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;     
} 

 @Entity --声明为一个实体bean

@Table (name= "promotion_info" )    --为实体bean映射指定表(表名="promotion_info)    
@Id                               --声明了该实体bean的标识属性    
@GeneratedValue                   --可以定义标识字段的生成策略.    
@Transient                        --将忽略这些字段和属性,不用持久化到数据库    
@Column (name= "promotion_remark" )--声明列(字段名= "promotion_total" ) 属性还包括(length= 200 等)    
@Temporal (TemporalType.TIMESTAMP)--声明时间格式    
@Enumerated                       --声明枚举    
@Version                          --声明添加对乐观锁定的支持    
@OneToOne                         --可以建立实体bean之间的一对一的关联    
@OneToMany                        --可以建立实体bean之间的一对多的关联    
@ManyToOne                        --可以建立实体bean之间的多对一的关联    
@ManyToMany                       --可以建立实体bean之间的多对多的关联    
@Formula                          --一个SQL表达式，这种属性是只读的,不在数据库生成属性(可以使用sum、average、max等)   



@Entity    
@Table (name= "promotion_info" )    
public class Promotion implements Serializable {    
   
     //AUTO--可以是identity类型的字段,或者sequence类型或者table类型,取决于不同的底层数据库    
     @Id    
     @GeneratedValue (strategy = GenerationType.AUTO)    
     private Long id;    
   
     @Column (name= "group_start_amount" )    
     private Integer groupStartAmount= 0 ;    
        
     @Column (name= "promotion_remark" ,length= 200 )    
     //@Lob 如果是文章内容可以使用 只需要把length=200去掉就可以了    
     private String remark;    
     //DATE       - java.sql.Date    
     //TIME       - java.sql.Time    
     //TIMESTAMP - java.sql.Timestamp    
     @Temporal (TemporalType.TIMESTAMP)    
     @Column (name= "start_time" )    
     private Date startTime;    
   
     //显示0 隐藏1    
     public static enum DisplayType {    
         显示,隐藏    
     }    
     @Enumerated (value = EnumType.ORDINAL) //ORDINAL序数    
     private DisplayType displayType = DisplayType.显示;    
   
     @Version    
     private Integer version;    
   
     //CascadeType.PERSIST    -- 触发级联创建(create)    
     //CascadeType.MERGE      -- 触发级联合并(update)    
     //FetchType.LAZY         -- 延迟加载    
     @ManyToOne (cascade = {CascadeType.PERSIST,CascadeType.MERGE},fetch = FetchType.LAZY)    
     private PromotionGroup promotionGroup;    
   
     //单向ManyToMany    
     //@JoinTable(关联的表名)    
     //joinColumns -- promotion关联的列的外键    
     //inverseJoinColumns -- largess 关联列的外键    
     @ManyToMany (cascade = {CascadeType.PERSIST,CascadeType.MERGE})    
     @JoinTable (name= "promotion_largess" ,joinColumns={ @JoinColumn (name= "promotion_id" )},inverseJoinColumns={ @JoinColumn (name= "largess_id" )})    
     private Set<Largess> largess;    
   
     //get set 省略....    
   
}    
@Entity    
@Table (name= "promotion_group" )    
public class PromotionGroup implements Serializable {    
     @Id    
     @GeneratedValue (strategy = GenerationType.AUTO)    
     private Long id;    
        
     //mappedBy的值"promotionGroup"指向owner(Promotion)端的关联属性,并且是双向关系    
     @OneToMany (mappedBy= "promotionGroup" ,cascade=CascadeType.ALL)    
     private List<Promotion> promotion;    
   
     //get set 省略....    
}    
@Entity    
@Table (name= "largess" )    
public class Largess implements Serializable {    
     @Id    
     @GeneratedValue (strategy = GenerationType.AUTO)    
     private Long id;    
   
     //1.sql语句中的字段和表名都应该和数据库相应,而不是类中的字段,    
     //若带有参数如la.id= id,这个=id才是类中属性    
     //2.操作字段一定要用别名    
     @Formula (select max(la.id) from largess as la)    
     private int maxId;    
   
     @Formula (select COUNT(la.id) from largess la)    
     private int count;    
   
     @Transient    
     private String img    
   
     //get set 省略....    
}  

 http://guyinglong.iteye.com/blog/520461