JPA注解总结

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;    
}