Mybatis一对多延迟加载实现代码解析

需求：一个用户可以有多个账户，查询用户时实现账户的延迟加载

1、用户实体类

package com.yl.bean;import java.io.Serializable;import java.sql.Date;import java.util.List;/** * 用户实体类 */public class User implements Serializable { private Integer id;//id private String username;//用户名 private Date birthday;//生日 private String sex;//性别 private String address;//地址 private List<Account> accountList;//用户所拥有的账户 public User() { } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public Date getBirthday() { return birthday; } public void setBirthday(Date birthday) { this.birthday = birthday; } public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } public List<Account> getAccountList() { return accountList; } public void setAccountList(List<Account> accountList) { this.accountList = accountList; } @Override public String toString() { return 'User{' +'id=' + id +', username=’' + username + ’’’ +', birthday=' + birthday +', sex=’' + sex + ’’’ +', address=’' + address + ’’’ +’}’; }}

2、账户实体类

package com.yl.bean;import java.io.Serializable;/** * 账户实体类 */public class Account implements Serializable { private Integer id;//账户id private Integer uid;//用户id private Double money;//余额 private User user;//账户所属用户 public Account() { } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public Integer getUid() { return uid; } public void setUid(Integer uid) { this.uid = uid; } public Double getMoney() { return money; } public void setMoney(Double money) { this.money = money; } public User getUser() { return user; } public void setUser(User user) { this.user = user; } @Override public String toString() { return 'Account{' +'id=' + id +', uid=' + uid +', money=' + money +’}’; }}

3、用户持久层接口

package com.yl.dao;import com.yl.bean.User;import java.util.List;/** * 用户持久层接口 */public interface IUserDao { /** * 查询所有用户 */ List<User> queryAll(); /** * 根据id查询用户 */ User queryById(int id);}

4、账户持久层接口

package com.yl.dao;import com.yl.bean.Account;import com.yl.bean.User;import java.util.List;/** * 账户持久层接口 */public interface IAccountDao { /** * 查询所有账户 */ List<Account> queryAll(); /** * 根据用户id查询账户 */ List<Account> queryByUid(int uid);}

5、全局配置文件

<?xml version='1.0' encoding='UTF-8'?><!DOCTYPE configuration PUBLIC '-//mybatis.org//DTD Config 3.0//EN' 'http://mybatis.org/dtd/mybatis-3-config.dtd'><configuration> <settings> <!--开启延迟加载--> <setting name='lazyLoadingEnabled' value='true'/> <setting name='aggressiveLazyLoading' value='false'/> </settings> <!--配置别名--> <typeAliases> <package name='com.yl.bean'/> </typeAliases> <!--配置mybatis环境--> <environments default='mysql'> <!--配置mysql环境--> <environment id='mysql'> <!--事务类型--> <transactionManager type='JDBC'></transactionManager> <!--数据源--> <dataSource type='POOLED'><property name='driver' value='com.mysql.cj.jdbc.Driver'/><property name='url' value='jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf8&amp;serverTimezone=GMT%2B8'/><property name='username' value='root'/><property name='password' value='123456'/> </dataSource> </environment> </environments> <!--指定映射配置文件位置--> <mappers> <mapper resource='IUserDao.xml'></mapper> <mapper resource='IAccountDao.xml'></mapper> </mappers></configuration>

6、用户映射配置文件

<?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.yl.dao.IUserDao'> <!--配置用户实体类和数据库的对应信息--> <resultMap type='com.yl.bean.User'> <id property='id' column='id'></id> <result property='username' column='username'></result> <result property='birthday' column='birthday'></result> <result property='sex' column='sex'></result> <result property='address' column='address'></result> <!--根据id查询账户--> <collection property='accountList' ofType='com.yl.bean.Account' select='com.yl.dao.IAccountDao.queryByUid' column='id'></collection> </resultMap> <!--查询所有用户--> <select resultMap='userMap'> SELECT * FROM USER </select> <!--根据id查询用户--> <select resultType='user' parameterType='int'> select * from user where id=#{id} </select></mapper>

7、账户映射配置文件

<?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.yl.dao.IAccountDao'> <!--配置Account属性对应数据库信息--> <resultMap type='account'> <id property='id' column='id'></id> <result property='uid' column='uid'></result> <result property='money' column='money'></result> <!--通过id查询用户--> <association property='user' column='uid' javaType='user' select='com.yl.dao.IUserDao.queryById'></association> </resultMap> <!--查询所有账户--> <select resultMap='accountUserMap'> SELECT * FROM account </select> <!--根据用户id查询账户--> <select resultType='account' parameterType='int'> select * from account where uid=#{uid} </select></mapper>

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