MyBatis——MyBatis的延迟加载

MyBatis的延迟加载（一对多查询案例）

1.什么是延迟加载？

开启延迟加载后，在真正使用数据的时候才发起级联查询，不用的时候不查询。

2.pojo

User类：

package com.wt.pojo;import java.io.Serializable;
import java.util.Date;
import java.util.List;public class User implements Serializable {private Integer id;private String username;private Date birthday;private String sex;private String address;//加入List<Account>存储用户所拥有的账户private List<Account> accounts;public List<Account> getAccounts() {return accounts;}public void setAccounts(List<Account> accounts) {this.accounts = accounts;}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;}@Overridepublic String toString() {return "User{" +"id=" + id +", username='" + username + '\'' +", birthday=" + birthday +", sex='" + sex + '\'' +", address='" + address + '\'' +", accounts=" + accounts +'}';}
}

Account类：略

3.mapper

UserDao:

public interface UserDao {public List<User> findAll();
}

AccountDao:

public interface AccountDao {Account findAccountById(Integer id);
}

UserDao.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wt.dao.UserDao"><resultMap type="User" id="findAllResultMap"><id column="id" property="id"></id><result column="username" property="username"/><result column="address" property="address"/><result column="sex" property="sex"/><result column="birthday" property="birthday"/><!--property：属性名ofType：泛型select： 要调用的 select 映射的 idcolumn ： 传递给 select 映射的参数fetchType="lazy"：懒加载，默认情况下是没有开启延迟加载的--><collection property="accounts" ofType="Account"select="com.by.dao.AccountDao.findAccountById" column="id"fetchType="lazy">	</collection></resultMap><select id="findAll" resultMap="findAllResultMap">select * from user</select>
</mapper>

AccountDao.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wt.dao.AccountDao"><select id="findAccountById" parameterType="int" resultType="account">select * from account where uid = #{id}</select>
</mapper>

4.测试

    @Testpublic void testFindAll() {UserDao userDao = sqlSession.getMapper(UserDao.class);List<User> userList = userDao.findAll();for(User user : userList){System.out.println(user.getUsername());//不查询accountSystem.out.println(user.getAccounts());//查询account}}