Mybatis

1.第一个Mybatis程序

思路：搭建环境->导入Mybatis->编写代码->测试

1.1.1 搭建环境

1.1.1 创建普通maven项目

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1.1.2 删除src

1.2 导入Mybatis

1.2.1 在pox.xml里导入依赖

<?xml version="1.0" encoding="UTF-8"?>
    <!--父工程-->
<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>com.kuang</groupId>
    <artifactId>mybatis_day1</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>
    <modules>
        <module>mybatis_01</module>
    </modules>
    <dependencies>
        <!-- mybatis-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.2</version>
        </dependency>
        <!-- mysql-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
        <!-- junit-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
    </dependencies>

</project>

注意用crtl+shift+o刷新maven依赖

1.2.2 新建模块moduel

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1.2.3 Mybatis系统的核心设置 -- mybatis-config.xml

进入mybatis的官网，入门https://mybatis.org/mybatis-3/zh/getting-started.html
在子模块的src->main->resources里添加mybatis-config.xml并做相应配置

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1.3 编写代码

1.3.1 根据官方文档，我们写一个工具类utils，来简化SqlSession的创建

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package com.kuang.utils;

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;

public class MybatisUtils {
    private static  SqlSessionFactory sqlSessionFactory = null;

    static {
        try {
            //使用Mybatis第一步：获取SqlSessionFactory对象
            String resource = "mybatis-config.xml";
            InputStream inputStream = null;
            inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }

}

1.3.2 根据数据库表创建相应的实体类和Dao

实体类的Constructor，getter，setter可以用alt+insert来快速构建
Dao中的UsersDao和之后的Mapper是一样的意思，都用Interface
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1.3.3 在dao包下再建一个UsersMapper.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.kuang.dao.UsersDao"> <!--绑定一个Dao接口-->
    <select id="getUsersList" resultType="com.kuang.pojo.Users"> <!--id是接口函数名， 然后是返回类型-->
    select * from mybatis.users
  </select>
</mapper>

1.4 测试

1.4.1 在src->test下创建相同结构的测试

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1.4.2 发现错误并且解决错误

错误1
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org.apache.ibatis.binding.BindingException: Type interface com.kuang.dao.UsersDao is not known to the MapperRegistry.

在核心配置mybatis-config.xml里要设置mappers
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错误2
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java.lang.ExceptionInInitializerError
maven由于他的约定大于配置，我们之后可能遇到我们写的配置文件，无法被到处或者生效的问题，解决方案如下
在pom.xml(父工程和子工程都要)中的build中配置resources,来防止我们资源导出失败问题
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<!--在build中配置resources，来防止我们资源导出失败的问题-->
    <build>
        <resources>
            <resource>
                <directory>
                    src/main/resources
                </directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>