mybatis 如何实现对象的相互关联
多端采用collection来描述“一端”；
一端采用association来描述“多端”
建立javaBean如下
/**
* @author shenzm
* @date 2019-10-22 11:08
* @since 1.0
*/
public class Depart {
private Long did;
private String departName;
private List<Employee> employees;
public Long getDid() {
return did;
}
public void setDid(Long did) {
this.did = did;
}
public String getDepartName() {
return departName;
}
public void setDepartName(String departName) {
this.departName = departName;
}
public List<Employee> getEmployees() {
return employees;
}
public void setEmployees(List<Employee> employees) {
this.employees = employees;
}
@Override
public String toString() {
return "Depart{" +
"did=" + did +
", departName='" + departName + '\'' +
", employees=" + employees +
'}';
}
}
/**
* @author shenzm
* @date 2019-10-22 11:09
* @since 1.0
*/
public class Employee {
private Long eid;
private String name;
private int age;
private Depart depart;
public Long getEid() {
return eid;
}
public void setEid(Long eid) {
this.eid = eid;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public Depart getDepart() {
return depart;
}
public void setDepart(Depart depart) {
this.depart = depart;
}
@Override
public String toString() {
return "Employee{" +
"eid=" + eid +
", name='" + name + '\'' +
", age=" + age +
'}';
}
/*
如果需要重写toString方法， 不可相互定义， 不然会造成栈调用递归 stackOverFlowError
@Override
public String toString() {
return "Employee{" +
"eid=" + eid +
", name='" + name + '\'' +
", age=" + age +
", depart=" + depart +
'}';
}
*/
}
mapper的xml配置
<?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.example.study.mapper.DepartMapper">
<resultMap type="com.example.study.bean.Depart" id="departResultMap">
<id column="did" property="did" javaType="java.lang.Long"/>
<result column="departName" property="departName" javaType="java.lang.String"/>
<!--通过collection来映射list类型的属性， 通过select来查询关联的对象-->
<collection property="employees"
ofType="com.example.study.bean.Employee"
select="com.example.study.mapper.EmployeeMapper.getEmployees"
column="did"/>
</resultMap>
<select id="findAll" resultType="java.util.List" resultMap="departResultMap">
<![CDATA[
select * from depart
]]>
</select>
<select id="selectRowById" resultType="com.example.study.bean.Depart" resultMap="departResultMap">
<![CDATA[
select * from depart where did=#{did}
]]>
</select>
</mapper>
<?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.example.study.mapper.EmployeeMapper">
<resultMap type="com.example.study.bean.Employee" id="employeeResultMap">
<id column="eid" property="eid" javaType="java.lang.Long"/>
<result column="name" property="name" javaType="java.lang.String"/>
<result column="age" property="age" javaType="int"/>
<!-- 通过association来进行一对多， 来描述“多”的对象的属性， 通过select来查询对象-->
<association property="depart" javaType="com.example.study.bean.Depart"
select="com.example.study.mapper.DepartMapper.selectRowById" column="did"/>
<!--
<association property="depart" javaType="com.example.study.bean.Depart"
select="com.example.study.mapper.DepartMapper.selectRowById" column="did">
没有必选定义Depart类的信息
<id column="did" property="did" javaType="java.lang.Long"/>
<result column="departName" property="departName" javaType="java.lang.String"/>
</association>
-->
</resultMap>
<select id="getEmployees" resultType="java.util.List" resultMap="employeeResultMap">
<![CDATA[
select * from employee where did = #{did}
]]>
</select>
</mapper>
需要准备数据
/*
Navicat MySQL Data Transfer
Source Server : mysql
Source Server Version : 50556
Source Host : 10.1.51.96:3306
Source Database : test
Target Server Type : MYSQL
Target Server Version : 50556
File Encoding : 65001
Date: 2019-10-22 13:44:36
*/
SET FOREIGN_KEY_CHECKS=0;
-- ----------------------------
-- Table structure for depart
-- ----------------------------
DROP TABLE IF EXISTS `depart`;
CREATE TABLE `depart` (
`did` bigint(11) NOT NULL,
`departName` varchar(255) DEFAULT NULL,
PRIMARY KEY (`did`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of depart
-- ----------------------------
INSERT INTO `depart` VALUES ('1', '研发部');
INSERT INTO `depart` VALUES ('2', '销售部');
-- ----------------------------
-- Table structure for employee
-- ----------------------------
DROP TABLE IF EXISTS `employee`;
CREATE TABLE `employee` (
`eid` bigint(20) NOT NULL,
`name` varchar(255) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`did` bigint(20) DEFAULT NULL,
PRIMARY KEY (`eid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of employee
-- ----------------------------
INSERT INTO `employee` VALUES ('1', '张三', '11', '1');
INSERT INTO `employee` VALUES ('2', '李四', '12', '1');
INSERT INTO `employee` VALUES ('3', '王五', '13', '2');
INSERT INTO `employee` VALUES ('4', '赵柳', '11', '3');
重点
在一端的表里面需要持续多的一端的id； toString方法避免栈溢出，循环打印对象；