1.1.1. 一对多关联
一个Person可以拥有一个或多个Address，而每个Address对应于唯一一个Person，因此Person和Address之间存在一对多的关系。本文介绍使用MyBatis如何进行基于List的一对多映射。
Person类：
/**
* @Title: Person.java
* @Package com.test.mybatis3.pojo
* @Description:
* @author http://www.cnblogs.com/coe2coe/
* @date 2017年4月9日 下午3:21:32
* @version V1.0
*/
package com.test.mybatis3.pojo;
import java.util.List;
/**
* @ClassName: Person
* @Description:
* @author http://www.cnblogs.com/coe2coe/
* @date 2017年4月9日 下午3:21:32
*
*/
public class Person {
private String id;
private String name;
private int status;
//用于满足POJO要求的无参数构造函数。
public Person()
{
}
public String getId() {
return id;
}
public String getName() {
return name;
}
public int getStatus() {
return status;
}
public void setId(String id) {
this.id = id;
}
public void setName(String name) {
this.name = name;
}
public void setStatus(int status) {
this.status = status;
}
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + ", status=" + status +"]";
}
private List<Address> addressList;
public List<Address> getAddressList() {
return addressList;
}
public void setAddressList(List<Address> addressList) {
this.addressList = addressList;
}
}
Address类：
/**
* @Title: Address.java
* @Package com.test.mybatis3.pojo
* @Description:
* @author http://www.cnblogs.com/coe2coe/
* @date 2017年4月17日 下午10:56:26
* @version V1.0
*/
package com.test.mybatis3.pojo;
/**
* @ClassName: Address
* @Description:
* @author http://www.cnblogs.com/coe2coe/
* @date 2017年4月17日 下午10:56:26
*
*/
public class Address {
private int addressid;
private String personid;
private String addressname;
public int getAddressid() {
return addressid;
}
public void setAddressid(int addressid) {
this.addressid = addressid;
}
public String getPersonid() {
return personid;
}
public void setPersonid(String personid) {
this.personid = personid;
}
public String getAddressname() {
return addressname;
}
public void setAddressname(String addressname) {
this.addressname = addressname;
}
@Override
public String toString() {
return "Address [addressid=" + addressid + ", personid=" + personid + ", addressname=" + addressname + "]";
}
private Person person;
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
}
Mapper配置文件：
有两种映射方式：
(1)一种是使用一条JOIN语句将Person和对应的Address集合全部查询出来。
(2)另外一种是先查询出Person列表，再根据personid使用SELECT子查询查询出Address集合。
使用JOIN语句的Mapper配置方式：
<!-- 查询Person和对应的Address集合 --> <select id="findPersonAndAddress" resultMap="personResultMap" > select a.* ,b.* from t_person a left outer join t_address b on (a.id = b.personid) order by a.id asc , b.addressid asc </select> <!-- 将t_person映射到Person类 --> <resultMap type="com.test.mybatis3.pojo.Person" id="personResultMap"> <id property="id" column="id" /> <result property="name" column="name" /> <result property="status" column="status" /> <!-- 将该Person对应的t_address记录集映射到addressList属性 --> <collection property="addressList" javaType="java.util.ArrayList" resultMap="addressResultMap" /> </resultMap> <!-- 将t_address记录映射到Address类 --> <resultMap type="com.test.mybatis3.pojo.Address" id="addressResultMap"> <id property="addressid" column="addressid" /> <result property="personid" column="personid" /> <result property="addressname" column="addressname" /> </resultMap>
使用嵌套查询的配置方式：
<!-- 查询Person和对应的Address集合 -->
<!-- 计划采用select子查询的方式来获取Address集合，因此此处仅查询person -->
<select id="findPersonAndAddress" resultMap="personResultMap" >
select * from t_person order by id asc
</select>
<!-- 将t_person映射到Person类 -->
<resultMap type="com.test.mybatis3.pojo.Person" id="personResultMap">
<id property="id" column="id" />
<result property="name" column="name" />
<result property="status" column="status" />
<!-- 将该Person对应的t_address记录集映射到addressList属性 -->
<!-- select:一个子查询的id -->
<collection select="findAddressByPersonid"
column="id" property="addressList"
javaType="java.util.ArrayList"
/>
</resultMap>
<!-- 将t_address记录映射到Address类 -->
<resultMap type="com.test.mybatis3.pojo.Address" id="addressResultMap">
<id property="addressid" column="addressid" />
<result property="personid" column="personid" />
<result property="addressname" column="addressname" />
</resultMap>
<!-- 根据personid查询Address集合 -->
<select parameterType="string" resultMap="addressResultMap" id="findAddressByPersonid" >
select * from t_address where personid=#{id}
</select>
Mapper接口的方法：
public interface PersonMapper {
List<Person> findPersonAndAddress() throws Exception;
}
单元测试代码如下：
//打开Session。
session = sessionBuilder.openSession();
//找到MyBatis自动实现的PersonMapper接口的代理对象。
PersonMapper personMapper = session.getMapper(PersonMapper.class);
//查询Person和相应的Address集合。
List<Person> persons = personMapper.findPersonAndAddress();
for(Person person : persons){
System.out.println(person);
for(Address address: person.getAddressList()){
System.out.println(address);
}
System.out.println("----------------------");
}
运行结果如下：
0 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - ==> Preparing: select a.* ,b.* from t_person a left outer join t_address b on (a.id = b.personid) order by a.id asc , b.addressid asc
42 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - ==> Parameters:
68 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Columns: id, name, status, addressid, personid, addressname
68 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Row: lisi, li si, 0, 4, lisi, road1
72 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Row: lisi, li si, 0, 5, lisi, road2
74 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Row: lisi, li si, 0, 6, lisi, road3
74 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Row: zhangsan, zhang san, 0, 1, zhangsan, street1
76 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Row: zhangsan, zhang san, 0, 2, zhangsan, street2
76 [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Row: zhangsan, zhang san, 0, 3, zhangsan, street3
76 [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress - <== Total: 6
Person [id=lisi, name=li si, status=0]
Address [addressid=4, personid=lisi, addressname=road1]
Address [addressid=5, personid=lisi, addressname=road2]
Address [addressid=6, personid=lisi, addressname=road3]
----------------------
Person [id=zhangsan, name=zhang san, status=0]
Address [addressid=1, personid=zhangsan, addressname=street1]
Address [addressid=2, personid=zhangsan, addressname=street2]
Address [addressid=3, personid=zhangsan, addressname=street3]
----------------------