JPA註解介紹
阿新 • • 發佈：2019-02-19
JPA 註解的幾個要點
1.設定Pojo為實體 @Entity //標識這個pojo是一個jpa實體 public class Users implements Serializable{ }
2.設定表名 @Entity @Table (name= users ) //指定表名為users public class Users implements Serializable{ }
3.設定主鍵 public
JPA 註解的幾個要點
1.設定Pojo為實體
@Entity //標識這個pojo是一個jpa實體
public class Users implements Serializable {}
2.設定表名
@Entity
@Table(name = "users") //指定表名為users
public class Users implements Serializable {
}
3.設定主鍵
public class Users implements Serializable {
@Id
private String userCode;
4. 設定欄位型別
通過@Column註解設定，包含的設定如下
.name：欄位名
.unique：是否唯一
.nullable：是否可以為空
.inserttable：是否可以插入.updateable：是否可以更新
.columnDefinition: 定義建表時建立此列的DDL
.secondaryTable: 從表名。如果此列不建在主表上（預設建在主表），該屬性定義該列所在從表的名字。
@Column(name = "user_code", nullable = false, length=32)//設定屬性userCode對應的欄位為user_code，長度為32，非空
private String userCode;
@Column(name = "user_wages", nullable = true, precision=12, scale=2)//設定屬性wages對應的欄位為user_wages，12位數字可保留兩位小數，可以為空private double wages;
@Temporal(TemporalType.DATE)//設定為時間型別
private Date joinDate;
5.欄位排序
在載入資料的時候可以為其指定順序，使用@OrderBy註解實現
@Table(name = "USERS")
public class User {
@OrderBy(name = "group_name ASC, name DESC")
private List books = new ArrayList();
}
6.主鍵生成策略
public class Users implements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)//主鍵自增，注意，這種方式依賴於具體的資料庫，如果資料庫不支援自增主鍵，那麼這個型別是沒法用的
@Column(name = "user_id", nullable = false)
private int userId;
public class Users implements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.TABLE)//通過一個表來實現主鍵id的自增，這種方式不依賴於具體的資料庫，可以解決資料遷移的問題
@Column(name = "user_code", nullable = false)
private String userCode;
public class Users implements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)//通過Sequence來實現表主鍵自增，這種方式依賴於資料庫是否有SEQUENCE，如果沒有就不能用
@SequenceGenerator(name="seq_user")
@Column(name = "user_id", nullable = false)
private int userId;
7.一對多對映關係
有T_One和T_Many兩個表，他們是一對多的關係，註解範例如下
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的關聯外來鍵欄位
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")//設定對應資料表的列名和引用的資料表的列名
@ManyToOne//設定在“一方”pojo的外來鍵欄位上
private One oneId;
8.多對多對映關係
貌似多對多關係不需要設定級聯，以前用hibernate的時候著實為多對多的級聯頭疼了一陣子，JPA的多對多還需要實際的嘗試一下才能有所體會。
估計JPA的多對多也是可以轉換成兩個一對多的。
第一個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;
第二個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;
9.一對一對映關係
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這方的設定比較簡單，只要設定好級聯和對映到從Pojo的外來鍵就可以了。
private OneB oneB;
從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)//設定從方指向主方的關聯外來鍵，這個ONEA_ID其實是表T_ONEA的主鍵
@OneToOne
private OneA oneA;
10 大欄位
@Lob //對應Blob欄位型別
@Column(name = "PHOTO")
private Serializable photo;
@Lob //對應Clob欄位型別
@Column(name = "DESCRIPTION")
private String description;
11.瞬時欄位
不需要與資料庫對映的欄位，在儲存的時候不需要儲存倒資料庫
@Transient
private int tempValue;
public int getTempValue(){
get tempValue;
}
public void setTempValue(int value){
this.tempValue = value;
}
@Entity --宣告為一個實體bean @Table (name= "promotion_info" ) --為實體bean對映指定表(表名="promotion_info) @Id --聲明瞭該實體bean的標識屬性 @GeneratedValue --可以定義標識欄位的生成策略. @Transient --將忽略這些欄位和屬性,不用持久化到資料庫 @Column (name= "promotion_remark" )--宣告列(欄位名= "promotion_total" ) 屬性還包括(length= 200 等) @Temporal (TemporalType.TIMESTAMP)--宣告時間格式 @Enumerated --宣告列舉 @Version --宣告新增對樂觀鎖定的支援 @OneToOne --可以建立實體bean之間的一對一的關聯 @OneToMany --可以建立實體bean之間的一對多的關聯 @ManyToOne --可以建立實體bean之間的多對一的關聯 @ManyToMany --可以建立實體bean之間的多對多的關聯 @Formula --一個SQL表示式，這種屬性是隻讀的,不在資料庫生成屬性(可以使用sum、average、max等) @Entity @Table (name= "promotion_info" ) public class Promotion implements Serializable { //AUTO--可以是identity型別的欄位,或者sequence型別或者table型別,取決於不同的底層資料庫 @Id @GeneratedValue (strategy = GenerationType.AUTO) private Long id; @Column (name= "group_start_amount" ) private Integer groupStartAmount= 0 ; @Column (name= "promotion_remark" ,length= 200 ) //@Lob 如果是文章內容可以使用 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; //顯示0 隱藏1 public static enum DisplayType { 顯示,隱藏 } @Enumerated (value = EnumType.ORDINAL) //ORDINAL序數 private DisplayType displayType = DisplayType.顯示; @Version private Integer version; //CascadeType.PERSIST -- 觸發級聯建立(create) //CascadeType.MERGE -- 觸發級聯合並(update) //FetchType.LAZY -- 延遲載入 @ManyToOne (cascade = {CascadeType.PERSIST,CascadeType.MERGE},fetch = FetchType.LAZY) private PromotionGroup promotionGroup; //單向ManyToMany //@JoinTable(關聯的表名) //joinColumns -- promotion關聯的列的外來鍵 //inverseJoinColumns -- largess 關聯列的外來鍵 @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)端的關聯屬性,並且是雙向關係 @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語句中的欄位和表名都應該和資料庫相應,而不是類中的欄位, //若帶有引數如la.id= id,這個=id才是類中屬性 //2.操作欄位一定要用別名 @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.... }