Java web（二）MyBatis

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一、概述

MyBatis是一款持久层框架，用于简化JDBC开发。JavaEE三层架构：表现层[页面展示]、业务层[逻辑处理]、持久层[负责将数据保存到数据库]。
由于JDBC存在一些缺点：硬编码（例如，参数、SQL语句写入到代码中）、操作繁琐（手动设置参数和封装结果集），MyBatis就会设置配置文件、自动完成繁琐操作来解决上述问题。

1.1 快速入门（普通映射方式）

1.创建user表tb_user

create database mybatis;
use mybatis;

drop table if exists tb_user;

create table tb_user(
	id int primary key auto_increment,
	username varchar(20),
	password varchar(20),
	gender char(1),
	addr varchar(30)
);



INSERT INTO tb_user VALUES (1, 'zhangsan', '123', '男', '北京');
INSERT INTO tb_user VALUES (2, '李四', '234', '女', '天津');
INSERT INTO tb_user VALUES (3, '王五', '11', '男', '西安');

2.创建模块导入坐标，配置pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>Mybatis-demo</groupId>
    <artifactId>Mybatis-demo</artifactId>
    <version>1.0-SNAPSHOT</version>

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

    <dependencies>
        <!--mybatis-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.3</version>
        </dependency>
        <!--数据库连接-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.46</version>
        </dependency>
        <!--junit单元测试-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
        <!--添加日志slf4j的api-->
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.25</version>
        </dependency>
        <!--添加logback-classic-->
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>1.2.3</version>
        </dependency>
        <!--添加日志logback-core-->
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-core</artifactId>
            <version>1.2.3</version>
        </dependency>
    </dependencies>

</project>

3.编写MyBatis核心配置文件mybatis-config.xml（替换链接信息）

<?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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <!--数据库连接信息-->
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!-- 加载sql映射文件 -->
        <mapper resource="UserMapper.xml"/>
    </mappers>
</configuration>

4.编写SQL映射文件UserMapper.xml（统一管理sql语句）

<?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">

<!--
    namespace:名称空间
-->
<mapper namespace="test">
    <select id="selectAll" resultType="com.bhy.pojo.User">
    select * from tb_user;
    </select>
</mapper>

5.定义User类，连接数据库

package com.bhy;

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

/**
 * @author 神代言
 * @version 1.0
 */
public class MybatisDemo {
    public static void main(String[] args) throws IOException {
        // 1.加载myatis的核心配置文件，获取SqlSessionFactory
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        // 2. 获取SqlSession对象，其执行sql
        SqlSession sqlSession = sqlSessionFactory.openSession();

        // 3.执行sql
        List<Object> users = sqlSession.selectList("test.selectAll");
        System.out.println(users);
        // 4.释放资源
        sqlSession.close();
    }
}

1.2 IDEA操作MySQL数据库

输入数据库名、用户名、密码，即可连接数据库。

连接成功如图所示，点击右侧笔的图标，即可在IDEA中编写SQL语句。

1.3 Mapper代理方式

与1.1方式不同的是，设置了一个UserMapper接口与UserMapper.xml对应
**注意：**创建UserMapper.xml的目录时一定要用com/bhy/mapper，不能用com.bhy.mapper!!!
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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <!--数据库连接信息-->
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!-- 1.加载sql映射文件 -->
        <!--<mapper resource="com\bhy\mapper\UserMapper.xml"/>-->
        
        <!-- 2.Mapper代理方式 -->
        <package name="com.bhy.mapper"/>
    </mappers>
</configuration>

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">

<!--
    namespace:名称空间,   代理方式保证命名空间namesapce与Mapper接口对应
-->
<mapper namespace="com.bhy.mapper.UserMapper">
    <select id="selectAll" resultType="com.bhy.pojo.User">
    select * from tb_user;
    </select>
</mapper>

UserMapper.java

package com.bhy.mapper;

import com.bhy.pojo.User;

import java.util.List;

/**
 * @author 神代言
 * @version 1.0
 */
public interface UserMapper {
    List<User> selectAll();
}

MybatisDemo2.java

package com.bhy;

import com.bhy.mapper.UserMapper;
import com.bhy.pojo.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 java.io.IOException;
import java.io.InputStream;
import java.util.List;

/**
 * @author 神代言
 * @version 1.0
 */
public class MybatisDemo2 {
    public static void main(String[] args) throws IOException {
        // 1.加载myatis的核心配置文件，获取SqlSessionFactory
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        // 2. 获取SqlSession对象，其执行sql
        SqlSession sqlSession = sqlSessionFactory.openSession();
        
//        // 传统方式仍存在硬编码问题
//        List