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。