spring-mybatis源码分析之spring如何获取到UserMapper实例的？

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原创 2022-03-05 14:47:04 661 阅读 AI 写同款· GEO 优化 ›

MyBatis面试核心知识点与实战解析 ORM框架是Java开发中数据库操作的核心组件，MyBatis作为轻量级ORM工具通过XML/注解配置实现SQL与代码解耦。其核心原理包括SqlSession工厂模式、动态SQL解析和分级缓存机制，在性能优化和复杂查询场景展现独特优势。本文深入剖析MyBatis的15个高频面试考点，涵盖插件开发、事务管理、分页实现等工程实践，特别针对一级缓存数据一致性、动态SQL注入防护等常见陷阱提供解决方案。结合互联网大厂真实面试数据，详解PageHelper分页原理和批量插入优化等热点技术，帮助开发者掌握MyBatis 阅读详情

spring-mybatis是如何获取到UserMapper实例的？

1.项目结构

2.pom.xml

<groupId>org.example</groupId>
    <artifactId>spring-mybatis-source</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.5</version>
    </parent>

    <dependencies>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.3</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.38</version>
        </dependency>

        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.12</version>
        </dependency>

        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>1.7.12</version>
        </dependency>

        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>

        <dependency>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-surefire-plugin</artifactId>
            <version>2.4.2</version>
        </dependency>

        <dependency>
            <groupId>cn.hutool</groupId>
            <artifactId>hutool-core</artifactId>
            <version>4.4.5</version>
        </dependency>

        <dependency>
            <groupId>cn.hutool</groupId>
            <artifactId>hutool-cache</artifactId>
            <version>4.4.5</version>
        </dependency>

        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis-spring</artifactId>
            <version>2.0.3</version>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-jdbc</artifactId>
            <version>5.3.9</version>
        </dependency>

        <dependency>
            <groupId>commons-dbcp</groupId>
            <artifactId>commons-dbcp</artifactId>
            <version>1.4</version>
        </dependency>

        <dependency>
            <groupId>commons-pool</groupId>
            <artifactId>commons-pool</artifactId>
            <version>1.6</version>
        </dependency>
    </dependencies>

3.基础配置

spring.xml

  <?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
	       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource">
        <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
        <property name="maxActive" value="100"/>
        <property name="maxIdle" value="30"/>
        <property name="maxWait" value="500"/>
        <property name="defaultAutoCommit" value="true"/>
    </bean>

    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="configLocation" value="classpath:mybatis-config.xml"/>
        <property name="dataSource" ref="dataSource"/>
    </bean>

    <bean id="userMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
        <property name="mapperInterface" value="com.lmh.dao.UserMapper"/>
        <property name="sqlSessionFactory" ref="sqlSessionFactory"/>
    </bean>
</beans>

mybatis-config.xml

<?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>
    <typeAliases>
        <typeAlias alias="User" type="com.lmh.entity.User"/>
    </typeAliases>
    
    <mappers>
        <mapper resource="dao/UserMapper.xml"/>
    </mappers>
</configuration>

4.测试代码

User实体类数据库字段随便建几个吧

package com.lmh.entity;

public class User {

    private Long id;

    private String username;

    private String password;

    private String email;

    private String headImage;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", email='" + email + '\'' +
                ", headImage='" + headImage + '\'' +
                '}';
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getHeadImage() {
        return headImage;
    }

    public void setHeadImage(String headImage) {
        this.headImage = headImage;
    }
}

UserMapper

import com.lmh.entity.User;

public interface UserMapper {

    User findUserById();
}

UserMapper.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.lmh.dao.UserMapper">

    <select id="findUserById" resultType="User">
        select
            id id,
            user_name username,
            user_password password,
            user_email email,
            head_image headImage
        from sys_user where id = 1
    </select>
</mapper>

测试类

package com.lmh.test;

import com.lmh.dao.UserMapper;
import com.lmh.entity.User;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class UserTest {
    public static void main(String[] args) {

        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
        //测试sqlSessionFactoryBean获取sqlSessionFactory
//        Object sqlSessionFactory = context.getBean("sqlSessionFactory");
//        System.out.println("sqlSessionFactory = " + sqlSessionFactory);

        //通过name获取。
        //结论 首先会获取到MapperFactoryBean, 然后调用getObject方法获取到UserMapper
        UserMapper userMapper = (UserMapper)context.getBean("userMapper");
        User userById = userMapper.findUserById();
        System.out.println("userById = " + userById);

        //通过class获取
        //结论 先遍历所有的beanDefinition，在判断用是否属于FactoryBean，如果属于FactoryBean对象，那么FactoryBean.getObjectType获取到Bean的类型
        //在与UserMapper匹配
        UserMapper bean = context.getBean(UserMapper.class);
        User user = bean.findUserById();
        System.out.println("user = " + user);

//        System.out.println("userById = " + userById);
//        context.getBean("dataSource");
    }
}

5.源码解析

在开始读懂源码之前，大家要懂得FactoryBean是什么东西。这里简单介绍一下，后面会直接跟源码。首先，FactoryBean是一个接口，当调用getBean方法获取Spring单例池中的bean的时候。如果在单例池中这个bean是一个FactoryBean，那么Spring中会调用FactoryBean的getObject方法返回bean信息。

接下来我们就真正开始研究源码！
断点查看一下单例池的信息。也就是context.beanFactory.singletonOjects属性如下。
在这里插入图片描述

第一个问题

我们可以看到，在单例池中(也就是singletonObject，下面统一用 ‘单例池’)，这两种重要的属性和我们在spring.xml中定义的是一致的。那么第一个问题来了，为什么单例池中没有UserMapper实例而我们却可以通过getBean(String name)方法获取到，mybatis的UserMapper实例呢？

我们debug进入当前断点，最终跟到AbstractBeanFactory#doGetBean
在这里插入图片描述
getSingleton方法相信大家都很熟悉了，这个方法从单例池中(singletonObject)获取到的bean信息，也就是xml中定义的MapperFactoryBean。

然后继续下面走到了org.springframework.beans.factory.support.AbstractBeanFactory#getObjectForBeanInstance
在这里插入图片描述

这和我们所学到的知识一样，如果这个bean不是FactoryBean，就直接返回了这个bean。但是MapperFactoryBean是FactoryBean

沒什么好说的，我们继续往下跟代码
org.springframework.beans.factory.support.FactoryBeanRegistrySupport#doGetObjectFromFactoryBean
在这里插入图片描述
这个和我之前说的结论一样，最终调用FactoryBean的getObject方法。