本片博文从传统jdbc方式引出mybatis,进而讲解mybatis如何使用。

1.jdbc使用方式

(此处仅列出核心代码)

/**
jdbc工具类
**/
public class DBUtils {
 public static Connection openConnection() {
            Class.forName(DRIVERNAME);
            return DriverManager.getConnection(URL, USERNAME, PASSWORD);
           }
}
/**
userJdbc
**/
public class UserJdbc {
    public void insertUser(User user){
        Connection connection = DBUtils.openConnection();
        String sql = "insert into user(username,age) value(?,?)";
        try {
            PreparedStatement preparedStatement = connection.prepareStatement(sql);
            preparedStatement.setString(1,user.getUsername());
            preparedStatement.setInt(2,user.getAge());
            preparedStatement.executeUpdate();
        } catch (SQLException e) {
            logger.error("预编译sql出错",e);
        } finally {
            DBUtils.closeConnection(connection);
        }
    }
    public User selectUserOne(int id) throws SQLException {
        Connection connection = DBUtils.openConnection();
        String sql = "select id,username,age,phone,`desc` from user where id =  ?";
        PreparedStatement preparedStatement = connection.prepareStatement(sql);
        preparedStatement.setInt(1,id);
        ResultSet resultSet = preparedStatement.executeQuery();
         /**
          * @notice 此处需要手动将结果集映射
         **/
        if(resultSet.next()) {
            User user = new User();
            user.setId(resultSet.getInt(1));
            user.setUsername(resultSet.getString(2));
            user.setAge(resultSet.getInt(3));
            user.setPhone(resultSet.getString(4));
            user.setDesc(resultSet.getString(5));
            return user;
        }
        return null;
    }
}

2.mybatis与jdbc对比

• 传统jdbc没有用数据库连接池，需要频繁创建连接，关闭连接,消耗系统资源。
• jdbc无法将结果集映射成对象。
• 使用 PreparedStatement设置参数时，需要1/2/3等数字标识参数顺序
• 总体来说api操作比较麻烦.

3.mybatis基本使用

• 引入maven依赖

<dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.4.4</version>
    </dependency>
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>5.1.30</version>
    </dependency>

• 配置mybatis-config.xml

<?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>
    <properties resource="jdbc.properties"/>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/UserMapper.xml"/>
        <mapper class="com.dck.mapper.UserMapperAnnotation"/>
    </mappers>
</configuration>

• mapper文件
  (举例UserMapper.xml)

<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dck.mapper.UserMapper">
    <select id="selectUserById" resultType="com.dck.pojo.User">
        select id ,username,age,phone,`desc` from user where id = #{id}
    </select>
</mapper>

• 创建sqlsessionfactory

String resource = "mybatis-config.xml";
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory =
                new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //以下任意一种方式均可
        User user1 = (User)sqlSession.selectOne("com.dck.mapper.UserMapper.selectUserById", 1);
       //推荐此种方式
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.selectUserById(1);

至此完成mybatis的部署

4.mybatis全局配置详解

5. #{}与${}的区别

• #{}采用的是预编译方式,可以防止sql注入,推荐使用这个
• ${}也有其用处,比如动态拼接sql时,对于表名的处理可以使用${}