序言
MyBatis,大家都知道，半自動的ORM框架，原來叫ibatis，后來好像是10年apache軟件基金組織把它托管給了goole code，就重新命名了MyBatis，功能相對以前更強大了。它相對全自動的持久層框架Hibernate，更加靈活，更輕量級，這點我還是深有體會的。
MyBatis的一個強大特性之一就是動態SQL能力了，能省去我們很多串聯判斷拼接SQL的痛苦，根據項目而定，在一定的場合下使用，能大大減少程序的代碼量和復雜程度，不過還是不是過度太過復雜的使用，以免不利于后期的維護和擴展。
動態SQL
動態SQL提供了對SQL語句的靈活操作,通過表達式進行判斷,對SQL進行拼接/組裝.
if
對查詢條件進行判斷,如果輸入參數不為空才進行查詢條件的拼接.
mapper
<code class=" hljs vbnet"><select id="selectUser" resulttype="com.fq.domain.User" parametertype="com.fq.domain.User">SELECT * FROM userAND id = #{id}AND name = #{name}AND password = #{password}</select></code> 會自動處理第一個AND(MyBatis還提供了自定義行為的元素, 詳細可參考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">@Testpublic 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, 因此這兩個條件不會拼接在SQL中,這一點可以調試時日志中看出.
choose/when/otherwise
有些時候,我們并不想用到所有的條件語句,而只想從中選擇一二.針對這種情況,MyBatis提供了元素,他有點像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 userAND id = #{id}AND name = #{name}AND password = #{password}</select></code></code></code></code> set
用于動態更新語句的解決方案為,set元素可以被用于動態包含需要更新的列, 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可以實現向SQL中傳遞數組或List:
傳入List
查詢多個id的用戶信息, 可以由下面兩種SQL實現:
<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 userid = #{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解析的參數名
index 循環下標
item 單個元素的名
open 循環開始輸出
close 循環結束輸出
separator 中間分隔輸出
傳遞List作為parameterType時,SQL解析參數名固定為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">@Testpublic 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,傳遞數組作為parameterType時,SQL解析參數名固定為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語句抽取出來, 作為一個SQL片段, 供其他SQL調用:
<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> 經驗:最好基于單表定義SQL片段,而且在SQL片段中不要包含/之類的標簽,這樣可以保證SQL片段重用度更高.
關聯查詢
數據模型分析思路
每張表的數據內容:分模塊對每張表記錄的內容進行熟悉,相當于學習系統需求/功能. 每張表重要的字段:非空字段/外鍵字段等. 表與表之間的數據庫級別關系: 外鍵關系. 表與表之間的業務關系:建立在某個業務的基礎上去分析.
訂單/商品數據模型
表內容
user: 購買商品的用戶信息 order: 用戶創建的訂單 orderdetail: 訂單詳細(購買商品信息) item: 商品信息 表與表之間的業務關系:
user/order:
user -> order: 一對多 order -> user: 一對一 order/orderdetail:
order -> orderdetail:一對多 orderdetail -> order:一對一 orderdetail/item:
orderdetail -> item:一對一 item -> orderdetail:一對多
‘一對一'查詢
需求: 查詢訂單信息,關聯查詢(創建訂單的)用戶信息.
由以上分析可知主查詢為order表,而order -> user關系為一對一, 因此使用resultMap將查詢結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,addressFROM `order`, userWHERE `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: 映射關聯查詢的單條記錄(將關聯查詢信息映射到PO對象屬性). 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 {@Autowiredprivate OrderDAO dao;@Testpublic 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> ‘一對多'查詢
需求: 查詢訂單及訂單明細的信息(一對多).
PO: 定義OrderDetail,并在Order中添加List orderDetails訂單明細屬性:
<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_idFROM `order`, user, orderdetailWHERE `order`.user_id = user.id AND `order`.id = orderdetail.order_id AND `order`.id = #{0};</select></code></code> 元素 描述
property 指定關聯查詢的結果集存儲到的屬性
ofType 指定關聯查詢結果集中的對象類型OrderDAO
<code class="hljs java"><code class="hljs vbnet"><code class="hljs mathematica">Order selectOrderWithDetail(Integer id) throws Exception;</code></code></code>
‘多對多'查詢
需求: 查詢用戶及用戶購買商品信息.
由于User表與Item表沒有直接關聯,因此只能通過Order表與OrderDetail表進行關聯.
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,將用戶創建的訂單映射到orders.
3) 在Order中添加List訂單明細列表屬性,將訂單的明細映射到orderDetails.
4) 在OrderDetail中添加Item屬性,將訂單明細所對應的商品映射到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">SELECTuser.*,`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_picFROM user, item, `order`, orderdetailWHERE `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:
使用和可以完成一對一和一對多的高級映射.
association: 將關聯查詢信息映射到一個PO對象中. collection: 將關聯查詢信息映射到一個集合中.
延遲加載
關聯查詢時,使用MyBatis 延遲加載 特性可有效減輕數據庫壓力.首次查詢只查詢主表信息,等需要時再去查詢關聯表信息.的/具備延遲加載功能.
需求: 查詢訂單信息并關聯查詢用戶信息.
延遲加載開關
在MyBatis核心配置文件(mybatis-configuration.xml)中配置:
1) lazyLoadingEnabled : 設置是否懶加載.默認false,則所有關聯查詢都會被初始化加載.
2) aggressiveLazyLoading : 設置是否積極加載. 默認true,所有關聯屬性被初始化加載.
Settings配置:
<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"><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 指定關聯查詢Statement為com.fq.mybatis.UserDAO.selectUserById.
column 指定關聯查詢時將users_id值傳入selectUserById.關聯查詢用戶信息(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 userWHERE 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來關聯查詢用戶信息.
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 {@Autowiredprivate OrderDAO dao;@Testpublic 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,會發現只有當確實需要User信息時才會調用selectUserById.
以上所述是小編給大家介紹的mybatis實踐之動態sql及關聯查詢，希望對大家以上幫助！
