Mybatis中开发Dao的方法

• 方法一:我们只需要开发一个Dao接口，以及其实现类。

(1) UserDao.java

package test.dao;

import test.entity.User;

public interface UserDao {
    //根据用户信息查询用户信息
    public User findUserById(Integer id);
}

(2)  UserDaoImpl.java(UserDao接口的实现类)

       该实现类中需要注入SqlSessionFactory工厂，使用SqlSessionFactory生成SqlSession，如果使用Mybatis和Spring，直接让Spring容器管理SqlSessionFactory，SqlSessionFactory创建SqlSession的方法时线程安全的。

package dao;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;

import entity.User;

public class UserDaoImpl implements UserDao {
    
    private SqlSessionFactory sqlSessionFactory;
    

    //通过构造方法注入sqlSessionFactory
    public UserDaoImpl(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    @Override
    public User findUserById(Integer id) {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        String statement = "config.userMapper.getUser";
        User user = sqlSession.selectOne(statement, 1);
        return user;
    }

}

(3) SqlMapConfig.xml核心配置文件

<?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="com.mysql.jdbc.Driver" />
                <property name="url" value="jdbc:mysql:///mybatis" />
                <property name="username" value="root" />
                <property name="password" value="123" />
            </dataSource>
        </environment>
    </environments>
    
    <!-- 注册userMapper.xml文件  -->
    <mappers>
        <mapper resource="config/userMapper.xml" />
    </mappers>
</configuration>

(4) uerMapper.xml映射文件

<?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">

<!-- namespace:SQL映射文件所在的路路径 -->
<mapper namespace="config.userMapper">
    <!-- parameterType:参数id的值的类型 resultType:查询的结果返回的类型 -->
    <select id="getUser" parameterType="Integer" resultType="entity.User">
        select * from users where id=#{id}
    </select>
</mapper>

(5) JUnit测试

package test;

import java.io.InputStream;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;

import dao.UserDao;
import dao.UserDaoImpl;
import entity.User;

public class MyBatisTest {
    private SqlSessionFactory sqlSessionFactory;
    
    @Before
    public void Before() throws Exception {
        //注入SqlSessionFactory工厂
        String resource = "config/SqlMapConfig.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }
    
    @Test
    public void test() {
        UserDao userDao = new UserDaoImpl(sqlSessionFactory);
        User user = userDao.findUserById(1);
        System.out.println(user.getName());
    }
}

• 方法二：我们只需要编写mapper接口动态代理(就是Dao接口)，mapper接口是Mybatis的官方叫法，而不需要写其实现类。

(1) userMapper.xml映射文件

<?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">

<!-- namespace是UserMapper.java的全限定名 -->
<mapper namespace="way_two.dao.UserMapper">
    <!-- parameterType:参数id的值的类型 resultType:查询的结果返回的类型 -->
    <select id="getUser" parameterType="Integer" resultType="entity.User">
        select * from users where id=#{id}
    </select>
    
    <select id="getAllUser" resultType="entity.User">
        select * from users;
    </select>
</mapper>

(2) UserMapper.java(相当于UserDao接口)

    注意：a. userMapper.xml的namespace是UserMapper.java的全限定名

         b. userMapper.xml中的statement的id值是UserMapper.java中的方法名，parameterTyep是方法的参数列表，resultType是方法的返回值。

         statement设置：

             

              UserMapper接口中的方法：

             

package way_two.dao;

import java.util.List;

import entity.User;

public interface UserMapper {
    //根据用户信息查询用户信息
    public User getUser(Integer id);
    
    public List<User> getAllUser();
}

(3) JUint测试

@Test
    public void getUserTest() {
        //得到SqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //通过SqlSession得到UserMapper的动态代理对象
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.getUser(1);
        System.out.println(user.getName());
    }
    
    @Test
    public void getAllUserTest() {
        //得到SqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //通过SqlSession得到UserMapper的动态代理对象
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        List<User> user = userMapper.getAllUser();
        System.out.println(user.size());
    }

小结：通过上面的动态代理的例子，我们发现在userMapper.xml中定义的resultType的类型应是一条SQL语句的结果集映射的Java对象。是否返回一个List集合还是一个单独的Java对象，是有UserMapper接口中方法的返回值类型决定，当返回一个List集合时，代理对象内部调用

sqlSession.selectList()方法获取集合。如果返回的是一个Java对象时，代理对象内部调用sqlSession.selectOne()方法。