mybatis延遲加載的實現
1.關於延遲加載
(1)延遲加載也叫懶加載,根據需求加載數據,先加載主要顯示的數據,如果需要關聯的數據顯示,再加載關聯的數據顯示
2.爲什麼使用延遲加載
(1)假設場景:用戶表和訂單表,用戶表有多個用戶,每個用戶有多個訂單, 某些時候,我們需要同時查看用戶信息和用戶的訂單信息,這個時候用戶的信息和訂單的信息都需要顯示。但如果我們只查看訂單列表,就沒必要把用戶信息和用戶的訂單的數據全部查詢出來,只顯示訂單信息就夠了。如果想看訂單詳情,這時候需要顯示訂單的用戶信息,再來加載訂單的用戶數據。那這個時候很明顯 就需要用到延遲加載 (2)好處就是:避免資源浪費,提升查詢速度
3.如何實現
(1)在sqlMapConfig配置文件中加入以下節點:
<settings>
<setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> </settings>
lazyLoadingEnabled 這個是開啓懶加載的全局配置 aggressiveLazyLoading 這個屬性必須改爲false 否則不生效
(2) 1) 將原始的關聯查詢代碼改爲單表查詢 原始sql語句
<select id="findUserAndOrderResultMap" resultMap="orderUserMap"> SELECT orders.id, orders.number, orders.createtime, user.id userId, user.username, user.birthday, user.sex, user.address FROM orders,USER WHERE orders.user_id = user.id </select>
更改後sql語句
<select id="findUserAndOrderResultMap" resultMap="orderUserMap"> SELECT * FROM orders </select>
2) 更改關聯查詢的resultMap內的映射association節點加入懶加載需要的查詢條件 和sql語句statementId 原始resultMap映射
<resultMap type="orders" id="orderUserMap"> <id column="id" property="id"/> <result column="number" property="number"/> <result column="createtime" property="createtime"/> <!-- association節點標明唯一關係映射 --> <association property="user" javaType="user"> <id column="userId" property="id"/> <result column="username" property="username"/> <result column="birthday" property="birthday"/> <result column="sex" property="sex"/> <result column="address" property="address"/> </association> </resultMap>
更改後的resultMap映射
<resultMap type="orders" id="orderUserMap"> <id column="id" property="id"/> <result column="number" property="number"/> <result column="createtime" property="createtime"/> <!-- association節點標明唯一關係映射 --> <association property="user" javaType="user" column="user_id" select="findUserById"> </association> </resultMap>
3) 添加根據id查詢用戶的sql語句statement
<select id="findUserById" parameterType="int" resultType="user"> select * from user where id = #{id} </select>
##4.效果演示 查詢訂單信息輸出
List<Orders> list = userMapper.findOrderAndUser(); for(Orders orders:list){ System.out.println(orders.getNumber()); }
運行測試方法控制檯只會輸出一條sql語句查詢所有訂單 00:02:27,127 DEBUG findOrderAndUser:139 - > Preparing: SELECT * FROM orders 00:02:27,166 DEBUG findOrderAndUser:139 - ==> Parameters: 00:02:27,280 DEBUG findOrderAndUser:139 - < Total: 3 如果在訂單內部獲取用戶,纔會又輸出一條根據訂單的用戶id查詢用戶的sql語句
List<Orders> list = userMapper.findOrderAndUser(); for(Orders orders:list){ System.out.println(orders.getNumber()); System.out.println(orders.getUser()); }
00:09:22,927 DEBUG findOrderAndUser:139 - > Preparing: SELECT * FROM orders 00:09:22,964 DEBUG findOrderAndUser:139 - ==> Parameters: 00:09:23,078 DEBUG findOrderAndUser:139 - < Total: 3 1000010 00:09:23,079 DEBUG findUserById:139 - > Preparing: select * from user where id = ? 00:09:23,079 DEBUG findUserById:139 - ==> Parameters: 1(Integer) 00:09:23,081 DEBUG findUserById:139 - < Total: 1 User [id=1, username=王五, sex=男, birthday=null, address=null]