MyBatis First Demo

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实践中入门，MyBatis框架小程序

1. 基本学习

MyBatis的执行流程·：

  1. mybatis配置文件，包括Mybatis全局配置文件和Mybatis映射文件，其中全局配置文件配置了数据源、事务等信息；映射文件配置了SQL执行相关的信息
  2. mybatis通过读取配置文件信息（全局配置文件和映射文件），构造出SqlSessionFactory，即会话工厂
  3. 通过SqlSessionFactory，可以创建SqlSession(即会话,Mybatis是通过SqlSession来操作数据库的)
  4. SqlSession本身不能直接操作数据库，它是通过底层的Executor执行器接口来操作数据库的。Executor接口有两个实现类，一个是普通执行器，一个是缓存执行器（默认）
  5. Executor执行器要处理的SQL信息是封装到一个底层对象MappedStatement中。该对象包括：SQL语句、输入参数映射信息、输出结果集映射信息。其中输入参数和输出结果的映射类型包括java的简单类型、HashMap集合对象、POJO对象类型

2. 环境配置

IDEA Maven项目
数据库环境: MySQL
MyBatisMaven三维坐标

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<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.4.1</version>
</dependency>

3. demo编写

测试代码

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public class UserDaoImpl implements UserDao1 {

    // 此处选用namespace为"user"，选用此处方法需将UserMapper.xml文件中namespace改过来
    public User findUserById(int id) throws Exception {
        String resource = "mybatis-config1.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = factory.openSession();
        //---------------
        User user = session.selectOne("user.findUserById",id); //参数一：namespace.id
        //--------------
        session.close();
        return user;
    }
}

Mapper映射配置文件

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<!--<mapper namespace="user">-->
<mapper namespace="user">
    <select id="findAllUsers" resultType="com.yza.demo1.entity.User">
        SELECT * FROM USER
    </select>
    <select id="insertUser" parameterType="com.yza.demo1.entity.User">
        INSERT INTO USER(id,userName,password,age) VALUES(#{id},#{userName},#{password},#{age})
    </select>
    <select id="deleteUserById" parameterType="int" >
        DELETE FROM USER WHERE id=#{id}
    </select>
    <select id="updateUserPassword" parameterType="com.yza.demo1.entity.User">
        UPDATE USER SET password=#{password} WHERE id=#{id}
    </select>
    <select id="findUserById" parameterType="int" resultType="com.yza.demo1.entity.User">
        SELECT * FROM USER WHERE id=#{id}
    </select>
</mapper>

Mapper接口

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public interface UserDao1 {
    public User findUserById(int id) throws Exception;
    public List<User> findAllUsers() throws Exception;
    public void insertUser(User user) throws Exception;
    public void deleteUserById(int id) throws Exception;
    public void updateUserPassword(User user) throws Exception;

}

4. 升级版–Mapper代理开发模式

  • Mapper代理的开发方式，程序员只需要编写mapper接口（相当于dao接口），不需再编写dao实现类（Mybatis会自动的为mapper接口生成动态代理实现类）
  • 要实现mapper代理的开发方式，需要遵循一些开发规范（接口的包名，类名，参数，返回值分别对应着映射文件的namespace，id，parameterType，resultType）
  1. mapper接口的全限定名要和mapper映射文件的namespace的值相同
  2. mapper接口的方法名称要和mapper映射文件中的statement的id相同
  3. mapper接口的方法参数只能有一个，且类型要和mapper映射文件中statement的parameterType的值保持一致
  4. mapper接口的返回值类型要和mapper映射文件中statement的resultType值或resultMap中的type值保持一致

UserMapper映射文件

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<?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标签要指定namespace属性，设置为Mapper接口的全路径名-->
<mapper namespace="com.yza.demo1.dao.UserDao">
    <select id="findAllUsers" resultType="com.yza.demo1.entity.User">
        SELECT * FROM USER
    </select>
    <select id="insertUser" parameterType="com.yza.demo1.entity.User">
        INSERT INTO USER(id,userName,password,age) VALUES(#{id},#{userName},#{password},#{age})
    </select>
    <select id="deleteUserById" parameterType="int" >
        DELETE FROM USER WHERE id=#{id}
    </select>
    <select id="updateUserPassword" parameterType="com.yza.demo1.entity.User">
        UPDATE USER SET password=#{password} WHERE id=#{id}
    </select>
    <select id="findUserById" parameterType="int" resultType="com.yza.demo1.entity.User">
        SELECT * FROM USER WHERE id=#{id}
    </select>
</mapper>

Mapper接口

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public interface UserMapper {
    public User findUserById(int id);
    public List<User> findUserAll();
    public void insertUser(User user);
    public void deleteUserById(int id);
    public void updateUserPassword(User user);
}

报错1

报错具体信息

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### Error querying database.  Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for user.findUserById
### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for user.findUserById

本质原因–mapper文件的namespace和DaoImpl路径不一致
mapper文件

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<!-- mapper标签要指定namespace属性，不然会报错，可看做包名 -->
<mapper namespace="com.yza.demo1.dao.UserDao">
<!--<mapper namespace="user">-->
    <select id="findUserById" parameterType="int" resultType="com.yza.demo1.entity.User">
        select * from user where id=#{id}
    </select>
</mapper>

DaoImpl方法–>此处namespace为”user”

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public class UserDaoImpl implements UserDao {

    public User findUserById(int id) throws Exception {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = factory.openSession();
        //---------------
        User user = session.selectOne("user.findUserById",id); //参数一：namespace.id
        //--------------
        session.close();
        return user;
    }
}

总结：

  • namespace：必须与UserDao的全路径想匹配，这样才能使的接口与sql文件一一对应

报错2

  • 使用Java和MyBatis时遇到的数据无法插入问题
    解决办法–>手动commit

session关闭前不会自动提交

  • 手动commit、close，两次导入数据库，报错
    解决办法：关闭是必须的，就不再提交commit
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public static void main(String[] args) throws Exception {
        SqlSession sqlSession = SqlSessionFactoryUtil.openSession();
        UserDao userDao = sqlSession.getMapper(UserDao.class);

        User user = new User(3,"yinziyang","111111",22);
        userDao.insertUser(user);

        //sqlSession.commit();
        sqlSession.close();
    }

附录：

MyBatisUtil工具类

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import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.*;
import java.io.InputStream;

public class SqlSessionFactoryUtil {

    private static SqlSessionFactory sqlSessionFactory;

    /**
     * 初始化sqlSessionFactory
     */
    static {
        getSqlSessionFactory();
    }

    /**
     * 获取SqlSessionFactory
     * @return
     */
    public static SqlSessionFactory getSqlSessionFactory() {
        if(sqlSessionFactory==null) {
            InputStream inputStream = null;
            try {
                inputStream = Resources.getResourceAsStream("mybatis-config.xml");
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            } catch(Exception e) {
                e.printStackTrace();
            }
        }
        return sqlSessionFactory;
    }

    /**
     * 获取SqlSession
     * @return
     */
    public static SqlSession openSession() {
        return sqlSessionFactory.openSession();
    }

    /**
     * 关闭SqlSession
      * @param sqlSession
     */
    public static void clSSoseSession(SqlSession sqlSession) {
        if(sqlSession != null) {
            sqlSession.close();
        }
    }

}

参考

MyBatis学习（1）：Mybatis使用详解和入门案例