java技術

1.設置Pojo為實體

01.@Entity //標識這個pojo是一個jpa實體   
02.public class Users implements Serializable {   
03.}  

2.設置表名

01.@Entity   
02.@Table(name = "users") //指定表名為users   
03.public class Users implements Serializable {   
04.}  

3.設置主鍵

01.public class Users implements Serializable {   
02.@Id   
03.private String userCode;  

4. 設置字段類型
通過@Column注解設置，包含的設置如下
.name：字段名
.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: 定義建表時創建此列的DDL
.secondaryTable: 從表名。如果此列不建在主表上（默認建在主表），該屬性定義該列所在從表的名字。

01.@Column(name = "user_code", nullable = false, length=32)//設置屬性userCode對應的字段為user_code，長度為32，非空   
02.private String userCode;   
03.@Column(name = "user_wages", nullable = true, precision=12, scale=2)//設置屬性wages對應的字段為user_wages，12位數字可保留兩位小數，可以為空   
04.private double wages;   
05.@Temporal(TemporalType.DATE)//設置為時間類型   
06.private Date joinDate;  

5.字段排序
在加載數據的時候可以為其指定順序，使用@OrderBy注解實現

01.@Table(name = "USERS")   
02.public class User {   
03.@OrderBy(name = "group_name ASC, name DESC")   
04.private List books = new ArrayList();   
05.}  

6.主鍵生成策略

01.public class Users implements Serializable {   
02.@Id   
03.@GeneratedValue(strategy=GenerationType.IDENTITY)//主鍵自增，注意，這種方式依賴于具體的數據庫，如果數據庫不支持自增主鍵，那么這個類型是沒法用的   
04.@Column(name = "user_id", nullable = false)   
05.private int userId;   
06. 
07. 
08.public class Users implements Serializable {   
09.@Id   
10.@GeneratedValue(strategy=GenerationType.TABLE)//通過一個表來實現主鍵id的自增，這種方式不依賴于具體的數據庫，可以解決數據遷移的問題   
11.@Column(name = "user_code", nullable = false)   
12.private String userCode;   
13. 
14. 
15.public class Users implements Serializable {   
16.@Id   
17.@GeneratedValue(strategy=GenerationType.SEQUENCE)//通過Sequence來實現表主鍵自增，這種方式依賴于數據庫是否有SEQUENCE，如果沒有就不能用   
18.@SequenceGenerator(name="seq_user")   
19.@Column(name = "user_id", nullable = false)   
20.private int userId;  

7.一對多映射關系
有T_One和T_Many兩個表，他們是一對多的關系，注解范例如下
主Pojo

01.@Entity   
02.@Table(name = "T_ONE")   
03.public class One implements Serializable {   
04.private static final long serialVersionUID = 1L;   
05.@Id   
06.@Column(name = "ONE_ID", nullable = false)   
07.private String oneId;   
08.@Column(name = "DESCRIPTION")   
09.private String description;   
10.@OneToMany(cascade = CascadeType.ALL, mappedBy = "oneId")//指向多的那方的pojo的關聯外鍵字段   
11.private Collection<Many> manyCollection;   

子Pojo

01.@Entity   
02.@Table(name = "T_MANY")   
03.public class Many implements Serializable {   
04.private static final long serialVersionUID = 1L;   
05.@Id   
06.@Column(name = "MANY_ID", nullable = false)   
07.private String manyId;   
08.@Column(name = "DESCRIPTION")   
09.private String description;   
10. 
11.@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   
12.@ManyToOne//設置在“一方”pojo的外鍵字段上   
13.private One oneId;   

8.多對多映射關系
貌似多對多關系不需要設置級聯，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

01.@Entity   
02.@Table(name = "T_MANYA")   
03.public class ManyA implements Serializable {   
04.private static final long serialVersionUID = 1L;   
05.@Id   
06.@Column(name = "MANYA_ID", nullable = false)   
07.private String manyaId;   
08.@Column(name = "DESCRIPTION")   
09.private String description;   
10.@ManyToMany   
11.@JoinTable(name = "TMANY1_TMANY2", joinColumns = {@JoinColumn(name = "MANYA_ID", referencedColumnName = "MANYA_ID")}, inverseJoinColumns = {@JoinColumn(name = "MANYB_ID", referencedColumnName = "MANYB_ID")})   
12.private Collection<ManyB> manybIdCollection;   

第二個Pojo

01.@Entity   
02.@Table(name = "T_MANYB")   
03.public class ManyB implements Serializable {   
04.private static final long serialVersionUID = 1L;   
05.@Id   
06.@Column(name = "MANYB_ID", nullable = false)   
07.private String manybId;   
08.@Column(name = "DESCRIPTION")   
09.private String description;   
10.@ManyToMany(mappedBy = "manybIdCollection")   
11.private Collection<ManyA> manyaIdCollection;   

9.一對一映射關系
主Pojo

01.@Entity   
02.@Table(name = "T_ONEA")   
03.public class OneA implements Serializable {   
04.private static final long serialVersionUID = 1L;   
05.@Id   
06.@Column(name = "ONEA_ID", nullable = false)   
07.private String oneaId;   
08.@Column(name = "DESCRIPTION")   
09.private String description;   
10.@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的外鍵就可以了。   
11.private OneB oneB;  
從Pojo

01.@Entity   
02.@Table(name = "T_ONEB")   
03.public class OneB implements Serializable {   
04.private static final long serialVersionUID = 1L;   
05.@Id   
06.@Column(name = "ONEA_ID", nullable = false)   
07.private String oneaId;   
08.@Column(name = "DESCRIPTION")   
09.private String description;   
10.@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其實是表T_ONEA的主鍵   
11.@OneToOne   
12.private OneA oneA;   

10 大字段

01.@Lob //對應Blob字段類型   
02.@Column(name = "PHOTO")   
03.private Serializable photo;   
04.@Lob //對應Clob字段類型   
05.@Column(name = "DESCRIPTION")   
06.private String description;  

11.瞬時字段
不需要與數據庫映射的字段，在保存的時候不需要保存倒數據庫

01.@Transient   
02.private int tempValue;   
03. 
04.public int getTempValue(){   
05.get tempValue;   
06.}   
07. 
08.public void setTempValue(int value){   
09.this.tempValue = value;   
10.}