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;     
}