Idea搭建Mybatis入门案例-----遇到的小问题

1.创建一个maven项目，学习mybatis的框架功能的话，建议一个空的maven工程项目。

二，在pom.xml中导入相应的jar包依赖

<?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>

    <groupId>cn.waggag</groupId>
    <artifactId>MybatisDemo1</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <!--Mybatis的依赖-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>
        <!--mysql数据库连接器-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.16</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.16</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
    </dependencies>
</project>

三、创建实体类和接口

3.1、实体类

package cn.waggag.domain;

import java.io.Serializable;
import java.util.Date;

/**
 * @description: User的实体类
 * @author: waggag
 * @time: 2019/7/15 0:14
 * @Company http://www.waggag.cn
 */
public class User implements Serializable {

    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;
    //setter和getter方法
}

3.2、接口

package cn.waggag.mapper;
import cn.waggag.domain.User;
import java.util.List;
/**
 * @description:用户的持久层接口
 * @author: waggag
 * @time: 2019/7/15 0:18
 * @Company http://www.waggag.cn
 */
public interface UserMapper {
    /**
     * 查询所有用户操作
     */
    List<User> findAll();
}

3.3、接口的配置文件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="cn.waggag.mapper.UserMapper">
    <!--配置查询所有-->
    <select id="findAll" resultType="cn.waggag.domain.User">
        select * from user;
    </select>
</mapper>

四、创建Mybatis的配置文件，放在resources下。

<?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">
<!--mybatis主配置文件-->
<configuration>
    <!--配置环境-->
    <environments default="mysql">
        <!--配置mysql的环境-->
        <environment id="mysql">
            <!--配置事务的类型-->
            <transactionManager type="JDBC"></transactionManager>
            <!--配置数据源（连接池）-->
            <dataSource type="POOLED">
                <!--配置连接数据库的基本信息-->
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/ssm"/>
                <property name="username" value="username"/>
                <property name="password" value="password"/>
            </dataSource>
        </environment>
    </environments>
    <!-- 指定映射配置文件的位置，映射配置文件指的是每个dao独立的配置文件 -->
    <mappers>
        <mapper resource="cn/waggag/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

注意： <mapper resource="cn/waggag/mapper/UserMapper.xml"/>的配置为文件的relative Path

五、创建一个测试类进行测试

package cn.waggag.test;

import cn.waggag.mapper.UserMapper;
import cn.waggag.domain.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;
/**
 * @description: mybatis的入门
 * @author: waggag
 * @time: 2019/7/15 0:43
 * @Company http://www.waggag.cn
 */
public class MybatisTest {
    /**
     * mybatis的入门
     * @param args
     */
    public static void main(String[] args) throws IOException {
        //1.读取配置文件
        InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.创建一个SqlSessionFactory工厂
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = builder.build(inputStream);
        //3.使用工厂生成SqlSession对象
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //4.使用SqlSession创建Dao接口的代理对象
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        //5.使用代理对象执行方法
        List<User> userList = mapper.findAll();
        for (User user : userList) {
            System.out.println(user);
        }
        //6.释放资源
        sqlSession.close();
        inputStream.close();
    }
}

六、入门案例中碰到的错误

6.1、数据库时区问题。

### Error querying database. Cause: java.sql.SQLException: The server time zone value '�й��׼ʱ��' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the serverTimezone configuration property) to use a more specifc time zone value if you want to utilize time zone support.

解决办法：

//在SqlSessionConfig.xml中配置数据库时区
<property name="url" value="jdbc:mysql://localhost:3306/ssm?serverTimezone=UTC"/>

6.2、Error Opening Session 空指针问题。

解决方法：< environments default=”mysql“> 的默认值需要指定下面配置的环境<environment id="mysql">

<!--mybatis主配置文件-->
<configuration>
    <!--配置环境-->
    <environments default="mysql">
        <!--配置mysql的环境-->
        <environment id="mysql">
            <!--配置事务的类型-->
            <transactionManager type="JDBC"></transactionManager>
            <!--配置数据源（连接池）-->
            <dataSource type="POOLED">
                <!--配置连接数据库的基本信息-->
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/ssm?serverTimezone=UTC"/>
                <property name="username" value="waggag"/>
                <property name="password" value="225514"/>
            </dataSource>
        </environment>
    </environments>
    <!-- 指定映射配置文件的位置，映射配置文件指的是每个dao独立的配置文件 -->
    <mappers>
        <mapper resource="cn/waggag/mapper/UserMapper.xml"/>
    </mappers>
</configuration>

6.3、无法找到接口的映射文件

解决方法1：

在pom文件中加入下面代码，发布src/main/java和src/main/resources下的资源文件到classes中

    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.xml</include>
                </includes>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.xml</include>
                    <include>**/*.properties</include>
                </includes>
            </resource>
        </resources>
    </build>

注意：添加src/main/java的resource,,必须配置src/main/resources,否则resources下的配置文件无法正常访问

解决方法2：

在resources下创建和接口相同的路径，放入接口的映射文件，目录结构如下，配置文件可正常发布到classes下。

原文链接：https://blog.csdn.net/qq_38264999/article/details/95937478