在 idea 下搭建的第一个MyBatis项目及增删改查用法

Posted 2021-03-06 wangjr1994

Mybatis搭建第一个项目

1. 简介

1.1 什么是 MyBatis？

• MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生类型、接口和 Java 的 POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

如何获取Mybatis？

• GitHub 源码：https://github.com/mybatis/mybatis-3/releases

• 中文文档：https://mybatis.org/mybatis-3/zh/index.html

• 对应的 maven 依赖

  <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.2</version>
  </dependency>

1.2 持久化

数据持久化

• 持久化就是将程序的数据在持久状态和瞬时状态转化的过程
• 内存：断电即失--->数据库，io 文件持久化

1.3 持久层

Dao 层

• 完成持久化工作的代码块
• 层界限十分明显

1.4 为什么需要Mybatis?

• 传统的 JDBC 代码太复杂，简化操作，减少代码冗余。
• 帮助开发人员将数据存入到数据库中
• 灵活，易于使用
• 解除 sql 与程序代码的耦合，sql 和代码的分离，提高了可维护性
• 提供 xml 标签，支持编写动态 sql

2. 搭建第一个Mybatis程序

• 配置 mybatis-confgi.xml 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="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${password}"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
      <mapper resource="com/youzi/dao/UserMapper.xml"/>
  </mappers>
</configuration>

• 编写工具类

public class MybatisUtils {
    
    private static SqlSessionFactory sqlSessionFactory;
    
    static {
        try {
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
    }
}

• 编写 pojo

package com.youzi.pojo;

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

• DAO 接口

public interface UserMapper {
    List<User> getUserList();

    User getUserById(int uId);

    int addUser(User user);

    int updateUser(User user);

    int deleteUser(int id);
}

• 接口实现类由原来 JDBC 中的 UserDaoImpl 转变为配置文件

<?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.youzi.dao.UserDao">
    <select id="getUserList" resultType="com.youzi.pojo.User">
        select * from mybatis_db.user;
    </select>
</mapper>

3. CRUD

• select

<select id="getUserList" resultType="com.youzi.pojo.User">
    select * from mybatis_db.user;
</select>

<select id="getUserById" parameterType="int" resultType="com.youzi.pojo.User">
    select * from user where id = #{uId};
</select>

@Test
public void test_getUser() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    List<User> userList = mapper.getUserList();
    for (User user : userList) {
        System.out.println(user);
    }
    sqlSession.close();
}

@Test
public void test_getUserById() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    User user = mapper.getUserById(1);
    System.out.println(user);

    sqlSession.close();
}

修改，添加，删除因为有事务的存在所以必须 commit

• insert

<insert id="addUser" parameterType="com.youzi.pojo.User">
    insert into user (id, name, pwd) values (#{id},#{name},#{pwd});
</insert>

 @Test
public void test_addUser() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    int i = mapper.addUser(new User(4, "王五", "2323223"));
    System.out.println(i);

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

}

• update

<update id="updateUser" parameterType="com.youzi.pojo.User">
    update user set name = #{name},pwd = #{pwd} where id = #{id};
</update>

@Test
public void test_updateUser() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);

    int i = mapper.updateUser(new User(2, "李四", "123123"));
    System.out.println(i);

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

• delete

<delete id="deleteUser" parameterType="int">
    delete from user where id = #{id};
</delete>

@Test
public void test_deleteUser() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    int i = mapper.deleteUser(4);
    System.out.println(i);

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

上面的 sqlSession 也可以使用 try() 语句创建 即 try-with-resources 自动关闭资源

try(SqlSession sqlSession = MybatisUtils.getSqlSession()){
    ...
}

过程中出现的错误

org.apache.ibatis.binding.BindingException: Type interface com.youzi.dao.UserDao is not known to the MapperRegistry. 没有绑定 mapper

<mappers>
    <mapper resource="com/youzi/dao/UserMap.xml"/>
</mappers>

idea 放在 resources 目录下报错 java.io.IOException: Could not find resource mybatis-config.xml

新建一个文件目录指定为资源目录然后把 mybatis-config.xml 放到这个下面再运行就好了。

只要命名为 resources 就不能正常读取资源。。