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(chuàng)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(shù)字可保留兩位小數(shù)，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(shù)據(jù)的時候可以為其指定順序，使用@OrderBy注解實現(xiàn)
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(shù)據(jù)庫，如果數(shù)據(jù)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(xiàn)主鍵id的自增，這種方式不依賴于具體的數(shù)據(jù)庫，可以解決數(shù)據(jù)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(xiàn)表主鍵自增，這種方式依賴于數(shù)據(jù)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(lián)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(shù)據(jù)表的列名和引用的數(shù)據(jù)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(lián)，以前用hibernate的時候著實為多對多的級聯(lián)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(lián)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(lián)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(shù)據(jù)庫映射的字段，在保存的時候不需要保存倒數(shù)據(jù)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.}