JPA注解
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http://blog.csdn.net/sensego/article/details/6082420
JPA 注解的几个要点
1.设置Pojo为实体
1. @Entity //标识这个pojo是一个jpa实体
2. public class Users implements Serializable {
3. }
2.设置表名
1. @Entity
2. @Table(name = "users") //指定表名为users
3. public class Users implements Serializable {
4. }
3.设置主键
1. public class Users implements Serializable {
2. @Id
3. 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。
1. @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
2. private String userCode;
3. @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
4. private double wages;
5. @Temporal(TemporalType.DATE)//设置为时间类型
6. 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
1. @Table(name = "USERS")
2. public class User {
3. @OrderBy(name = "group_name ASC, name DESC")
4. private List books = new ArrayList();
5. }
6.主键生成策略
1. public class Users implements Serializable {
2. @Id
3. @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
4. @Column(name = "user_id", nullable = false)
5. private int userId;
6.
7.
8. public class Users implements Serializable {
9. @Id
10. @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
11. @Column(name = "user_code", nullable = false)
12. private String userCode;
13.
14.
15. public class Users implements Serializable {
16. @Id
17. @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
18. @SequenceGenerator(name="seq_user")
19. @Column(name = "user_id", nullable = false)
20. 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
1. @Entity
2. @Table(name = "T_ONE")
3. public class One implements Serializable {
4. private static final long serialVersionUID = 1L;
5. @Id
6. @Column(name = "ONE_ID", nullable = false)
7. private String oneId;
8. @Column(name = "DESCRIPTION")
9. 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
1. @Entity
2. @Table(name = "T_MANY")
3. public class Many implements Serializable {
4. private static final long serialVersionUID = 1L;
5. @Id
6. @Column(name = "MANY_ID", nullable = false)
7. private String manyId;
8. @Column(name = "DESCRIPTION")
9. 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
1. @Entity
2. @Table(name = "T_MANYA")
3. public class ManyA implements Serializable {
4. private static final long serialVersionUID = 1L;
5. @Id
6. @Column(name = "MANYA_ID", nullable = false)
7. private String manyaId;
8. @Column(name = "DESCRIPTION")
9. 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
1. @Entity
2. @Table(name = "T_MANYB")
3. public class ManyB implements Serializable {
4. private static final long serialVersionUID = 1L;
5. @Id
6. @Column(name = "MANYB_ID", nullable = false)
7. private String manybId;
8. @Column(name = "DESCRIPTION")
9. private String description;
10. @ManyToMany(mappedBy = "manybIdCollection")
11. private Collection<ManyA> manyaIdCollection;
9.一对一映射关系
主Pojo
1. @Entity
2. @Table(name = "T_ONEA")
3. public class OneA implements Serializable {
4. private static final long serialVersionUID = 1L;
5. @Id
6. @Column(name = "ONEA_ID", nullable = false)
7. private String oneaId;
8. @Column(name = "DESCRIPTION")
9. 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
1. @Entity
2. @Table(name = "T_ONEB")
3. public class OneB implements Serializable {
4. private static final long serialVersionUID = 1L;
5. @Id
6. @Column(name = "ONEA_ID", nullable = false)
7. private String oneaId;
8. @Column(name = "DESCRIPTION")
9. 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
1. @Lob //对应Blob字段类型
2. @Column(name = "PHOTO")
3. private Serializable photo;
4. @Lob //对应Clob字段类型
5. @Column(name = "DESCRIPTION")
6. 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
1. @Transient
2. private int tempValue;
3.
4. public int getTempValue(){
5. get tempValue;
6. }
7.
8. public void setTempValue(int value){
9. this.tempValue = value;
10. }
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