第二章 第一個Mybatis程式

思路：

搭建環境--->匯入Mybatis--->編寫程式碼----->測試

1.搭建環境

2.建立專案

1. 新建一個普通Maven專案

2. 匯入Maven依賴

   <dependencies>
       <!--mysql驅動-->
       <dependency>
           <groupId>mysql</groupId>
           <artifactId>mysql-connector-java</artifactId>
           <
    
   version>8.0.19</version>
       </dependency>
       <!--mybatis-->
       <dependency>
           <groupId>org.mybatis</groupId>
           <artifactId>mybatis</artifactId>
           <version>3.5.4</version>
       </dependency>
       <!--junit-->
       <
    
   dependency>
           <groupId>junit</groupId>
           <artifactId>junit</artifactId>
           <version>4.13</version>
           <scope>test</scope>
       </dependency>
   ​
   </dependencies>

3. 建立模組

   • 編寫mybatis的核心配置檔案

     <?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核心配置檔案-->
     <configuration>
         <environments default="development">
             <environment id="development">
                 <transactionManager type="JDBC"/>
                 <dataSource type="POOLED">
                     <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                     <property name="url" value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=Asia/Shanghai&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                     <property name="username" value="root"/>
                     <property name="password" value="123456"/>
                 </dataSource>
             </environment>
         </environments>
     ​
     </configuration>

• 編寫mybatis工具類

  package com.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   -->sqlSessioin
  public class MybatisUtils {
  ​
      private static SqlSessionFactory sqlSessionFactory;
  ​
      static {
          try {
              //獲取sqlSessionFactory物件
              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，顧名思義，我們可以從中獲得 SqlSession 的例項。
      // SqlSession 提供了在資料庫執行 SQL 命令所需的所有方法。你可以通過 SqlSession 例項來直接執行已對映的 SQL 語句。
      public static SqlSession getSqlSession(){
         return sqlSessionFactory.openSession();
      }
  }

  • 編寫程式碼

    1. 實現類

       package com.pojo;
       ​
       public class User {
           private int id;
           private String name;
           private String pwd;
       ​
       ​
           @Override
           public String toString() {
               return "User{" +
                       "id=" + id +
                       ", name='" + name + '\'' +
                       ", 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;
           }
       }

       2. DAO介面

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

       3. 介面實現類 由原來的UserDaoImpl轉變為一個Mapper配置檔案

          <?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繫結一個對應的Dao/Mapper介面-->
          <mapper namespace="com.dao.UserDao">
             <select id="getUserList" resultType="com.pojo.User">
                 select * from mybatis.user
             </select>
          </mapper>

• 測試

  注意點：org.apache.ibatis.binding.BindingException: Type interface com.dao.UserDao is not known to the MapperRegistry.

  public void test(){
      //第一步：獲得SqlSession物件
      SqlSession sqlSession = MybatisUtils.getSqlSession();
      //方式一：執行SQL
      UserDao userDao = sqlSession.getMapper(UserDao.class);
      List<User> userList = userDao.getUserList();
      for (User user : userList) {
          System.out.println(user);
      }
      //關閉SqlSession
      sqlSession.close();
  }

可能遇見的問題：

1. 配置檔案沒有註冊

2. 繫結介面錯誤

3. 方法名不對

4. 返回型別不對

5. Maven匯出資源型別