mybatis延迟加载

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1.需求

1.1有一个订单表以及一个用户表，一个用户可以创建多个订单，现在需要查询出所有订单，但是订单所属的用户不能一起查询出来，当需要的时候再进行查询，这样就用到mybatis的延迟查询。

2.实现

2.1ordermapper.xml

     <!-- 实现延迟加载 -->
     <resultMap type="order" id="ordersUserLazyLoadingResultMap">
        <id column="id" property="id"/>
        <result column="user_id" property="user_id"/>
        <result column="number" property="number"/>
        <result column="create_time" property="create_time"/>
        <result column="note" property="note"/>
        <!-- property =Order里面的user变量  javatype =pojo类型  select=查询用户使用的statement column= 传递的参数  select不在同一个包下需要命名空间-->
        <association property="user" javaType="user" select="com.ssm.mapping.UserMapperDAO.findUserById" column="user_id">

        </association>
     </resultMap>
     <select id="findOrdersUserLazyLoading" resultMap="ordersUserLazyLoadingResultMap">
        select * from orders
     </select>

2.2usermapperdao.xml

<!-- ${}表示拼接sql字符串 根据姓名模糊查询用户 
        可能会造成sql注入为题，如果接收的参数为简单类型，只能为${value} 与 #{}不一样，可是使用任何名称
    -->
    <select id="findUserByName" parameterType="java.lang.String" resultType="com.ssm.model.User">
        SELECT * FROM USER WHERE USERNAME LIKE '%${value}%'
    </select>

2.3ordermapper.java

public List<Order> findOrdersUserLazyLoading();

2.4springmvc控制器

@RequestMapping(value="/lazyload",method=RequestMethod.GET)
    public String ordersUserLazyLoading(HttpServletRequest request)
    {
        List<Order> list = ordermapper.findOrdersUserLazyLoading();
        request.setAttribute("orderlist", list);
        return "order_detail";
    }

2.5mybatis配置文件

<settings>
        <!-- 开启延迟加载 -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!-- 是否主动加载 -->
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>
    <typeAliases>
        <!-- 批量定义别名 别名为类名 首字母大小写均可-->
        <package name="com.ssm.model"/>
    </typeAliases>
    <!-- 引入资源 -->

2.6jsp页面

<c:forEach var="order" items="${requestScope.orderlist}">
        <p>订单id：${order.id}</p>
        <p>订单number：${order.number}</p>
        <p>订单user_id：${order.user_id}</p>
        <p>订单create_time：${order.create_time}</p>
        <p>订单note：${order.note}</p>
        <p>用户名：${order.user.username}</p>
   </c:forEach>

3.结果

3.1控制台输出

DEBUG [http-bio-8080-exec-12] - DispatcherServlet with name 'springmvc' processing GET request for [/SSM/order/lazyload]
DEBUG [http-bio-8080-exec-12] - Mapping [/order/lazyload] to HandlerExecutionChain with handler [com.ssm.controller.OrderController@32bef7ff] and 1 interceptor
DEBUG [http-bio-8080-exec-12] - Last-Modified value for [/SSM/order/lazyload] is: -1
DEBUG [http-bio-8080-exec-12] - Invoking request handler method: public java.lang.String com.ssm.controller.OrderController.ordersUserLazyLoading(javax.servlet.http.HttpServletRequest)
DEBUG [http-bio-8080-exec-12] - Creating a new SqlSession
DEBUG [http-bio-8080-exec-12] - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@50c31fd4] was not registered for synchronization because synchronization is not active
DEBUG [http-bio-8080-exec-12] - Fetching JDBC Connection from DataSource
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@11fe15ef] will not be managed by Spring
DEBUG [http-bio-8080-exec-12] - ==>  Preparing: select * from orders 
DEBUG [http-bio-8080-exec-12] - ==> Parameters: 
DEBUG [http-bio-8080-exec-12] - <==      Total: 3
DEBUG [http-bio-8080-exec-12] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@50c31fd4]
DEBUG [http-bio-8080-exec-12] - Returning JDBC Connection to DataSource
DEBUG [http-bio-8080-exec-12] - com.mchange.v2.async.ThreadPoolAsynchronousRunner@48a767dd: Adding task to queue -- com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@1ca4c610
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - Rendering view [org.springframework.web.servlet.view.JstlView: name 'order_detail'; URL [/WEB-INF/views/order_detail.jsp]] in DispatcherServlet with name 'springmvc'
DEBUG [http-bio-8080-exec-12] - Forwarding to resource [/WEB-INF/views/order_detail.jsp] in InternalResourceView 'order_detail'
DEBUG [http-bio-8080-exec-12] - Fetching JDBC Connection from DataSource
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@7122bae3] will not be managed by Spring
DEBUG [http-bio-8080-exec-12] - ==>  Preparing: SELECT * FROM USER WHERE id=? 
DEBUG [http-bio-8080-exec-12] - ==> Parameters: 1(Integer)
DEBUG [http-bio-8080-exec-12] - <==      Total: 1
DEBUG [http-bio-8080-exec-12] - Returning JDBC Connection to DataSource
DEBUG [http-bio-8080-exec-12] - com.mchange.v2.async.ThreadPoolAsynchronousRunner@48a767dd: Adding task to queue -- com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@77e0abe
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - Fetching JDBC Connection from DataSource
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@43e14471] will not be managed by Spring
DEBUG [http-bio-8080-exec-12] - ==>  Preparing: SELECT * FROM USER WHERE id=? 
DEBUG [http-bio-8080-exec-12] - ==> Parameters: 1(Integer)
DEBUG [http-bio-8080-exec-12] - <==      Total: 1
DEBUG [http-bio-8080-exec-12] - Returning JDBC Connection to DataSource
DEBUG [http-bio-8080-exec-12] - com.mchange.v2.async.ThreadPoolAsynchronousRunner@48a767dd: Adding task to queue -- com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@29fbdf8b
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - Fetching JDBC Connection from DataSource
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@671c4bc1] will not be managed by Spring
DEBUG [http-bio-8080-exec-12] - ==>  Preparing: SELECT * FROM USER WHERE id=? 
DEBUG [http-bio-8080-exec-12] - ==> Parameters: 10(Integer)
DEBUG [http-bio-8080-exec-12] - <==      Total: 1
DEBUG [http-bio-8080-exec-12] - Returning JDBC Connection to DataSource
DEBUG [http-bio-8080-exec-12] - com.mchange.v2.async.ThreadPoolAsynchronousRunner@48a767dd: Adding task to queue -- com.mchange.v2.resourcepool.BasicResourcePool$1RefurbishCheckinResourceTask@50fee334
DEBUG [http-bio-8080-exec-12] - trace com.mchange.v2.resourcepool.BasicResourcePool@161a1f7f [managed: 3, unused: 2, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@514d00d3)
DEBUG [http-bio-8080-exec-12] - Successfully completed request
DEBUG [C3P0PooledConnectionPoolManager[identityToken->1hge8029

3.2结论

1.可以看到mybatis并没有一次性查询出订单以及所属用户，而是等到jsp页面取order的变量user时才发出的查询语句根据user_id查询用户名