JPA注解学习总结

核心提示：JPA 注解的几个要点 1.设置Pojo为实体 @Entity //标识这个pojo是一个jpa实体 public class Users implements Serializable{ } 2.设置表名 @Entity @Table (name= users ) //指定表名为users public class Users implements Serializable{ } 3.设置主键 public

JPA 注解的几个要点

1.设置Pojo为实体

@Entity //标识这个pojo是一个jpa实体

public class Users implements Serializable {

}

2.设置表名

@Entity

@Table(name = "users") //指定表名为users

public class Users implements Serializable {

}

3.设置主键

public class Users implements Serializable {

@Id

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。

@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

private String userCode;

@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

private double wages;

@Temporal(TemporalType.DATE)//设置为时间类型

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

@Table(name = "USERS")

public class User {

@OrderBy(name = "group_name ASC, name DESC")

private List books = new ArrayList();

}

6.主键生成策略

public class Users implements Serializable {

@Id

@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

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

private int userId;

public class Users implements Serializable {

@Id

@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

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

private String userCode;

public class Users implements Serializable {

@Id

@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

@SequenceGenerator(name="seq_user")

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

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

@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的关联外键字段

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")//设置对应数据表的列名和引用的数据表的列名

@ManyToOne//设置在“一方”pojo的外键字段上

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

@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;

第二个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;

9.一对一映射关系

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。

private OneB oneB;

从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)//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

@OneToOne

private OneA oneA;

10 大字段

@Lob //对应Blob字段类型

@Column(name = "PHOTO")

private Serializable photo;

@Lob //对应Clob字段类型

@Column(name = "DESCRIPTION")

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

@Transient

private int tempValue;

public int getTempValue(){

get tempValue;

}

public void setTempValue(int value){

this.tempValue = value;

}