[Java知识库]mybatis入门学习及搭建

Mybatis学习

一、第一个mybatis程序（查询数据库中的数据）

1、新建maven项目

2、导入Maven依赖

<?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>

    <!--父工程-->
    <groupId>com.kuang</groupId>
    <artifactId>mybatis-study</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>

    <!--子工程-->
    <modules>
        <module>mybatis-01</module>
    </modules>

    <dependencies>

        <!--mysql驱动-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.25</version>
        </dependency>

        <!--mybatis-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.7</version>
        </dependency>

        <!--junit-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13.2</version>
        </dependency>

    </dependencies>

    <!--在build中配置resources，来防止我们资源导出失败问题-->
    <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>
</project>

3、编写mybatis的核心配置文件 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_kuang?useSSL=true&amp;
                                            useUnicode=true&amp;characterEncoding=UTF-8 "/>
                <property name="username" value="root"/>
                <property name="password" value="000"/>
            </dataSource>
        </environment>
    </environments>
     <!--每一个mapper.xml文件都需要在mybatis核心配置文件中注册-->
    <mappers>
        <mapper resource="com/kuang/dao/UserMapper.xml"/>
    </mappers>
</configuration>

4、编写mybatis工具类 MybatisUtils.java

//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 = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    /**
     * 既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例。
     * SqlSession 提供了在数据库执行 SQL 命令所需的所有方法。你可以通过 SqlSession 实例来直接执行已映射的 SQL 语句。
     */

    public static SqlSession getSqlSession(){

        return sqlSessionFactory.openSession();
    }
}

5、编写代码

①实体类

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

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", pwd='" + pwd + '\'' +
                '}';
    }
}

②dao接口 

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

③接口实现类：由原来的UserDaoImpl 接口实现类转变成Mapper配置文件

<?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: 绑定一个对应的Dao（Mapper）接口；-->
<mapper namespace="com.kuang.dao.UserDao">

    <!--select查询语句-->
    <select id="getUserList" resultType="com.kuang.pojo.User">
        select * from mybatis_kuang.user
    </select>

</mapper>

6、测试
junit测试

public class UserMapperTest {

    @Test
    public void test(){
        //第一步：获取sqlSession对象
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        //方式一：getMapper
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = userMapper.getUserList();

        for (User user : userList){
            System.out.println(user);
        }

        //关闭SqlSession
        sqlSession.close();
    }
}

注意：
遇到的错误及解决方法；
Caused by: org.apache.ibatis.exceptions.PersistenceException:
Error building SqlSession.
The error may exist in com/kuang/dao/UserMapper.xml
Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.io.IOException: Could not find resource com/kuang/dao/UserMapper.xml

**解决方法：**在pom.xml配置文件中加入一下代码！！！

    <!--在build中配置resources，来防止我们资源导出失败问题-->
    <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>