JPA註解

 JPA 註解的幾個要點

1.設置Pojo爲實體

@Entity //標識這個pojo是一個jpa實體
public class Usersimplements Serializable {
}

2.設置表名

@Entity
@Table(name = "users")//指定表名爲users
public class Usersimplements Serializable {
}

3.設置主鍵

public class Usersimplements Serializable {
@Id
private String userCode;

4. 設置字段類型
通過@Column註解設置，包含的設置如下
.name：字段名
.unique：是否唯一
.nullable：是否可以爲空
.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。

@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，可以爲空
private double wages;
@Temporal(TemporalType.DATE)//設置爲時間類型
private Date joinDate;

5.字段排序
在加載數據的時候可以爲其指定順序，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 Usersimplements 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，如果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 Usersimplements 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數據庫，可以解決數據遷移的問題
@Column(name = "user_code", nullable =false)
private String userCode;


public class Usersimplements Serializable {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)//通過Sequence來實現表主鍵自增，這種方式依賴於數據庫是否有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，註解範例如下
主Pojo

@Entity
@Table(name = "T_ONE")
public class Oneimplements Serializable {
private staticfinallong 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 Manyimplements Serializable {
private staticfinallong 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
貌似多對多關係不需要設置級聯，以前用hibernate的時候着實爲多對多的級聯頭疼了一陣子，JPA的多對多還需要實際的嘗試一下纔能有所體會。
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 ManyAimplements Serializable {
private staticfinallong 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 ManyBimplements Serializable {
private staticfinallong 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 OneAimplements Serializable {
private staticfinallong 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的外鍵就可以了。
private OneB oneB;

從Pojo

@Entity
@Table(name = "T_ONEB")
public class OneBimplements Serializable {
private staticfinallong 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其實是表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

@Transient
private int tempValue;

public int getTempValue(){
get tempValue;
}

public void setTempValue(int value){
this.tempValue = value;
}