MyBatis 多表聯合查詢及優化方法

這篇文章我打算來簡單的談談 mybatis 的多表聯合查詢。起初是覺得挺簡單的,沒必要拿出來寫,畢竟 mybatis 這東西現在是個開發的都會用,而且網上的文章也是一搜羅一大堆,根本就用不著我來重復。但是吧,就我前幾天在做一個多表聯合查詢的時候,竟然出瞭很多意想不到的問題,而且這些問題的出現,並不是對 mybatis 不瞭解,而是在用的過程中會或多或少的忽略一些東西,導致提示各種錯誤。

背景

老規矩,開始之前,還是要先說說這件事的背景。也就是最近幾天,公司要做一個後臺的管理平臺,由於之前的一些限制,這次要做成單獨的項目進行部署,因此就要重新考慮很多東西。索性這幾天有時間,就做瞭一個小 Demo ,實現 mybatis 的多表聯合查詢的,由於之前用的是 Hibernate 做的聯合查詢,眾所周知,Hibernate 是全自動的數據庫持久層框架,它可以通過實體來映射數據庫,通過設置一對多、多對一、一對一、多對多的關聯來實現聯合查詢。

正文

下面就來說一下 mybatis 是通過什麼來實現多表聯合查詢的。首先看一下表關系,如圖:

這裡,我已經搭好瞭開發的環境,用到的是 SpringMVC + Spring + MyBatis,當然,為瞭簡單期間,你可以不用搭前端的框架,隻使用 Spring + MyBatis 就可以,外加 junit 測試即可。環境我就不帶大傢搭瞭,這裡隻說涉及到聯合查詢的操作。
設計好表之後,我用到瞭 mybatis 的自動生成工具 mybatis generator 生成的實體類、mapper 接口、以及 mapper xml 文件。由於是測試多表聯合查詢,因此需要自己稍加改動。
下面是 User 和 Role 的實體類代碼:

User

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.domain;
import java.io.Serializable;
import java.util.List;
public class User implements Serializable {
    private String id;
    private String username;
    private String password;
    private List<Role> roles;
    private static final long serialVersionUID = 1L;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id == null ? null : id.trim();
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username == null ? null : username.trim();
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password == null ? null : password.trim();
    }
    public List<Role> getRoles() {
        return roles;
    }
    public void setRoles(List<Role> roles) {
        this.roles = roles;
    }
    @Override
    public boolean equals(Object that) {
        if (this == that) {
            return true;
        }
        if (that == null) {
            return false;
        }
        if (getClass() != that.getClass()) {
            return false;
        }
        User other = (User) that;
        return (this.getId() == null ? other.getId() == null : this.getId().equals(other.getId()))
            && (this.getUsername() == null ? other.getUsername() == null : this.getUsername().equals(other.getUsername()))
            && (this.getPassword() == null ? other.getPassword() == null : this.getPassword().equals(other.getPassword()));
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((getId() == null) ? 0 : getId().hashCode());
        result = prime * result + ((getUsername() == null) ? 0 : getUsername().hashCode());
        result = prime * result + ((getPassword() == null) ? 0 : getPassword().hashCode());
        return result;
    }
}</span>

Role

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.domain;
import java.io.Serializable;
public class Role implements Serializable {
    private String id;
    private String name;
    private String jsms;
    private String bz;
    private Integer jlzt;
    private String glbm;
    private String userid;
    private static final long serialVersionUID = 1L;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id == null ? null : id.trim();
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name == null ? null : name.trim();
    }
    public String getJsms() {
        return jsms;
    }
    public void setJsms(String jsms) {
        this.jsms = jsms == null ? null : jsms.trim();
    }
    public String getBz() {
        return bz;
    }
    public void setBz(String bz) {
        this.bz = bz == null ? null : bz.trim();
    }
    public Integer getJlzt() {
        return jlzt;
    }
    public void setJlzt(Integer jlzt) {
        this.jlzt = jlzt;
    }
    public String getGlbm() {
        return glbm;
    }
    public void setGlbm(String glbm) {
        this.glbm = glbm == null ? null : glbm.trim();
    }
    public String getUserid() {
        return userid;
    }
    public void setUserid(String userid) {
        this.userid = userid == null ? null : userid.trim();
    }
    @Override
    public boolean equals(Object that) {
        if (this == that) {
            return true;
        }
        if (that == null) {
            return false;
        }
        if (getClass() != that.getClass()) {
            return false;
        }
        Role other = (Role) that;
        return (this.getId() == null ? other.getId() == null : this.getId().equals(other.getId()))
            && (this.getName() == null ? other.getName() == null : this.getName().equals(other.getName()))
            && (this.getJsms() == null ? other.getJsms() == null : this.getJsms().equals(other.getJsms()))
            && (this.getBz() == null ? other.getBz() == null : this.getBz().equals(other.getBz()))
            && (this.getJlzt() == null ? other.getJlzt() == null : this.getJlzt().equals(other.getJlzt()))
            && (this.getGlbm() == null ? other.getGlbm() == null : this.getGlbm().equals(other.getGlbm()))
            && (this.getUserid() == null ? other.getUserid() == null : this.getUserid().equals(other.getUserid()));
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((getId() == null) ? 0 : getId().hashCode());
        result = prime * result + ((getName() == null) ? 0 : getName().hashCode());
        result = prime * result + ((getJsms() == null) ? 0 : getJsms().hashCode());
        result = prime * result + ((getBz() == null) ? 0 : getBz().hashCode());
        result = prime * result + ((getJlzt() == null) ? 0 : getJlzt().hashCode());
        result = prime * result + ((getGlbm() == null) ? 0 : getGlbm().hashCode());
        result = prime * result + ((getUserid() == null) ? 0 : getUserid().hashCode());
        return result;
    }
}</span>

首先講一下業務,這裡用到的 User 、Role 的對應關系是,一個用戶有多個角色,也就是 User : Role 是 1 : n 的關系。因此,在 User 的實體中加入一個 Role 的屬性,對應一對多的關系。
然後就是 mapper 接口和 xml 文件瞭:
mapper接口
UserMapper

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.mapper;
import com.sica.domain.User;
import java.util.List;
public interface UserMapper {
    int deleteByPrimaryKey(String id);
    int insert(User record);
    int insertSelective(User record);
    User selectByPrimaryKey(String id);
    int updateByPrimaryKeySelective(User record);
    int updateByPrimaryKey(User record);
    List<User> queryForList();
}</span>

mapper xml文件
UserMapper

<span style="font-family:Comic Sans MS;font-size:12px;"><?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.sica.mapper.UserMapper">
    <resultMap id="BaseResultMap" type="com.sica.domain.User">
        <id column="id" property="id" jdbcType="VARCHAR"/>
        <result column="username" property="username" jdbcType="VARCHAR"/>
        <result column="password" property="password" jdbcType="VARCHAR"/>
    </resultMap>
 
    <resultMap id="queryForListMap" type="com.sica.domain.User">
        <id column="id" property="id" jdbcType="VARCHAR"/>
        <result column="username" property="username" jdbcType="VARCHAR"/>
        <result column="password" property="password" jdbcType="VARCHAR"/>
        <collection property="roles" javaType="java.util.List" ofType="com.sica.domain.Role">
            <id column="r_id" property="id" jdbcType="VARCHAR" />
            <result column="r_name" property="name" jdbcType="VARCHAR" />
            <result column="r_jsms" property="jsms" jdbcType="VARCHAR" />
            <result column="r_bz" property="bz" jdbcType="VARCHAR" />
            <result column="r_jlzt" property="jlzt" jdbcType="INTEGER" />
            <result column="r_glbm" property="glbm" jdbcType="VARCHAR" />
        </collection>
    </resultMap>
    <select id="queryForList" resultMap="queryForListMap">
        SELECT
          u.id,
          u.username,
          u.password,
          r.id r_id,
          r.name r_name,
          r.jsms r_jsms,
          r.bz r_bz,
          r.jlzt r_jlzt,
          r.glbm r_glbm
        FROM
          user u
        LEFT JOIN
          role r
        ON
          u.id = r.userid
    </select>
    <sql id="Base_Column_List">
      id, username, password
    </sql>
    <select id="selectByPrimaryKey" resultMap="BaseResultMap" parameterType="java.lang.String">
        select
        <include refid="Base_Column_List"/>
        from user
        where id = #{id,jdbcType=VARCHAR}
    </select>
    <delete id="deleteByPrimaryKey" parameterType="java.lang.String">
    delete from user
    where id = #{id,jdbcType=VARCHAR}
    </delete>
    <insert id="insert" parameterType="com.sica.domain.User">
    insert into user (id, username, password
      )
    values (#{id,jdbcType=VARCHAR}, #{username,jdbcType=VARCHAR}, #{password,jdbcType=VARCHAR}
      )
    </insert>
    <insert id="insertSelective" parameterType="com.sica.domain.User">
        insert into user
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="id != null">
                id,
            </if>
            <if test="username != null">
                username,
            </if>
            <if test="password != null">
                password,
            </if>
        </trim>
        <trim prefix="values (" suffix=")" suffixOverrides=",">
            <if test="id != null">
                #{id,jdbcType=VARCHAR},
            </if>
            <if test="username != null">
                #{username,jdbcType=VARCHAR},
            </if>
            <if test="password != null">
                #{password,jdbcType=VARCHAR},
            </if>
        </trim>
    </insert>
    <update id="updateByPrimaryKeySelective" parameterType="com.sica.domain.User">
        update user
        <set>
            <if test="username != null">
                username = #{username,jdbcType=VARCHAR},
            </if>
            <if test="password != null">
                password = #{password,jdbcType=VARCHAR},
            </if>
        </set>
        where id = #{id,jdbcType=VARCHAR}
    </update>
    <update id="updateByPrimaryKey" parameterType="com.sica.domain.User">
    update user
    set username = #{username,jdbcType=VARCHAR},
      password = #{password,jdbcType=VARCHAR}
    where id = #{id,jdbcType=VARCHAR}
  </update>
</mapper></span>

之後,我擴展瞭一個 Dao 接口,當然,你也可以直接使用 mapper 接口,都是一樣的。
Dao 接口
IUserDao

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.dao;
 
import com.sica.mapper.UserMapper;
 
/**
 * Created by IntelliJ IDEA.
 * Package: com.sica.dao
 * Name: IUserDao
 * User: xiang.li
 * Date: 2015/5/22
 * Time: 15:25
 * Desc: To change this template use File | Settings | File Templates.
 */
public interface IUserDao extends UserMapper {
 
}</span>

下面就是 service 和實現層的代碼瞭。
IUserService

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.service;
import com.sica.domain.User;
import java.util.List;
/**
 * Created by xiang.li on 2015/1/31.
 */
public interface IUserService {
    /**
     * 根據Id查詢用戶對象
     * @param id 編號
     * @return 用戶對象
     */
    User getUserById(String id);
    /**
     * 根據用戶名查詢用戶對象
     * @return  List
     */
    List<User> queryUserList();
}</span>

UserServiceImpl

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.service.impl;
import com.sica.dao.IUserDao;
import com.sica.domain.User;
import com.sica.service.IUserService;
import org.springframework.stereotype.Service;
import javax.annotation.Resource;
import java.util.List;
/**
 * Created by xiang.li on 2015/1/31.
 */
@Service("userService")
public class UserServiceImpl implements IUserService {
    @Resource
    public IUserDao userDao;
    @Override
    public User getUserById(String id) {
        return this.userDao.selectByPrimaryKey(id);
    }
    @Override
    public List<User> queryUserList() {
        return userDao.queryForList();
    }
}</span>

當然,還有所謂的 applicationContext.xml 配置,不過,我這裡叫 spring-mybatis.xml。

<span style="font-family:Comic Sans MS;font-size:12px;"><?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc.xsd">
 
    <!-- 自動掃描 -->
    <context:component-scan base-package="com.sica"/>
    <!-- 引入配置文件 -->
    <bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"
          p:location="classpath:jdbc.properties"
          />
 
    <!-- 配置數據庫連接池 -->
    <!-- 初始化連接大小 -->
    <!-- 連接池最大數量 -->
    <!-- 連接池最大空閑 -->
    <!-- 連接池最小空閑 -->
    <!-- 獲取連接最大等待時間 -->
    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"
          p:driverClassName="${jdbc.driver}"
          p:url="${jdbc.url}"
          p:username="${jdbc.username}"
          p:password="${jdbc.password}"
          p:initialSize="${jdbc.initialSize}"
          p:maxActive="${jdbc.maxActive}"
          p:maxIdle="${jdbc.maxIdle}"
          p:minIdle="${jdbc.minIdle}"
          p:maxWait="${jdbc.maxWait}"
          />
 
    <!-- spring和MyBatis完美整合,不需要mybatis的配置映射文件 -->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean" lazy-init="default"
          p:dataSource-ref="dataSource"
          p:mapperLocations="classpath:com/sica/mapping/*.xml"
          />
 
    <!-- DAO接口所在包名,Spring會自動查找其下的類 -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer"
          p:basePackage="com.sica.dao"
          p:sqlSessionFactoryBeanName="sqlSessionFactory"
          />
 
    <!-- (事務管理)transaction manager, use JtaTransactionManager for global tx -->
    <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"
          p:dataSource-ref="dataSource"
          />
</beans></span>

最後,我用到的是 junit 進行的測試,測試代碼如下。
GetUserTest

<span style="font-family:Comic Sans MS;font-size:12px;">package com.sica.user;
 
import com.alibaba.fastjson.JSON;
import com.sica.domain.User;
import com.sica.service.IUserService;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 
import javax.annotation.Resource;
import java.util.List;
 
/**
 * Created by xiang.li on 2015/2/1.
 */
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "classpath:spring-mybatis.xml")
public class GetUserTest {
 
    private static String UUID = "3";
    @Resource
    private IUserService userService;
    private static Logger logger = LoggerFactory.getLogger(GetUserTest.class);
 
    @Test
    public void test() {
        User user = userService.getUserById(UUID);
        logger.info(JSON.toJSONString(user));
    }
 
    /**
     * 測試聯合查詢
     */
    @Test
    public void test2() {
        List<User> users = userService.queryUserList();
        logger.info(JSON.toJSONString(users));
    }
}</span>

測試結果

可以看到,所有的用戶和用戶對應的角色都全部查出來瞭,這說明,這次的測試很成功。

關於優化

對於優化嘛,我這裡簡單的提幾點,大傢可以考慮一下。首先,就是對表的設計,在設計表初期,不僅僅要考慮到數據庫的規范性,還好考慮到所謂的業務,以及對性能的影響,比如,如果從規范性角度考慮的話,可能就會分多個表,但是如果從性能角度來考慮的話,龐大的數據量在多表聯合查詢的時候,相對於單表來說,就會慢很多,這時,如果字段不是很多的話,可以考慮冗餘幾個字段采用單表的設計。
其次嘛,就是在 sql 上下功夫瞭,對於聯合查詢,sql 的優化是很有必要的,到底是采用 INNER JOIN,還是采用 LEFT JOIN 亦或是 RIGHT JOIN 、OUTTER JOIN 等,都是要在滿足業務需求之後,通過測試性能得出的結論。
再次嘛,就是在程序中調用的時候瞭,是采用懶加載,還是采用非懶加載的方式,這也算是一個因素吧,具體的還是要考慮業務的需要。
最後嘛,就要用到數據庫的緩存瞭,或者在數據庫與程序的中間再加一層緩存。不過,還是建議用好數據庫本身自帶的緩存功能。

到此這篇關於MyBatis 多表聯合查詢及優化的文章就介紹到這瞭,更多相關MyBatis 多表聯合查詢內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet!

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