（转）jpa 注解

转自：jpa 注解

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
有可能值AUTO,INDENTITY,SEQUENCE 和 TABLE 4种,分别表示让ORM框架自动选择,
根据数据库的Identity字段生成,根据数据库表的Sequence字段生成,以有根据一个额外的表生成主键,默认为AUTO。

    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;    
    }