<dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.2.6</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>6.0.6</version>
        </dependency>

        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
</dependencies>

<?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>

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

    <typeAliases>
        <typeAlias type="com.justin.pojo.User" alias="user"/>
    </typeAliases>

    <environments default="local">
        <environment id="local">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <!-- mysql连接的依赖包是高版本的情况下,后面要加上具体的配置信息-->
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useUnicode=true&amp;useJDBCCompliantTimezoneShift=true&amp;useLegacyDatetimeCode=false&amp;serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>


    <mappers>
        <mapper resource="user-mapper.xml"/>
    </mappers>

</configuration>

public class TestFirst {

    private static Logger logger = Logger.getLogger(TestFirst.class);
    public static void main(String[] args){

        // 记录debug级别的信息
        logger.debug("This is debug message.");
        // 记录info级别的信息
        logger.info("This is info message.");
        // 记录error级别的信息
        logger.error("This is error message.");
    }
}

<?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">


<!-- 命名空间为绑定接口的全类名 -->
<mapper namespace="com.justin.first.UserDao">

    <resultMap id="userListResultMap" type="user">
        <!--id是主键 -->
        <id column="id" property="id"/>
        <result column="username" property="username"/>
        <result column="sex" property="sex"/>
    </resultMap>
    <!-- id 为接口定义的方法名  resultType 为实体类的全类名  #{userId} 中的参数和方法名一致 -->
    <select id="getOne" resultType="com.justin.pojo.User">
        select * from users where id = #{userId}
    </select>
    <select id="findUserListMap" resultMap="userListResultMap">
        SELECT id,username,sex FROM users;
    </select>
</mapper>

public interface UserDao {

    User getOne(int userId);

    List<User> findUserListMap();

}

public class TestFirst {

    public static void main(String[] args) {

        try {
            testUserSelect();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void testUserSelect() throws IOException {

        SqlSessionFactory factory = new SqlSessionFactoryBuilder()
                .build(Resources.getResourceAsStream("mybatis-config.xml"));
        SqlSession session = factory.openSession();
        //mybatis的映射文件
        UserDao dao = session.getMapper(UserDao.class);
        User one = dao.getOne(1);
        System.out.println(one);
    }
}

<!--定义sql片段  
    id是sql片段的唯一标识，便于下面语句的引用。
    通常把SQL片段设计成基于单表来定义sql片段，这样话这个sql片段可重用性才高。  
    注意：在sql片段中不要包括 where  
    -->
<sql id="query_user_where">  
        <if test="userCustom!=null">  
            <if test="userCustom.sex!=null and userCustom.sex!=‘‘">  
                and user.sex = #{userCustom.sex}  
            </if>  
            <if test="userCustom.username!=null and userCustom.username!=‘‘">  
                and user.username LIKE ‘%${userCustom.username}%‘  
            </if>  
        </if>  
</sql>  

<select id="findUserWithDynamic" resultType="user">

        <where>
            <!-- 引用sql片段 的id，如果refid指定的id不在本mapper文件中，需要前边加namespace -->
            <include refid="query_user_where"/>
            <!-- 在这里还可以引用其它的sql片段  -->
        </where>

    </select>

<select id="findUserList" parameterType="com.justin.pojo.UserQueryVo"
            resultType="com.justin.pojo.User">
        SELECT * FROM users
        <!--
        where可以自动去掉条件中的第一个and
         -->
        <where>
            <include refid="query_user_where"/>
        </where>
</select>