = @JoinColumn(name = "employee_id",referencedColumnName="eid"))
    /*
        通過設置外鍵關聯
        設置"did"字段做為另一方的外鍵"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、多對多實體映射：老師與學生

@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; 
...............
}