Mybatis懒加载机制

懒加载会按需要是否执行关联查询语句。如果用到了关联查询语句的信息，就需要执行关联查询，否则就只执行主查询

1.在mybatis的全局配置文件中加入两个属性：

<settings>
                <setting name="mapUnderscoreToCamelCase" value="true"/>
                <!-- 开启懒加载机制 ,默认值为true-->
                <setting name="lazyLoadingEnabled" value="true"/>
                <!-- 开启的话，每个属性都会直接全部加载出来；禁用的话，只会按需加载出来 -->
                <setting name="aggressiveLazyLoading" value="false"/>
            </settings>

Mapper接口：

    public tbl_employee findTblEmployeeById(int id);

xml文件：

<!--关联查询-->
<select id="selectDepartment" resultType="com.neuedu.bean.tbl_dept">
        select * from tbl_dept where id=#{id}
    </select>
    <resultMap type="com.neuedu.bean.tbl_employee" id="findTblEmployeeByIdMap">
        <id  column="id" property="id" />
        <result column="email" property="email" />
        <result column="gender" property="gender" />
        <result column="user_name" property="userName" />
    <!--     <result column="d_id" property="dId"/> -->
        <association property="dept" select="selectDepartment" column="d_id"></association>
    </resultMap>

<!--主查询-->

<select id="findTblEmployeeById" resultMap="findTblEmployeeByIdMap" parameterType="int"> select e.id,e.email,e.gender,e.user_name,e.d_id from tbl_employee e where e.id=#{id} </select>

测试：

　　只用到主查询的信息

    @Test
    public void testFindEmployee() throws IOException{
        SqlSessionFactory sqlSessionFactory = sqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession();
        Tbl_employeeMapper mapper = openSession.getMapper(Tbl_employeeMapper.class);
        tbl_employee findTblEmployeeById = mapper.findTblEmployeeById(1);
      System.out.println("名字："+findTblEmployeeById.getUserName());
    }

　　下面是打印的日志：

DEBUG 09-05 20:13:14,178 ==> Preparing: select e.id,e.email,e.gender,e.user_name,e.d_id from tbl_employee e where e.id=? (BaseJdbcLogger.java:145)
DEBUG 09-05 20:13:14,452 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145)
DEBUG 09-05 20:13:14,861 <== Total: 1 (BaseJdbcLogger.java:145)
名字：yr

通过日志可知只打印了一条SQL语句，就以为着只执行了一条主查询

　　使用了关联查询的信息：

    @Test
    public void testFindEmployee() throws IOException{
        SqlSessionFactory sqlSessionFactory = sqlSessionFactory();
        SqlSession openSession = sqlSessionFactory.openSession();
        Tbl_employeeMapper mapper = openSession.getMapper(Tbl_employeeMapper.class);
        tbl_employee findTblEmployeeById = mapper.findTblEmployeeById(1);
        System.out.println(findTblEmployeeById);
    }

　　打印的日志：

DEBUG 09-05 20:17:13,873 ==> Preparing: select e.id,e.email,e.gender,e.user_name,e.d_id from tbl_employee e where e.id=? (BaseJdbcLogger.java:145)
DEBUG 09-05 20:17:14,107 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145)
DEBUG 09-05 20:17:14,449 <== Total: 1 (BaseJdbcLogger.java:145)
DEBUG 09-05 20:17:14,454 ==> Preparing: select * from tbl_dept where id=? (BaseJdbcLogger.java:145)
DEBUG 09-05 20:17:14,455 ==> Parameters: 1(Integer) (BaseJdbcLogger.java:145)
DEBUG 09-05 20:17:14,460 <== Total: 1 (BaseJdbcLogger.java:145)
tbl_employee [id=1, userName=yr, gender=0, email=907339416@qq.com, dId=0, dept=tbl_dept [id=1, deptName=秘书部, empList=null]]

这里用到了关联查询的信息，所以用到了两个查询语句。打印了两个。