hibernate中映射文件中的关联关系——多对一/一对多

双向关联：

员工知道所属部门，部门知道包括哪些员工

1.  package oneTomany;

3.  import java.util.HashSet;
4.  import java.util.Set;

6.  public class Department {
7.  private int id;
8.  private String name;
9.  private Set<Employee> employees=new HashSet<Employee>();
10.  public int getId() {
11.  return id;
12.  }
13.  public void setId(int id) {
14.  this.id = id;
15.  }
16.  public String getName() {
17.  return name;
18.  }
19.  public void setName(String name) {
20.  this.name = name;
21.  }
22.  public Set<Employee> getEmployees() {
23.  return employees;
24.  }
25.  public void setEmployees(Set<Employee> employees) {
26.  this.employees = employees;
27.  }
28.  @Override
29.  public String toString() {
30.  // TODO Auto-generated method stub
31.  return "[Department:id="+id+",]";
32.  }

34.  }

1.  package oneTomany;

3.  public class Employee {
4.  private int id;
5.  private String name;
6.  private Department department; // 关联的对象
7.  public int getId() {
8.  return id;
9.  }
10.  public void setId(int id) {
11.  this.id = id;
12.  }
13.  public String getName() {
14.  return name;
15.  }
16.  public void setName(String name) {
17.  this.name = name;
18.  }
19.  public Department getDepartment() {
20.  return department;
21.  }
22.  public void setDepartment(Department department) {
23.  this.department = department;
24.  }
25.  @Override
26.  public String toString() {
27.  // TODO Auto-generated method stub
28.  return "[Employee:id="+id+",]";
29.  }
30.  }

Department.hbm.xml

<?xml version="1.0" encoding="gbk"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<!--

This mapping demonstrates

(1) a table-per-subclass mapping strategy

(2) a simple component mapping

(3) recursive associations withing an inheritance tree

-->

<hibernate-mapping   package="oneTomany">
<!--
name:类名
table:表名
-->
<class name="Department" table="t_department">

<id name="id" type="int" column="id" >
<generator class="native"/> <!--表示自动增长-->
</id>

<property name="name" type="string" column="name" />
<!--employees属性 ，表达的是本类与Employee类的一对多的关系-->
<set name="employees" >
<key column="departmentId"></key>
<one-to-many class="Employee"></one-to-many>
</set>

</class>

</hibernate-mapping>

Employee.hbm.xml

<?xml version="1.0" encoding="gbk"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<!--

This mapping demonstrates

(1) a table-per-subclass mapping strategy

(2) a simple component mapping

(3) recursive associations withing an inheritance tree

-->

<hibernate-mapping   package="oneTomany">
<!--
name:类名
table:表名
-->
<class name="Employee" table="t_employee">

<id name="id" type="int" column="id" >
<generator class="native"/> <!--表示自动增长-->
</id>

<property name="name" type="string" column="name" />

<!--Department属性，表达的是本类与Department的多对一的关系-->
<many-to-one name="department" class="Department" column="departmentId"></many-to-one>
</class>

</hibernate-mapping>

也可以单向关联，可以让一对多的一方放弃维护关联关系，在 set 标签中添加属性 inverse=”true”,inverse 默认为 false，为 true 时表示由对方维护关联关系，本方不维护。可以减少 sql 语句。

在保存的时候，被依赖的在前边保存，不被依赖的在后边保存

1.  // 保存
2.  session.save(department);
3.  session.save(employee1);
4.  session.save(employee2);

department 保存后，保存 employee 时就直接保存了外键的值，当先保存 employee 是，不知道外键的值，多以保存完 depatment 后，再保存外键的值。