MyBatis的Dao层实现

1.传统开发方式

<dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.29</version>
    </dependency>
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.9</version>
    </dependency>
</dependencies>

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

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

import com.caoyan.dao.UserMapper;
import com.caoyan.domain.User;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class UserMapperImpl implements UserMapper {

    @Override
    public List<User> findAll() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sessionFactory.openSession();
        List<User> userList = sqlSession.selectList("userMapper.findAll");
        return userList;
    }

}

import java.io.IOException;
import java.util.List;

import com.caoyan.dao.UserMapper;
import com.caoyan.dao.impl.UserMapperImpl;
import com.caoyan.domain.User;

public class ServiceDemo {
    public static void main(String[] args) throws IOException {
        // 创建dao层对象当前dao层实现是手动编写的
        UserMapper userMapper = new UserMapperImpl();
        List<User> all = userMapper.findAll();
        System.out.println(all);
    }
}

<?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="userMapper">
    <!-- 查询操作 -->
    <select id="findAll" resultType="user">
        select * from user
    </select>
</mapper>

2.代理开发方式

采用MyBatis的代理开发方式实现DAO层的开发，这种方式使我们后面进入企业的主流。

Mapper接口开发方式方法只需要程序员编写mapper接口（相当于Dao接口），由MyBatis框架根据接口定义创建接口的动态代理对象，代理对象的方法同上边Dao接口实现类方法。

Mapper接口开发需要遵循以下规范：

• Mapper.xml文件的namespace与mapper接口的全限定名相同

• Mapper接口方法名和Mapper.xml中定义的每个statement的id相同
• Mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同
• Mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同

<?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.caoyan.dao.UserMapper">
    <!-- 查询操作 -->
    <select id="findAll" resultType="user">
        select * from user
    </select>
    <!-- 根据id查询 -->
    <select id="findById" parameterType="int" resultType="user">
        select * from user where id = #{id}
    </select>
</mapper>

public interface UserMapper {
    public List<User> findAll() throws IOException;

    public User findById(int id);
}

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

import com.caoyan.dao.UserMapper;
import com.caoyan.domain.User;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class ServiceDemo {
    public static void main(String[] args) throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        List<User> all = userMapper.findAll();
        System.out.println(all);
        User user = userMapper.findById(1);
        System.out.println(user);
    }
}