Hibernate的关联关系注解映射（一对一、一对多、多对一、多对多、自连接）
Hibernate注解的关联关系映射，分为2种，即单向关联映射和双向关联映射，它们最大的区别就是在查询数据时，单向关联只能通过一边进行查询，而双向关联两边都可以进行查询。
单向关联是指只在一方加上注解，
双向关联是指双方都加上注解。这里主要记录双向关联。
1. 一对一关联：
如有2张表，公民表Person和身份证表IdCard
一方（IdCard）：
@OneToOne(fetch=FetchType.LAZY,//延迟加载
targetEntity=IdCards.class,//目标对象
mappedBy="person",//指定由哪边维护关系（在这指有Person表维护，‘person’指IdCards标中关联的Person对象）
cascade=CascadeType.ALL)
另一方（Person）：
@OneToOne(fetch=FetchType.LAZY,
targetEntity=Person.class)//维护关系表
@JoinColumn(name="personId",referencedColumnName="personId",unique=true)
具体如下：
@Entity
@Table(name = "tb_person")
public class Person implements Serializable {
private static final long serialVersionUID = 576770011136181361L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long personId; @OneToOne(fetch=FetchType.LAZY,targetEntity=IdCards.class,mappedBy="person",cascade=CascadeType.ALL) private IdCard idcard;
@Entity
@Table(name = "tb_idcard")
public class Idcard implements Serializable {
private static final long serialVersionUID = 576770011136181369L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long idcardId; @OneToOne(fetch=FetchType.LAZY,
targetEntity=Person.class)
@JoinColumn(name="personId",referencedColumnName="personId",unique=true)
private Person person;2. 一对多关系：
一方：
@OneToMany(fetch=FetchType.LAZY,
targetEntity=User.class,
cascade=CascadeType.ALL,
orphanRemoval=true,//孤儿删除
mappedBy="department")
cascade=CascadeType.ALL，,即交由多的一方控制；@ManyToOne(fetch = FetchType.LAZY,targetEntity = Department.class)@JoinColumn(name = "departmentId",referencedColumnName="departmentId")
如：一个用户只能属于一个部门，一个部门里可以有很多用户，其注解如下，部门实体类：
@Entity
@Table(name = "tb_department")
public class Department implements Serializable {
private static final long serialVersionUID = -3489189232910430049L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long departmentId;
private String name;
private String description;
@OneToMany(fetch=FetchType.LAZY,targetEntity=User.class,cascade=CascadeType.ALL,orphanRemoval=true,mappedBy="department")
private Set users;
........
}
用户实体类：
@Entity
@Table(name="tb_user")
public class User implements Serializable {
private static final long serialVersionUID = 576770011136181361L;
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Long userId;
@ManyToOne(fetch = FetchType.LAZY,targetEntity = Department.class)
@JoinColumn(name = "departmentId",referencedColumnName="departmentId")
private Department department;
private String loginName;
........
........2. 多对多关系：
多方：
@ManyToMany(fetch=FetchType.LAZY,targetEntity=Role.class)@JoinTable(name = "tb_user_role", joinColumns = @JoinColumn(name = "userId",referencedColumnName="userId") , inverseJoinColumns = @JoinColumn(name = "roleId",referencedColumnName="roleId"))
其中“tb_user_role”是两者的关系表，由两者的主键ID组成。通过@JoinColumn(name = "userId")指定在关系表中的字段名。
另一个多方：
@ManyToMany(fetch = FetchType.LAZY, targetEntity = User.class, cascade = CascadeType.ALL, mappedBy = "roles")
joinColumns ：指主表，inverseJoinColumns ：指匹配表如：一个用户可以有多个角色，一个角色可以适用于多个用户。
用户实体类：
@Entity
@Table(name="tb_user")
public class User implements Serializable {
private static final long serialVersionUID = 576770011136181361L;
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Long userId;
private String loginName;
private String name;
private String gender;
private String phone;
private String email;
private String description;
@ManyToMany(fetch=FetchType.LAZY,targetEntity=Role.class) @JoinTable(name = "tb_user_role", joinColumns = @JoinColumn(name = "userId",referencedColumnName="userId") , inverseJoinColumns = @JoinColumn(name = "roleId",referencedColumnName="roleId"))
private Set roles = new HashSet(); .....
}角色实体类：
@Entity
@Table(name = "tb_role")
public class Role implements Serializable {
private static final long serialVersionUID = -7625883839295666830L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long roleId;
private String name;
private String description;
@ManyToMany(fetch = FetchType.LAZY, targetEntity = User.class, cascade = CascadeType.ALL, mappedBy = "roles")
private Set users = new HashSet();
.....
.....
}
2. 自连接关系：
自连接一方：
@ManyToOne(fetch = FetchType.LAZY, targetEntity = Department.class)
@JoinColumn(name = "parentId",referencedColumnName="departmentId")
自连接多方：
@OneToMany(fetch=FetchType.LAZY,targetEntity=Department.class,cascade=CascadeType.ALL,orphanRemoval=true,mappedBy="parent")
@OrderBy("departmentId ASC")
因为是自连接，在一个表里操作，表中要增加一个父类id，用户建立连接。
如：部门管理，一个部门可以有很多个下级部门，但是一个上级部门只能有一个上级部门，这时需要建立自连接。
部门实体类：：
@Entity
@Table(name = "tb_department")
public class Department implements Serializable {
private static final long serialVersionUID = -3489189232910430049L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long departmentId;
private String name;
private String description;
@ManyToOne(fetch = FetchType.LAZY, targetEntity = Department.class)
@JoinColumn(name = "parentId",referencedColumnName="departmentId")
private Department parent;
@OneToMany(fetch=FetchType.LAZY,targetEntity=Department.class,cascade=CascadeType.ALL,orphanRemoval=true,mappedBy="parent")
@OrderBy("departmentId ASC")
private Set children = new HashSet(); ....
.....
}