1.设置Pojo为实体
01.@Entity //标识这个pojo是一个jpa实体
02.public class Users implements Serializable {
03.}
2.设置表名
01.@Entity
02.@Table(name = "users") //指定表名为users
03.public class Users implements Serializable {
04.}
3.设置主键
01.public class Users implements Serializable {
02.@Id
03.private String userCode;
4. 设置字段类型
通过@Column注解设置，包含的设置如下
.name：字段名
.unique：是否唯一
.nullable：是否可以为空
.inserttable：是否可以插入
.updateable：是否可以更新
.columnDefinition: 定义建表时创建此列的DDL
.secondaryTable: 从表名。如果此列不建在主表上（默认建在主表），该属性定义该列所在从表的名字。
01.@Column(name = "user_code", nullable = false, length=32)//设置属性userCode对应的字段为user_code，长度为32，非空
02.private String userCode;
03.@Column(name = "user_wages", nullable = true, precision=12, scale=2)//设置属性wages对应的字段为user_wages，12位数字可保留两位小数，可以为空
04.private double wages;
05.@Temporal(TemporalType.DATE)//设置为时间类型
06.private Date joinDate;
5.字段排序
在加载数据的时候可以为其指定顺序，使用@OrderBy注解实现
01.@Table(name = "USERS")
02.public class User {
03.@OrderBy(name = "group_name ASC, name DESC")
04.private List books = new ArrayList();
05.}
6.主键生成策略
01.public class Users implements Serializable {
02.@Id
03.@GeneratedValue(strategy=GenerationType.IDENTITY)//主键自增，注意，这种方式依赖于具体的数据库，如果数据库不支持自增主键，那么这个类型是没法用的
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)//通过一个表来实现主键id的自增，这种方式不依赖于具体的数据库，可以解决数据迁移的问题
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)//通过Sequence来实现表主键自增，这种方式依赖于数据库是否有SEQUENCE，如果没有就不能用
18.@SequenceGenerator(name="seq_user")
19.@Column(name = "user_id", nullable = false)
20.private int userId;
7.一对多映射关系
有T_One和T_Many两个表，他们是一对多的关系，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的关联外键字段
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")//设置对应数据表的列名和引用的数据表的列名
12.@ManyToOne//设置在“一方”pojo的外键字段上
13.private One oneId;
8.多对多映射关系
貌似多对多关系不需要设置级联，以前用hibernate的时候着实为多对多的级联头疼了一阵子，JPA的多对多还需要实际的尝试一下才能有所体会。
估计JPA的多对多也是可以转换成两个一对多的。
第一个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;
第二个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;
9.一对一映射关系
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这方的设置比较简单，只要设置好级联和映射到从Pojo的外键就可以了。
11.private OneB oneB;
从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)//设置从方指向主方的关联外键，这个ONEA_ID其实是表T_ONEA的主键
11.@OneToOne
12.private OneA oneA;
10 大字段
01.@Lob //对应Blob字段类型
02.@Column(name = "PHOTO")
03.private Serializable photo;
04.@Lob //对应Clob字段类型
05.@Column(name = "DESCRIPTION")
06.private String description;
11.瞬时字段
不需要与数据库映射的字段，在保存的时候不需要保存倒数据库
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.}