hibernate多对一的操作解析

在hibernate的关联操作中有很多关系，其中多对一关系是最常见的。我们看这两个表。

这里有部门表和员工表。

那么我们可以这么说一个部门可以有多个员工。这就是1对多的关系。这是我们站在部门表的角度上看的。

那么我们也可以说多个员工是一个部门的，这就是多对一的关系。这就是我们站在员工表的角度上说的。

下面我做一个多对一的例子，实现两表的增加和查询操作。

1.      部门对象映射类

package com.fish.testdao;

public class Department {

private Integer id;//部门编号

private String name;//部门名称

public Integer getId() {

return id;

}

public void setId(Integer id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

}

2.      员工对象映射对象类

package com.fish.testdao;

public class Employee {

private Integer id;//员工编号

private String name;//员工姓名

private Department department;//部门类（有这个我们就可以找到部门表的信息）

public Integer getId() {

return id;

}

public void setId(Integer id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public Department getDepartment() {

return department;

}

public void setDepartment(Department department) {

this.department = department;

}

}

接着我们写一个部门的映射xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/HibernateMapping DTD 3.0//EN"

"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

<class name="com.fish.testdao.Department" >

<id name="id" type="integer">

<generator class="increment"></generator>

</id>

<property name="name"></property>

</class>

</hibernate-mapping>

然后一写一个员工的映射xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/HibernateMapping DTD 3.0//EN"

"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

<class name="com.fish.testdao.Employee" >

<id name="id" column="id"type="integer">

<generator class="increment"></generator>

</id>

<property name="name" ></property>

<many-to-one name="department"    ></many-to-one>

</class>

</hibernate-mapping>

*注<many-to-onename="department"   ></many-to-one>这句话就说明了我们是站在员工表的角度操作部门表的。首先name是前面映射类的属性，后面如果不加property-ref="部门表中的字段"  则说明该表默认找部门的表主键为员工表的外键。这样我们可以在数据库生成的表，department就是在员工表的外键，且字段名是department

所以我一直的理解就是多对一就是通过外键来实现的。

然后我们写一个配置文件

<!DOCTYPE hibernate-configuration PUBLIC

"-//Hibernate/Hibernate Configuration DTD3.0//EN"

"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

<session-factory>

<property name="connection.driver_class">

com.mysql.jdbc.Driver

</property>

<property name="connection.url">jdbc:mysql://127.0.0.1:3306/test</property>

<property name="connection.username">root</property>

<property name="connection.password">root</property>

<property name="dialect">org.hibernate.dialect.HSQLDialect</property>

<property name="show_sql" >false</property>

<property name="hbm2ddl.auto">create</property>

<mapping  resource="com/fish/testdao/Employee.hbm.xml"/>

<mapping  resource="com/fish/testdao/Department.hbm.xml"/>

</session-factory>

</hibernate-configuration>

我们写一个测试类

package com.fish.domain;

import java.util.List;

import org.hibernate.Query;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

import com.fish.testdao.Department;

import com.fish.testdao.Employee;

public class Test {

//写一个得到session的模板

public staticSession getMySession() {

Configurationconfiguration = new Configuration();

configuration.configure("hibernate.cfg.xml");

SessionFactoryfactory = configuration.buildSessionFactory();

Sessionsession = factory.openSession();

returnsession;

}

//对两个表数据的添加

public staticvoid add() {

Sessionsession = getMySession();

Departmentdepartment = new Department();

department.setName("财务部");

Employeeemployee = new Employee();

employee.setName("yaku");

employee.setDepartment(department);

Transactiontransaction = session.beginTransaction();

transaction.begin();

session.save(department);

session.save(employee);

transaction.commit();

session.close();

}

//通过查员工表知道他是隶属那个部门的

public staticvoid query() {

String hql ="from Employee";

Sessionsession = getMySession();

Query query =session.createQuery(hql);

List<Employee>list = query.list();

for (Employeei : list) {

System.out.println(i.getName()+"归属"+i.getDepartment().getName());

//结果是：    yaku归属财务部

}

}

public staticvoid main(String[] args) throws Exception {

add();

query();

}

}