問題發現

在學習MyBatis框架的時候，不使用 XML 構建 SqlSessionFactory，調用Mapper的接口，報類型接口沒有注冊。

示例代碼如下：

public class Test {public static void main(String[] args) throws IOException {DataSource dataSource = new PooledDataSource("com.mysql.jdbc.Driver", "jdbc:mysql://localhost:3306/spring_data?characterEncoding=utf-8&useSSL=false", "root", "123456");TransactionFactory transactionFactory = new JdbcTransactionFactory();Environment environment = new Environment("development", transactionFactory, dataSource);org.apache.ibatis.session.Configuration configuration = new org.apache.ibatis.session.Configuration(environment);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);SqlSession sqlSession = sqlSessionFactory.openSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);List<User> users = userMapper.selectByUser();// 關閉 SqlSessionsqlSession.close();}
}
public interface UserMapper {List<User> selectByUser();
}

<?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.example.mybatisstudy.dao.UserMapper"><select id="selectByUser" resultType="com.example.mybatisstudy.User">select * from user;</select>
</mapper>

網上說都是命名空間的錯誤，發現命名空間也沒問題。然后進行一系列的問題排查。

問題解決

方法一：檢查Mapper文件的namespace路徑是否正確

這也是網上解決方法最多的一種，貌似大部分人都是遇到這種錯誤。

1. 先確認映射的類名是否正確。
2. 在確認包路徑使用的.點（com.*.*.Object）而不是/左斜杠（com/*/*/Object）或\右斜杠（com\*\*\Object），如果錯誤請修改為.點。
3. 如果鼠標懸浮到路徑上，顯示下劃線說明配置正確。

如圖所示

方法二：使用其他方法是否正確

后面切換到，使用XML 中構建 SqlSessionFactory方法，示例代碼如下：

public class Test {public static void main(String[] args) throws IOException {DataSource dataSource = new PooledDataSource("com.mysql.jdbc.Driver", "jdbc:mysql://localhost:3306/spring_data?characterEncoding=utf-8&useSSL=false", "root", "123456");TransactionFactory transactionFactory = new JdbcTransactionFactory();Environment environment = new Environment("development", transactionFactory, dataSource);org.apache.ibatis.session.Configuration configuration = new org.apache.ibatis.session.Configuration(environment);configuration.addMapper(UserMapper.class);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);SqlSession sqlSession = sqlSessionFactory.openSession();List<User> selectByUser = sqlSession.selectList("selectByUser");// 關閉 SqlSessionsqlSession.close();}
}

運行成功，如圖所示

然后逐個代碼排除，根據錯誤提示（未注冊）發現少了configuration.addMapper()方法的代碼，加上后，運行成功。

示例代碼如下

public class Test {public static void main(String[] args) throws IOException {DataSource dataSource = new PooledDataSource("com.mysql.jdbc.Driver", "jdbc:mysql://localhost:3306/spring_data?characterEncoding=utf-8&useSSL=false", "root", "123456");TransactionFactory transactionFactory = new JdbcTransactionFactory();Environment environment = new Environment("development", transactionFactory, dataSource);org.apache.ibatis.session.Configuration configuration = new org.apache.ibatis.session.Configuration(environment);configuration.addMapper(UserMapper.class);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);SqlSession sqlSession = sqlSessionFactory.openSession();UserMapper mapper = sqlSession.getMapper(UserMapper.class);List<User> users = mapper.selectByUser();// 關閉 SqlSessionsqlSession.close();}
}

問題解決。

最開始看官網給的代碼片段，以為可以去掉addMapper的步驟，后來才想起不注冊的話怎么獲取呢？嘖~還是太粗心了呀！！！