一、Mybatis 简介和上手体验

Mybatis官方中文文档：https://mybatis.org/mybatis-3/zh

1、简介

1.1、Mybatis是什么？

MyBatis 是一款优秀的持久层框架，它支持自定义 SQL、存储过程以及高级映射。MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。MyBatis 可以通过简单的== XML 或注解==来配置和映射原始类型、接口和 Java POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

1.2、Mybatis的特点

简单易学
灵活
解除 SQL 与代码的耦合
可实现 ORM 映射
支持对象关系映射
支持动态SQL

2、第一个Mybatis程序

思路：搭建环境-->导入Mybatis-->编写代码--->测试

2.1、环境搭建

搭建数据库

CREATE DATABASE mybatis;

USE mybatis;

CREATE TABLE `user`(
	`id` INT(20) NOT NULL PRIMARY KEY,
	`name` VARCHAR(40) NOT NULL,
	`pwd` VARCHAR(40) NOT NULL
)ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO `user`(`id`, `name`, `pwd`) VALUES
(1, '张三', '123456'),
(2, '李四', '654321'),
(3, '王五', '111111')

创建一个maven项目，导入Mybatis和mysql、junit包

<dependencies>
  <dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.25</version>
  </dependency>
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.13</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.5.7</version>
  </dependency>
</dependencies>

2.2、创建第一个模块

编写mybatis核心配置文件

在项目的resources下创建mybatis配置文件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">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&amp;useUnicodetrue&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="org/mybatis/example/BlogMapper.xml"/>
    </mappers>
</configuration>

在项目的utils包下创建MybatisUtils工具类

package com.luca.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;

//SqlSessionFactory --> sqlSession
public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    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();
        }
    }

    //既然有了sqlSessionFactory对象，顾名思义就可以使用它获取sqlSession对象了
    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
    }
}

2.3、编写代码

实体类

在pojo包下创建user表对应的实体类User

package com.luca.pojo;

public class User {
    private int id;
    private String name;
    private String pwd;

    public User() {
    }

    public User(int id, String name, String pwd) {
        this.id = id;
        this.name = name;
        this.pwd = pwd;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }
}

Dao接口

在dao包下创建UserMapper接口

package com.luca.dao;

import com.luca.pojo.User;
import java.util.List;

public interface UserMapper {
    List<User> getUserList();
}

接口实现类

又原来的Impl转变为xml文件

<?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绑定一个dao接口-->
<mapper namespace="com.luca.dao.UserMapper">
    <!--id相当于重写方法的名字-->
    <select id="getUserList" resultType="com.luca.pojo.User">
        select * from mybatis.user;
    </select>
</mapper>

截止目前，项目的结果如下，如果有没有说请清楚的地方可以参考下：

2.4、测试

在test中创建与java中对应的包路径，添加测试方法，点击运行会报错

package com.luca.dao;

import com.luca.pojo.User;
import com.luca.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class TestUserMapper {
    @Test
    public void test() {
        //第一步：获取sqlSession
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        //第二步：执行sql - 方式一
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = mapper.getUserList();
        for (User user : userList) {
            System.out.println(user.getName());
        }
        System.out.println("==========");
        //第二步：执行sql - 方式二
        List<User> userList2 = sqlSession.selectList("com.luca.dao.UserMapper.getUserList");
        for (User user : userList2) {
            System.out.println(user.getName());
        }

        //关闭连接
        sqlSession.close();
    }
}

因为我们创建的UserMapper没有在mybatis-config.xml中注册，在配置文件中跟新mapper内容如下
```
<mappers>
  <mapper resource="com/luca/dao/UserMapper.xml"/>
</mappers>
```

再次运行测试方法，发现还是报错，提示找不到资源（因为配置文件不在resources目录下）

Caused by: java.io.IOException: Could not find resource com/luca/dao/UserMapper.xml
	at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:114)
	at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:100)
	at org.apache.ibatis.builder.xml.XMLConfigBuilder.mapperElement(XMLConfigBuilder.java:376)
	at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:120)
	... 30 more

遇到这种情况，需要在maven的配置中增加资源过滤的片段，如下：

<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>

我们清理下maven后重新运行下测试方法，发现sql语句可以正常查询了，输出了我们期望的数据

Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
张三
李四
王五
==========
张三
李四
王五

Process finished with exit code 0

posted @ 2021-09-12 21:42 LucaZ 阅读(82) 评论(0) 编辑收藏举报

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Luca Z