简单的搭建。
MyBatis实战——前身iBatis、基础环境搭建和如何"钓鱼" 这篇写了基础的环境搭建和代码操作。
我这里是使用了xml加interface结合的方法进行操作。现有Employee类。如果是简单的select，可以看看下面简单的例子。
Employee.java
public class Employee {//省略get set 方法
private int id;
private String first_name;
private String last_name;
private int salary;
}
EmployeeMapper.java
package com.mybatis3.mappers;
import model.Employee;
public interface EmployeeMapper
{
Employee findEmployeeById(Integer a);
}
EmployeeMapper.xml(和EmployeeMapper.java位于同个package中
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis3.mappers.EmployeeMapper">
<select id="findEmployeeById" resultType="model.Employee">
select * from Employee where id = #{abc} <!-- 后面发现这个参数名这里填什么都无所谓，能映射到 -->
</select>
</mapper>
mybatis-config.xml的mappers映射改成：
<mapper class="com.mybatis3.mappers.EmployeeMapper" />
进行查询操作。
public class Factory {
public static void main(String[] args) {
String configxml = "mybatis-config.xml";
InputStream is = null;
try {
is = Resources.getResourceAsStream(configxml);
} catch (IOException e) {
e.printStackTrace();
}
SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);
SqlSession s = sf.openSession();
EmployeeMapper employeeMapper = s.getMapper(EmployeeMapper.class);
Employee e = employeeMapper.findEmployeeById(47);
System.out.println(e.getFirst_name());
System.out.println(e.getLast_name());
System.out.println(e.getSalary());
s.close();
}
}
这是单表查询的时候，那么问题就来了。如果是多表复杂的查询。现在有了一张新表Employer，假如你是框架的设计者，会让Employee的字段全部丢到Employer中吗，显然不合适。
这是MyBatis提供了resultmap的方法。先看看单独一个类也可以使用resultmap进行映射。
Employer类
public class Employer {
private int id;//省略set get 方法
private int employee_id;
private String name;
private Employee em;
}
EmployerMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis3.mappers.EmployerMapper">
<resultMap id="Employer" type="model.Employer">
<id property="id" column="id" /> <!-- id相当于resultmap的引用的唯一标识-->
<result property="name" column="name" /> <!-- property对应javabean的属性名，column对应数据库字段名 -->
<result property="employee_id" column="employee_id" />
</resultMap>
<select id="findEmployeeById" resultMap="Employer">
select * from Employer where id = #{abc}
</select>
</mapper>
测试：
public class Factory {
public static void main(String[] args) {
String configxml = "mybatis-config.xml";
InputStream is = null;
try {
is = Resources.getResourceAsStream(configxml);
} catch (IOException e) {
e.printStackTrace();
}
SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);
SqlSession s = sf.openSession();
EmployerMapper employerMapper = s.getMapper(EmployerMapper.class);
Employer e2 = employerMapper.findEmployeeById(1);
System.out.println(e2.getEmployee_id());
System.out.println(e2.getName());
s.close();
}
}
折腾resultMap的id,result属性：
接下来开始折腾了。将EmployerMapper.xml里面改成只剩：
<resultMap id="Employer" type="model.Employer">
</resultMap>
<select id="findEmployeeById" resultMap="Employer">
select * from Employer where id = #{abc}
</select>
发现还是能正常映射。但是如果改javabean的一个属性:
private int employeeid;
public int getEmployeeid() {
return employeeid;
}
public void setEmployeeid(int employee_id) {
this.employeeid = employee_id;
}
这样最后取值是取不到的，因为映射不到。那究竟是属性名还是通过getset方法的名字映射呢。再折腾：
private int employeeid;
public int getEmployee_id() {
return employeeid;
}
public void setEmployee_id(int employee_id) {
this.employeeid = employee_id;
}
employeeid属性名与数据库字段名不对应，但是get，set的方法名是和数据库字段名对应的。运行正常。
如果不想在配置这里瞎折腾，JavaBean字段名保持和数据库字段名是最好的选择。
折腾resultMap extends属性，表关联查询。
Employer.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis3.mappers.EmployerMapper">
<resultMap id="Employer" type="model.Employer">
</resultMap>
<resultMap type="model.Employer" id="EmployerWithEmployee"
extends="Employer">
<result property="em.id" column="id" />
<result property="em.first_name" column="first_name" />
<result property="em.last_name" column="last_name" />
<result property="em.salary" column="salary" />
</resultMap>
<select id="findEmployeeById2" resultMap="EmployerWithEmployee">
select employee_id,name,b.first_name,b.salary,b.last_name from Employer a
left join Employee b on a.employee_id=b.id where a.id = #{abc}
</select>
</mapper>
测试：
SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);
SqlSession s = sf.openSession();
EmployerMapper employerMapper = s.getMapper(EmployerMapper.class);
Employer e2 = employerMapper.findEmployeeById2(1);
System.out.println(e2.getEmployee_id());
System.out.println(e2.getName());
System.out.println(e2.getEm().getFirst_name());
System.out.println(e2.getEm().getLast_name());
注意这次的em不再是和上面第一个折腾的一样，是和方法名一致的。这次是和属性名一致。不然会报错。
而且MyBatis的关联查询就这么简单，还是一样的关联语句，只不过加了一些映射配置。
折腾association
extends拓展看起来挺好，但是讲到解耦的话，你extends出来是个不伦不类的东西。本身我就有Employee的存在。
所以，这时association出现了。
<mapper namespace="com.mybatis3.mappers.EmployerMapper">
<resultMap id="Employee" type="model.Employee">
<id property="id" column="id" />
<result property="first_name" column="first_name" />
<result property="last_name" column="last_name" />
<result property="salary" column="salary" />
</resultMap>
<resultMap id="Employer" type="model.Employer">
<id property="id" column="id" /> <!-- id相当于resultmap的引用的唯一标识-->
<result property="name" column="name" /> <!-- property对应javabean的属性名，column对应数据库字段名 -->
<result property="employee_id" column="employee_id" />
<association property="em" resultMap="Employee" />
</resultMap>
<select id="findEmployeeById2" resultMap="Employer">
select employee_id,name,b.first_name,b.salary,b.last_name from Employer a
left join Employee b on a.employee_id=b.id where a.id = #{abc}
</select>
</mapper>
测试代码不变：
SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is); SqlSession s = sf.openSession(); EmployerMapper employerMapper = s.getMapper(EmployerMapper.class); Employer e2 = employerMapper.findEmployeeById2(1); System.out.println(e2.getEmployee_id()); System.out.println(e2.getName()); System.out.println(e2.getEme().getFirst_name()); System.out.println(e2.getEme().getLast_name()); s.close();
但是不同于resultmap使用extends，result标签是不可少的，一旦少了，将获取不到其属性值。
经过上面种种折腾之后终于知道作者讲的作用是什么：最后就明白了书上所讲的ResultMap的作用，映射SQL选择声明的结果到JavaBean属性中。通过一对一和一对多associations可以将简单选择声明映射复杂的声明。
时间： 2024-11-07 12:33:45