JAP註釋詳解

時間 2019-11-20

標籤 jap 註釋詳解简体版

原文原文鏈接

1.設置Pojo爲實體

@Entity //標識這個pojo是一個jpa實體數據庫

public class Users implements Serializable { app

} this

2.設置表名

@Entity spa

@Table(name = "users") //指定表名爲users hibernate

public class Users implements Serializable { code

} 排序

3.設置主鍵

public class Users implements Serializable { ip

@Id ci

private String userCode;get

4. 設置字段類型

經過@Column註解設置，包含的設置以下

.name：字段名

.unique：是否惟一

.nullable：是否能夠爲空

.inserttable：是否能夠插入

.updateable：是否能夠更新

.columnDefinition: 定義建表時建立此列的DDL

.secondaryTable: 從表名。若是此列不建在主表上（默認建在主表），該屬性定義該列所在從表的名字。

@Column(name = "user_code", nullable = false, length=32)//設置屬性userCode對應的字段爲user_code，長度爲32，非空

private String userCode;

@Column(name = "user_wages", nullable = true, precision=12, scale=2)//設置屬性wages對應的字段爲user_wages，12位數字可保留兩位小數，能夠爲空

private double wages;

@Temporal(TemporalType.DATE)//設置爲時間類型

private Date joinDate;

5.字段排序

在加載數據的時候能夠爲其指定順序，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數據庫，若是數據庫不支持自增主鍵，那麼這個類型是沒法用的

@Column(name = "user_id", nullable = false)

private int userId;

public class Users implements Serializable {

@Id

@GeneratedValue(strategy=GenerationType.TABLE)//經過一個表來實現主鍵id的自增，這種方式不依賴於具體的數據庫，能夠解決數據遷移的問題

@Column(name = "user_code", nullable = false)

private String userCode;

public class Users implements Serializable {

@Id

@GeneratedValue(strategy=GenerationType.SEQUENCE)//經過Sequence來實現表主鍵自增，這種方式依賴於數據庫是否有SEQUENCE，若是沒有就不能用

@SequenceGenerator(name="seq_user")

@Column(name = "user_id", nullable = false)

private int userId;

7.一對多映射關係

有T_One和T_Many兩個表，他們是一對多的關係，註解範例以下

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的關聯外鍵字段

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")//設置對應數據表的列名和引用的數據表的列名

@ManyToOne//設置在「一方」pojo的外鍵字段上

private One oneId;

8.多對多映射關係

貌似多對多關係不須要設置級聯，之前用hibernate的時候着實爲多對多的級聯頭疼了一陣子，JPA的多對多還須要實際的嘗試一下才能有所體會。

估計JPA的多對多也是能夠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;

9.一對一映射關係

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這方的設置比較簡單，只要設置好級聯和映射到從Pojo的外鍵就能夠了。

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)//設置從方指向主方的關聯外鍵，這個ONEA_ID實際上是表T_ONEA的主鍵

@OneToOne

private OneA oneA;

10. 大字段

@Lob //對應Blob字段類型

@Column(name = "PHOTO")

private Serializable photo;

@Lob //對應Clob字段類型

@Column(name = "DESCRIPTION")

private String description;

11.瞬時字段

不須要與數據庫映射的字段，在保存的時候不須要保存倒數據庫

@Transient

private int tempValue;

public int getTempValue(){

get tempValue;

}

public void setTempValue(int value){

this.tempValue = value;

}

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