JPA 注解以及pojo转为表 (二)

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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.}