@Entity//标识这个pojo是一个jpa实体
@Table(name = "users") //指定表名为users
public class Users implements Serializable {
	@Id//设置主键
	private String userId;
	//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;

	@OrderBy(name = "group_name ASC, name DESC")//[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
	private List books = new ArrayList();
}

//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; 
}

//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; 

//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;

//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