MyBatis實(shí)踐之動(dòng)態(tài)SQL及關(guān)聯(lián)查詢

更新時(shí)間：2016年03月10日 13:52:26 作者：翡青

MyBatis,大家都知道，半自動(dòng)的ORM框架，原來(lái)叫ibatis，后來(lái)好像是10年apache軟件基金組織把它托管給了goole code，就重新命名了MyBatis，功能相對(duì)以前更強(qiáng)大了。本文給大家介紹MyBatis實(shí)踐之動(dòng)態(tài)SQL及關(guān)聯(lián)查詢，對(duì)mybatis動(dòng)態(tài)sql相關(guān)知識(shí)感興趣的朋友一起學(xué)習(xí)吧

序言

MyBatis,大家都知道，半自動(dòng)的ORM框架，原來(lái)叫ibatis，后來(lái)好像是10年apache軟件基金組織把它托管給了goole code，就重新命名了MyBatis，功能相對(duì)以前更強(qiáng)大了。它相對(duì)全自動(dòng)的持久層框架Hibernate，更加靈活，更輕量級(jí)，這點(diǎn)我還是深有體會(huì)的。

MyBatis的一個(gè)強(qiáng)大特性之一就是動(dòng)態(tài)SQL能力了，能省去我們很多串聯(lián)判斷拼接SQL的痛苦，根據(jù)項(xiàng)目而定，在一定的場(chǎng)合下使用，能大大減少程序的代碼量和復(fù)雜程度，不過(guò)還是不是過(guò)度太過(guò)復(fù)雜的使用，以免不利于后期的維護(hù)和擴(kuò)展。

動(dòng)態(tài)SQL

動(dòng)態(tài)SQL提供了對(duì)SQL語(yǔ)句的靈活操作,通過(guò)表達(dá)式進(jìn)行判斷,對(duì)SQL進(jìn)行拼接/組裝.

對(duì)查詢條件進(jìn)行判斷,如果輸入?yún)?shù)不為空才進(jìn)行查詢條件的拼接.

mapper

<code class=" hljs vbnet"><select id="selectUser" resulttype="com.fq.domain.User" parametertype="com.fq.domain.User">
SELECT * FROM user
AND id = #{id}
AND name = #{name}
AND password = #{password}
</select></code>

會(huì)自動(dòng)處理第一個(gè)AND(MyBatis還提供了自定義行為的元素, 詳細(xì)可參考MyBatis文檔).

UserDAO

<code class="hljs vbnet"><code class="hljs php">List<user> selectUser(User user) throws Exception;</user></code></code>

Client

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java">@Test
public void selectUserClient() throws Exception {
UserDAO dao = session.getMapper(UserDAO.class);
User user = dao.selectUser(new User(null, null, "new_password"));
System.out.println(user);
}</code></code></code>

由于id與name為null, 因此這兩個(gè)條件不會(huì)拼接在SQL中,這一點(diǎn)可以調(diào)試時(shí)日志中看出.

choose/when/otherwise

有些時(shí)候,我們并不想用到所有的條件語(yǔ)句,而只想從中選擇一二.針對(duì)這種情況,MyBatis提供了元素,他有點(diǎn)像Java中的switch.

<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><select id="selectUser" resulttype="com.fq.domain.User" parametertype="com.fq.domain.User">
SELECT * FROM user
AND id = #{id}
AND name = #{name}
AND password = #{password}
</select></code></code></code></code>

set

用于動(dòng)態(tài)更新語(yǔ)句的解決方案為,set元素可以被用于動(dòng)態(tài)包含需要更新的列, 而舍去其他的.

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><update id="updateUserById" parametertype="com.fq.domain.User">
UPDATE user
<set>
<if test="name != null">
name = #{name} ,
</if>
<if test="password != null">
password = #{password} ,
</if>
</set>
WHERE id = #{id};
</update></code></code></code></code></code>

foreach

使用foreach可以實(shí)現(xiàn)向SQL中傳遞數(shù)組或List:

傳入List

查詢多個(gè)id的用戶信息, 可以由下面兩種SQL實(shí)現(xiàn):

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql">SELECT * FROM user WHERE (id = ? OR id = ? OR id = ?);
SELECT * FROM user WHERE id IN (?, ?, ?, ?);</code></code></code></code></code></code>

因此其foreach的定義也有如下兩種方案:

<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><select id="selectUser" parametertype="java.util.List" resulttype="com.fq.domain.User">
SELECT *
FROM user
id = #{id}
</select></code></code></code></code></code></code></code>
<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><select id="selectUser" parametertype="java.util.List" resulttype="com.fq.domain.User">
SELECT *
FROM user
#{id}
</select></code></code></code></code></code></code></code></code>

元素描述

collection SQL解析的參數(shù)名
index 循環(huán)下標(biāo)
item 單個(gè)元素的名
open 循環(huán)開始輸出
close 循環(huán)結(jié)束輸出
separator 中間分隔輸出

傳遞List作為parameterType時(shí),SQL解析參數(shù)名固定為list.

UserDAO

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica">List<user> selectUser(List<integer> ids) throws Exception;</integer></user></code></code></code></code></code></code></code></code></code>

批量插入用戶案例

mapper

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><insert id="insertUserList" parametertype="java.util.List">
INSERT INTO user(name, password) VALUES
<if test="list != null and list.size != 0">
<foreach collection="list" item="user" separator=",">
(#{user.name}, #{user.password})
</foreach>
</if>
</insert></code></code></code></code></code></code></code></code></code></code>

UserDAO

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java">void insertUserList(List<user> users) throws Exception;</user></code></code></code></code></code></code></code></code></code></code></code>

Client

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java">@Test
public void insertUserListClient() throws Exception {
UserDAO dao = session.getMapper(UserDAO.class);
dao.insertUserList(Lists.newArrayList(new User(null, "mojia5", "mojia5"), new User(null, "mojia6", "mojia6"), new User(null, "mojia7", "mojia7")));
}</code></code></code></code></code></code></code></code></code></code></code></code>

傳入數(shù)組

mapper

<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs xml"><code class=" hljs java"><code class=" hljs java"><code class=" hljs cs"><select id="selectUser" parametertype="Object[]" resulttype="com.fq.domain.User">
SELECT *
FROM user
#{id}
</select></code></code></code></code></code></code></code></code></code></code></code></code></code>

與List類似,傳遞數(shù)組作為parameterType時(shí),SQL解析參數(shù)名固定為array.

UserDAO

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica">List<user> selectUser(Integer[] ids) throws Exception;</user></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

SQL片段

可以將一段公共的SQL語(yǔ)句抽取出來(lái), 作為一個(gè)SQL片段, 供其他SQL調(diào)用:

<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs xml"><code class=" hljs java"><code class=" hljs java"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs vbnet"><sql id="user_where">
<if test="id != null">
AND id = #{id}
</if>
<if test="name != null">
AND name = #{name}
</if>
<if test="password != null">
AND password = #{password}
</if>
</sql>
<select id="selectUser" resulttype="com.fq.domain.User" parametertype="com.fq.domain.User">
SELECT * FROM user
</select></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

經(jīng)驗(yàn):最好基于單表定義SQL片段,而且在SQL片段中不要包含/之類的標(biāo)簽,這樣可以保證SQL片段重用度更高.

關(guān)聯(lián)查詢

數(shù)據(jù)模型分析思路

每張表的數(shù)據(jù)內(nèi)容:分模塊對(duì)每張表記錄的內(nèi)容進(jìn)行熟悉,相當(dāng)于學(xué)習(xí)系統(tǒng)需求/功能. 每張表重要的字段:非空字段/外鍵字段等. 表與表之間的數(shù)據(jù)庫(kù)級(jí)別關(guān)系: 外鍵關(guān)系. 表與表之間的業(yè)務(wù)關(guān)系:建立在某個(gè)業(yè)務(wù)的基礎(chǔ)上去分析.

訂單/商品數(shù)據(jù)模型

表內(nèi)容

user: 購(gòu)買商品的用戶信息 order: 用戶創(chuàng)建的訂單 orderdetail: 訂單詳細(xì)(購(gòu)買商品信息) item: 商品信息表與表之間的業(yè)務(wù)關(guān)系:

user/order:
user -> order: 一對(duì)多 order -> user: 一對(duì)一 order/orderdetail:
order -> orderdetail:一對(duì)多 orderdetail -> order:一對(duì)一 orderdetail/item:
orderdetail -> item:一對(duì)一 item -> orderdetail:一對(duì)多

‘一對(duì)一'查詢

需求: 查詢訂單信息,關(guān)聯(lián)查詢(創(chuàng)建訂單的)用戶信息.

由以上分析可知主查詢?yōu)閛rder表,而order -> user關(guān)系為一對(duì)一, 因此使用resultMap將查詢結(jié)果的訂單信息映射到Order中,將用戶信息映射到Order中的User屬性.

PO: 改造User

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java">public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private Integer sex;
private String address;
// ..
}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

PO: 新增Order, 將User組合到Order中:

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java"><code class="hljs java">public class Order implements Serializable {
private Integer id;
private Integer userId;
private String number;
private Date createTime;
private String note;
private User user;
// ...
}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

mapper

<code class=" hljs vbnet"><code class=" hljs php"><code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs applescript"><code class=" hljs sql"><code class=" hljs cs"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs xml"><code class=" hljs java"><code class=" hljs java"><code class=" hljs cs"><code class=" hljs mathematica"><code class=" hljs vbnet"><code class=" hljs java"><code class=" hljs java"><code class=" hljs xml"><!--{cke_protected}{C}%3C!%2D%2D%3Fxml%20version%3D%221.0%22%20encoding%3D%22UTF-8%22%20%3F%2D%2D%3E-->
<mapper namespace="com.fq.mybatis.OrderDAO">
<cache type="org.mybatis.caches.ehcache.EhcacheCache">
<resultmap id="order_user_map" type="com.fq.domain.Order">
<id column="id" property="id">
<result column="user_id" property="userId">
<result column="number" property="number">
<result column="create_time" property="createTime">
<result column="note" property="note">
<association property="user" javatype="com.fq.domain.User">
<id column="user_id" property="id">
<result column="username" property="username">
<result column="birthday" property="birthday">
<result column="sex" property="sex">
<result column="address" property="address">
</result></result></result></result></id></association>
</result></result></result></result></id></resultmap>
<select id="selectOrderWithUser" resultmap="order_user_map">
SELECT
`order`.*,
username,
birthday,
sex,
address
FROM `order`, user
WHERE `order`.user_id = user.id AND `order`.id = #{0};
</select>
</cache></mapper></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>
association: 映射關(guān)聯(lián)查詢的單條記錄(將關(guān)聯(lián)查詢信息映射到PO對(duì)象屬性).

OrderDAO

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java"><code class="hljs java"><code class="hljs xml"><code class="hljs cs">public interface OrderDAO {
Order selectOrderWithUser(Integer id) throws Exception;
}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

Client

<code class="hljs vbnet"><code class="hljs php"><code class="hljs java"><code class="hljs vbnet"><code class="hljs applescript"><code class="hljs sql"><code class="hljs cs"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs xml"><code class="hljs java"><code class="hljs java"><code class="hljs cs"><code class="hljs mathematica"><code class="hljs vbnet"><code class="hljs java"><code class="hljs java"><code class="hljs xml"><code class="hljs cs"><code class="hljs java">@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "classpath:spring/applicationContext.xml")
public class OrderDAOClient {
@Autowired
private OrderDAO dao;
@Test
public void client() throws Exception {
Order order = dao.selectOrderWithUser(3);
System.out.println(order);
}
}</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

‘一對(duì)多'查詢

需求: 查詢訂單及訂單明細(xì)的信息(一對(duì)多).

PO: 定義OrderDetail,并在Order中添加List orderDetails訂單明細(xì)屬性:

<code class="hljs java">public class OrderDetail implements Serializable {
private Integer id;
private Integer orderId;
private Integer itemId;
private Integer itemNumber;
// ...
}</code>

mapper

<code class=" hljs java"><code class=" hljs vbnet"><resultmap id="order_user_detail_map" type="com.fq.domain.Order" extends="order_user_map">
<collection property="orderDetails" oftype="com.fq.domain.OrderDetail">
<id column="order_detail_id" property="id">
<result column="item_id" property="itemId">
<result column="item_num" property="itemNumber">
<result column="order_id" property="orderId">
</result></result></result></id></collection>
</resultmap>
<select id="selectOrderWithDetail" resultmap="order_user_detail_map">
SELECT
`order`.*,
username,
birthday,
sex,
address,
orderdetail.id order_detail_id,
item_id,
item_num,
order_id
FROM `order`, user, orderdetail
WHERE `order`.user_id = user.id AND `order`.id = orderdetail.order_id AND `order`.id = #{0};
</select></code></code>

元素描述

property 指定關(guān)聯(lián)查詢的結(jié)果集存儲(chǔ)到的屬性

ofType 指定關(guān)聯(lián)查詢結(jié)果集中的對(duì)象類型OrderDAO

<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica">Order selectOrderWithDetail(Integer id) throws Exception;</code></code></code>

‘多對(duì)多'查詢

需求: 查詢用戶及用戶購(gòu)買商品信息.

由于User表與Item表沒有直接關(guān)聯(lián),因此只能通過(guò)Order表與OrderDetail表進(jìn)行關(guān)聯(lián).

思路:

1) 將用戶信息映射到User中.
2) 在User中添加List訂單列表屬性,將用戶創(chuàng)建的訂單映射到orders.
3) 在Order中添加List訂單明細(xì)列表屬性,將訂單的明細(xì)映射到orderDetails.
4) 在OrderDetail中添加Item屬性,將訂單明細(xì)所對(duì)應(yīng)的商品映射到item.

PO: Item

<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs cs">public class Item {
private Integer id;
private String name;
private Float price;
private String detail;
private String pic;
private Date createTime;
//...
}</code></code></code></code>

mapper

<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><resultmap id="user_item_map" type="com.fq.domain.User">
<id column="id" property="id">
<result column="username" property="username">
<result column="birthday" property="birthday">
<result column="sex" property="sex">
<result column="address" property="address">
<collection property="orders" oftype="com.fq.domain.Order">
<id column="order_id" property="id">
<result column="id" property="userId">
<result column="order_create_time" property="createTime">
<result column="order_note" property="note">
<result column="order_number" property="number">
<collection property="orderDetails" oftype="com.fq.domain.OrderDetail">
<id column="order_detail_id" property="id">
<result column="order_id" property="orderId">
<result column="item_id" property="itemId">
<result column="order_item_num" property="itemNumber">
<association property="item" javatype="com.fq.domain.Item">
<id column="item_id" property="id">
<result column="item_create_time" property="createTime">
<result column="item_detail" property="detail">
<result column="item_name" property="name">
<result column="item_price" property="price">
<result column="item_pic" property="pic">
</result></result></result></result></result></id></association>
</result></result></result></id></collection>
</result></result></result></result></id></collection>
</result></result></result></result></id></resultmap>
<select id="selectUserItem" resultmap="user_item_map">
SELECT
user.*,
`order`.id order_id,
`order`.create_time order_create_time,
`order`.note order_note,
`order`.number order_number,
orderdetail.id order_detail_id,
orderdetail.item_num order_item_num,
item.id item_id,
item.create_time item_create_time,
item.detail item_detail,
item.name item_name,
item.price item_price,
item.pic item_pic
FROM user, item, `order`, orderdetail
WHERE `order`.user_id = user.id AND orderdetail.order_id = `order`.id AND orderdetail.item_id = item.id ;
</select></code></code></code></code></code>

OrderDAO

<code class="hljs java"><code class="hljs vbnet">
<code class="hljs mathematica">
<code class="hljs cs">
<code class="hljs applescript">
<code class="hljs php">List<user> selectUserItem() throws Exception;</user>
</code></code></code></code></code></code>

resultMap小結(jié):

使用和可以完成一對(duì)一和一對(duì)多的高級(jí)映射.

association: 將關(guān)聯(lián)查詢信息映射到一個(gè)PO對(duì)象中. collection: 將關(guān)聯(lián)查詢信息映射到一個(gè)集合中.

延遲加載

關(guān)聯(lián)查詢時(shí),使用MyBatis 延遲加載特性可有效減輕數(shù)據(jù)庫(kù)壓力.首次查詢只查詢主表信息,等需要時(shí)再去查詢關(guān)聯(lián)表信息.的/具備延遲加載功能.

需求: 查詢訂單信息并關(guān)聯(lián)查詢用戶信息.

延遲加載開關(guān)

在MyBatis核心配置文件(mybatis-configuration.xml)中配置:

1) lazyLoadingEnabled : 設(shè)置是否懶加載.默認(rèn)false,則所有關(guān)聯(lián)查詢都會(huì)被初始化加載.

2) aggressiveLazyLoading : 設(shè)置是否積極加載. 默認(rèn)true,所有關(guān)聯(lián)屬性被初始化加載.

Settings配置:

<code class=" hljs java">
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<code class=" hljs cs"><code class=" hljs applescript"><code class=" hljs php">
<code class=" hljs xml"><settings>
<setting name="cacheEnabled" value="true">
<setting name="lazyLoadingEnabled" value="true">
<setting name="aggressiveLazyLoading" value="false">
</setting></setting></setting></settings></code></code></code></code></code></code></code>

Mapper

只查詢訂單信息

<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><code class=" hljs php"><code class=" hljs xml"><code class=" hljs applescript"><resultmap id="order_user_map" type="com.fq.domain.Order">
<id column="id" property="id">
<result column="user_id" property="userId">
<result column="number" property="number">
<result column="create_time" property="createTime">
<result column="note" property="note">
<association property="user" javatype="com.fq.domain.User" select="com.fq.mybatis.UserDAO.selectUserById" column="user_id">
</association></result></result></result></result></id></resultmap>
<select id="selectOrderWithUser" resultmap="order_user_map">
SELECT *
FROM `order`;
</select></code></code></code></code></code></code></code></code>

元素描述

select 指定關(guān)聯(lián)查詢Statement為com.fq.mybatis.UserDAO.selectUserById.

column 指定關(guān)聯(lián)查詢時(shí)將users_id值傳入selectUserById.關(guān)聯(lián)查詢用戶信息(namespace為com.fq.mybatis.UserDAO)

<code class=" hljs java"><code class=" hljs vbnet"><code class=" hljs mathematica"><code class=" hljs cs"><code class=" hljs applescript"><code class=" hljs php"><code class=" hljs xml"><code class=" hljs applescript"><code class=" hljs vbnet"><select id="selectUserById" parametertype="java.lang.Integer" resulttype="com.fq.domain.User">
SELECT *
FROM user
WHERE id = #{id};
</select></code></code></code></code></code></code></code></code></code>

將上面查詢到的訂單信息中的user_id傳入selectUserById來(lái)關(guān)聯(lián)查詢用戶信息.

OrderDAO(同前)

<code class="hljs java">
<code class="hljs vbnet">
<code class="hljs mathematica">
<code class="hljs cs">
<code class="hljs applescript">
<code class="hljs php">
<code class="hljs xml">
<code class="hljs applescript">
<code class="hljs vbnet">
<code class="hljs mathematica">List<order> selectOrderWithUser() throws Exception;</order>
</code>
</code></code></code></code></code></code></code></code></code>

Client

<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs cs"><code class="hljs applescript"><code class="hljs php"><code class="hljs xml"><code class="hljs applescript"><code class="hljs vbnet"><code class="hljs mathematica"><code class="hljs java">@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "classpath:spring/applicationContext.xml")
public class OrderDAOClient {
@Autowired
private OrderDAO dao;
@Test
public void client() throws Exception {
List<order> orders = dao.selectOrderWithUser();
for (Order order : orders) {
System.out.println(order.getUser());
}
}
}</order></code></code></code></code></code></code></code></code></code></code></code>

debug上面Client, 觀察log信息,會(huì)發(fā)現(xiàn)只有當(dāng)確實(shí)需要User信息時(shí)才會(huì)調(diào)用selectUserById.

以上所述是小編給大家介紹的mybatis實(shí)踐之動(dòng)態(tài)sql及關(guān)聯(lián)查詢，希望對(duì)大家以上幫助！

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