namespace命名空间

namespace中的包名要和Dao/Mapper接口中的包名一致

select 查询

选择,查询语句

• id: 对应namespace中的方法名;
• resultType:Sql语句执行的返回值;
• parameterType: 参数类型;

注意:增删改需要提交事务才能成功

流程

• 编写接口
• 编写对应的mapper中的sql语句
• 测试

代码实现

实体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;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + ''' +
                ", pwd='" + pwd + ''' +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }
}

MabatisUtils工具类

public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;
    // 静态代码块
    static {
        // 使用mybatis第一步 获取sqlSessionFactory
        try {
            String resource = "application.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    // 既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例。SqlSession 提供了在数据库执行 SQL 命令所需的所有方法。你可以通过 SqlSession 实例来直接执行已映射的 SQL 语句。
    /**
     * sqlSession完全包含了面向数据库执行sql,命令所需的所有方法.
     */
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

配置文件application.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<!--configuration核心配置文件-->
<configuration>
    <environments default="development">
        <!--开发环境-->
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/sqlstudy?useSSL=true&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/mybatisstudy/dao/UserMapper.xml"/>
    </mappers>
</configuration>

UserMapper

public interface UserMapper {
    /**
     * 获取用户列表
     * @return
     */
    List<User> getUserList();

    /**
     * 根据id查询用户
     * @param id
     * @return
     */
    User getUserById(int id);

    /**
     * 添加用户
     * @param user
     * @return
     */
    int insertUser(User user);


    /**
     * 修改用户
     */
    int updateUser(User user);

    /**
     * 删除用户
     * @param id
     * @return
     */
    int removeUser(int id);
}

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--namespace绑定一个对应的dao/Mapper接口-->
<mapper namespace="com.mybatisstudy.dao.UserMapper">

    <!--添加用户-->
    <insert id="insertUser" parameterType="com.mybatisstudy.pojo.User">
        insert into user(id,name,pwd) values(#{id},#{name},#{pwd})
    </insert>

    <!--修改一个用户-->
    <update id="updateUser" parameterType="com.mybatisstudy.pojo.User">
        update user set name=#{name},pwd=#{pwd} where id=#{id}
    </update>

    <!--删除一个用户-->
    <delete id="removeUser" parameterType="int">
        delete from user where id=#{id}
    </delete>

    <!--resultType结果集-->
    <select id="getUserList" resultType="com.mybatisstudy.pojo.User">
        select * from sqlstudy.user
    </select>

    <!--根据id查询用户-->
    <select id="getUserById" parameterType="int" resultType="com.mybatisstudy.pojo.User">
        select * from user where id = #{id}
    </select>
</mapper>

测试类

public class UserMapperTest {
    @Test
    public void test(){
        // 第一步:获得sqlSession对象
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        // 方式一:执行SQL
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = mapper.getUserList();
        userList.forEach(e -> {
            System.out.println(e);
        });
        // 关闭sqlSession
        sqlSession.close();
    }

    @Test
    public void testGetUserById(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User userById = mapper.getUserById(1);
        System.out.println(userById);

        sqlSession.close();
    }

    /**
     * 增删改需要提交事务
     */
    @Test
    public void addUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        int i = mapper.insertUser(new User(2, "王亮", "123"));
        if (i > 0){
            System.out.println("添加成功");
        }
        // 提交事务
        sqlSession.commit();
        // 关流
        sqlSession.close();
    }


    @Test
    public void updateUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        int i = mapper.updateUser(new User(1, "hh", "hhhhh"));
        if (i > 0){
            System.out.println("修改成功");
        }

        // 提交事务并关流
        sqlSession.commit();
        sqlSession.close();
    }

    @Test
    public void deleteUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        int i = mapper.removeUser(1);
        if (i > 0){
            System.out.println("删除成功");
        }

        // 提交事务并关流
        sqlSession.commit();
        sqlSession.close();
    }

}

常见错误分析

• ..mapper.xml标签匹配
• 配置文件resource绑定mapper,需要使用路径,文件分级使用/连接
• 数据库连接,程序配置文件必须符合规范;
• 空指针异常,没有注册到资源;
• 输出的xml文件存在中文乱码,重新生成
• maven资源没有导出问题;添加pom中资源过滤坐标