使用idea创建第一个Mybatis程序及可能遇到的问题

第一个Mybatis程序

思路：搭建环境—>导入Mybatis—>编写代码—>执行

搭建环境

创建数据库

CREATE DATABASE `mybatis`
USE `mybatis`
CREATE TABLE `user`(
	`id` INT(20) NOT NULL PRIMARY KEY,
	`name` VARCHAR(30)  DEFAULT NULL ,
	`pwd`  VARCHAR(30)  DEFAULT NULL
)ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO `user`(`id`,`name`,`pwd`) VALUES
(1,'张三','zs123'),
(2,'李四','ls456'),
(3,'王五','ww789')

创建项目

****先看一下我的项目结构：

1. 创建项目
2. 导入Maven依赖

 <dependencies>
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>servlet-api</artifactId>
        <version>2.5</version>
    </dependency>
    <dependency>
        <groupId>javax.servlet.jsp</groupId>
        <artifactId>javax.servlet.jsp-api</artifactId>
        <version>2.3.3</version>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.47</version>
    </dependency>
        <!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.5</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/junit/junit -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13</version>
            <scope>test</scope>
        </dependency>

    </dependencies>

1. 编写代码

创建一个Module

• 编写mybatis的核心配置文件

  <?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;useUnicode=true&amp;CharacterEncoding=UTF-8"/>
                  <property name="username" value="root"/>
                  <property name="password" value="1234"/>
              </dataSource>
          </environment>
      </environments>
  
  </configuration>
  

• 编写Mybatis工具类

package com.vchicken.Dao;

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.InputStream;

public class MybatisUtils {

    public  static SqlSessionFactory sqlSessionFactory;
    static{
        try {
            String resource = "org/mybatis/example/mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

编写代码

创建实体类

package com.vchicken.pojo;

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

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

Dao接口

package com.vchicken.dao;

import com.vchicken.pojo.User;

import java.util.List;

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

接口实现类(由UserDaoImpl文件转变为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">
<mapper namespace="com.vchicken.dao.UserDao" >
    <select id="getUserList" resultType="com.vchicken.pojo.User">
    select * from mybatis.user
  </select>
</mapper>

测试

package com.vchicken.dao;

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

import java.util.List;

public class UserDaoTest {
    @Test
    public void test(){
        //获得sqlsession对象
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        //方法一
//        UserDao userDao = sqlSession.getMapper(UserDao.class);
//        List<User> userList = userDao.getUserList();

        //方法二，不推荐使用
        List<User> userList = sqlSession.selectList("com.vchicken.dao.UserDao.getUserList");
        for (User user : userList) {
            System.out.println(user);
        }

        //关闭sqlsession
        sqlSession.close();
    }

}

可能遇到的问题

1. 找不到config.xml文件

   java.io.IOException: Could not find resource org/mybatis/example/mybatis-config.xml
   

• pom.xml文件中缺少以下:

<!-- 如果不添加此节点mybatis的mapper.xml文件都会被漏掉。 -->
    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
        </resources>
    </build>

• 还有一种可能就是路径错误，在封装工具类的时候，直接复制官方Mapper.xml的路径，而实际不同，所以会导致找不到文件

   String resource = "org/mybatis/example/mybatis-config.xml";
  //改为自己的路径即可
   String resource = "mybatis-config.xml";
  

1. 空指针异常

java.lang.NullPointerException

• 在将从 XML 文件中构建 SqlSessionFactory 的实例的过程封装为工具类的时候，出现错误

public class MybatisUtils {

    public  static SqlSessionFactory sqlSessionFactory;
    static{
        try {
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

原因

在这里我已经把sqlSessionFactory设为一个全局的对象，而在下面又再次实例化对象，开辟新的栈空间，导致了空指针异常，修改后则代码运行正常。

sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

3.出现以下错误

org.apache.ibatis.exceptions.PersistenceException:
### Error building SqlSession.
### The error may exist in com/vchicken/dao/UserMapper.xml
### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error creating document instance.  Cause: com.sun.org.apache.xerces.internal.impl.io.MalformedByteSequenceException: 1 字节的 UTF-8 序列的字节 1 无效。

• 原因是：在UserMapping.xml文件中出现中文注释

<?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">
<!--绑定一个Dao/mapping接口-->
<mapper namespace="com.vchicken.dao.UserDao" >
    <select id="getUserList" resultType="com.vchicken.pojo.User">
    select * from mybatis.user
  </select>
</mapper>

将中文注释去掉即可

1. 出现以下错误：

org.apache.ibatis.exceptions.PersistenceException:
### Error querying database.  Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.vchicken.dao.UserDao.getUserList
### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.vchicken.dao.UserDao.getUserList

• 原因：没有在mybatis-config.xml文件中配置文件

<mappers>
  <mapper resource="com/vchicken/dao/UserMapper.xml"></mapper>//注意是自己的文件路径
</mappers>

1. 运行结果