多对一，一对多的关系：
@Entity @Table(name="employee") public class Employee implements java.io.Serializable{ @Id private String sn; @ManyToOne @JoinColumn(name="identity") private Dept dept; public String getSn() { return sn; } public void setSn(String sn) { this.sn = sn; } @Column(name="name") private String name; @Column(name="password") private String password; @Column(name="status") private String status; public Dept getDept() { return dept; } public void setDept(Dept dept) { this.dept = dept; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public String getStatus() { return status; } public void setStatus(String status) { this.status = status; } }employee里的joinColumn里的值必须要和一方实体类的id名称对应,Table(name="数据库里要生成的表名"),cloumn:是数据库里要生成的列名，记住：多对一的类型要写一方实体类的类型 @Entity @Table(name="dept") public class Dept implements java.io.Serializable{ @Id @GeneratedValue(strategy=GenerationType.IDENTITY) private Integer identity; @Column(name="name") private String name; @OneToMany(mappedBy="dept",fetch=FetchType.LAZY,cascade=CascadeType.ALL) private Set<Employee> emps=new HashSet(0); public Integer getIdentity() { return identity; } public void setIdentity(Integer identity) { this.identity = identity; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Set<Employee> getEmps() { return emps; } public void setEmps(Set<Employee> emps) { this.emps = emps; } }mappedBy:写一方要生成的表名，fetch=FetchType.LAZY:是延迟加载的意思,cascadeType:ALL：就是对所有的增，删，改查都在这实现一对一关系：
@Entity @Table(name="biz_leave") public class Leave implements java.io.Serializable{ @Id @GeneratedValue(strategy=GenerationType.IDENTITY) private Integer id; @JoinColumn(name="sn") //一对一，一张请假条对应一名员工 private Employee emp; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public Employee getEmp() { return emp; } public void setEmp(Employee emp) { this.emp = emp; }@GeneratedValue():这里因为我用的是mysql数据库，所有这样写，其他数据库是不一样的写法,记住在写多对一，和一对一的时候，千万别出现两个实体类的id名称一样，除非这两个类不用被实现多对一和一对一的关系，否则它将分不清，你是要哪个表。
