Hibernate註解的關聯關係映射，分爲2種，即單向關聯映射和雙向關聯映射，它們最大的區別就是在查詢數據時，單向關聯只能通過一邊進行查詢，而雙向關聯兩邊都可以進行查詢。
單向關聯是指只在一方加上註解，
雙向關聯是指雙方都加上註解。這裏主要記錄雙向關聯。
1. 一對一關聯：
如有2張表，公民表Person和身份證表IdCard
一方（IdCard）：
@OneToOne(fetch=FetchType.LAZY,//延遲加載
targetEntity=IdCards.class,//目標對象
mappedBy="person",//指定由哪邊維護關係（在這指有Person表維護，‘person’指IdCards標中關聯的Person對象）
cascade=CascadeType.ALL)
另一方（Person）：
@OneToOne(fetch=FetchType.LAZY,
targetEntity=Person.class)//維護關係表
@JoinColumn(name="personId",referencedColumnName="personId",unique=true)
具體如下：
@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. 一對多關係：
一方：
@OneToMany(fetch=FetchType.LAZY,
targetEntity=User.class,
cascade=CascadeType.ALL,
orphanRemoval=true,//孤兒刪除
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：一個用戶只能屬於一個部門，一個部門裏可以有很多用戶，其註解如下，部門實體類：
@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<User> users;
........
}
用戶實體類：
@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. 多對多關係：
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”是兩者的關係表，由兩者的主鍵ID組成。通過@JoinColumn(name = "userId")指定在關係表中的字段名。
另一個多方：
@ManyToMany(fetch = FetchType.LAZY, targetEntity = User.class, cascade = CascadeType.ALL, mappedBy = "roles")
joinColumns ：指主表，inverseJoinColumns ：指匹配表如：一個用戶可以有多個角色，一個角色可以適用於多個用戶。
用戶實體類：
@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<Role> roles = new HashSet<Role>(); .....
}角色實體類：
@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<User> users = new HashSet<User>();
.....
.....
}
2. 自連接關係：
自連接一方：
@ManyToOne(fetch = FetchType.LAZY, targetEntity = Department.class)
@JoinColumn(name = "parentId",referencedColumnName="departmentId")
自連接多方：
@OneToMany(fetch=FetchType.LAZY,targetEntity=Department.class,cascade=CascadeType.ALL,orphanRemoval=true,mappedBy="parent")
@OrderBy("departmentId ASC")
因爲是自連接，在一個表裏操作，表中要增加一個父類id，用戶建立連接。
如：部門管理，一個部門可以有很多個下級部門，但是一個上級部門只能有一個上級部門，這時需要建立自連接。
部門實體類：：
@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<Department> children = new HashSet<Department>(); ....
.....
}