Mybatis中一对多查询的实现

User类：

package com.qublog.domain;

import java.io.Serializable;
import java.util.Date;
import java.util.List;

public class User implements Serializable {
    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;

    //一对多关系映射，主表实体应该包含从表实体的集合引用
    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 String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", address='" + address + '\'' +
                ", sex='" + sex + '\'' +
                ", birthday=" + birthday +
                '}';
    }
}

UserDao类：

package com.qublog.dao;


import com.qublog.domain.User;

import java.util.List;

//用户的持久层接口
public interface UserDao {

    //查询所有用户，同时获取到用户下所有账户的信息
    List<User> findAll();
}

UserDao.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.qublog.dao.UserDao">
    <!-- 定义user的resultMap -->
    <resultMap id="userAccountMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <!-- 配置user对象中accounts集合的映射 -->
        <collection property="accounts" ofType="account">
            <id property="id" column="aid"></id>
            <result property="uid" column="uid"></result>
            <result property="money" column="money"></result>
        </collection>
    </resultMap>

    <!-- 配置查询所有 -->
    <select id="findAll" resultMap="userAccountMap">
        select u.*, a.id as aid, a.uid, a.money from user u left outer join account a on a.uid=u.id;
    </select>
</mapper>

UserTest类：

package com.qublog.test;

import com.qublog.dao.AccountDao;
import com.qublog.dao.UserDao;
import com.qublog.domain.Account;
import com.qublog.domain.AccountUser;
import com.qublog.domain.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

public class UserTest {
    private InputStream in = null;
    private SqlSession sqlSession = null;
    UserDao userDao = null;

    @Before //用于在测试方法执行之前执行
    public void init() throws Exception {
        //读取配置文件，生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //获取SqlSessionFactory对象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //获取SqlSession对象
        sqlSession = factory.openSession();
        //获取dao代理对象
        userDao = sqlSession.getMapper(UserDao.class);
    }

    @After //用于在测试方法执行之后执行
    public void destroy() throws Exception {
        //提交事务
        sqlSession.commit();
        //释放资源
        sqlSession.close();
        in.close();
    }

    @Test
    public void testFindAll() throws Exception {
        //执行查询所有方法
        List<User> users = userDao.findAll();
        for (User user:users) {
            System.out.println(user);
            System.out.println(user.getAccounts() );
        }
    }

}

结果：

从结果中可以看出mybatis自动将重复项合并了。