MyBatis 實踐

標簽： Java與存儲

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動態(tài)SQL

動態(tài)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.

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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

<select id="selectUser" resultType="com.fq.domain.User" parameterType="com.fq.domain.User">SELECT * FROM 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></where> </select>

<where/>會自動處理第一個AND(MyBatis還提供了自定義<where/>行為的<trim/>元素, 詳細可參考MyBatis文檔).

• UserDAO

List<User> selectUser(User user) throws Exception;

• Client

@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); }

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.

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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<choose/>元素,他有點像Java中的switch.

<select id="selectUser" resultType="com.fq.domain.User" parameterType="com.fq.domain.User">SELECT * FROM user<where><choose><when test="id != null">AND id = #{id}</when><when test="name != null">AND name = #{name}</when><otherwise>AND password = #{password}</otherwise></choose></where> </select>

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set

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

<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>

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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:

SELECT * FROM user WHERE (id = ? OR id = ? OR id = ?); SELECT * FROM user WHERE id IN (?, ?, ?, ?);

因此其foreach的定義也有如下兩種方案:

<select id="selectUser" parameterType="java.util.List" resultType="com.fq.domain.User">SELECT *FROM user<where><if test="list != null and list.size >= 1"><foreach collection="list" item="id" open="(" separator="or" close=")">id = #{id}</foreach></if></where> </select> <select id="selectUser" parameterType="java.util.List" resultType="com.fq.domain.User">SELECT *FROM user<where><if test="list != null and list.size >= 1"><foreach collection="list" item="id" open="id IN (" separator="," close=")">#{id}</foreach></if></where> </select> 元素描述

collection	SQL解析的參數名
index	循環(huán)下標
item	單個元素的名
open	循環(huán)開始輸出
close	循環(huán)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

List<User> selectUser(List<Integer> ids) throws Exception;

批量插入用戶案例

• mapper

<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>

• UserDAO

void insertUserList(List<User> users) throws Exception;

• Client

@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"))); }

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傳入數組

• mapper

<select id="selectUser" parameterType="Object[]" resultType="com.fq.domain.User">SELECT *FROM user<where><if test="array != null and array.length >= 1"><foreach collection="array" item="id" open="id IN (" separator="," close=")">#{id}</foreach></if></where> </select>

與List類似,傳遞數組作為parameterType時,SQL解析參數名固定為array.

• UserDAO

List<User> selectUser(Integer[] ids) throws Exception;

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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:

<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<where><include refid="user_where"/></where> </select>

經驗:最好基于單表定義SQL片段,而且在SQL片段中不要包含<where>/<set>之類的標簽,這樣可以保證SQL片段重用度更高.

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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(yè)務關系:建立在某個業(yè)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(chuàng)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(yè)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

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‘一對一’查詢

需求: 查詢訂單信息,關聯查詢(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

public class User implements Serializable {private Integer id;private String username;private Date birthday;private Integer sex;private String address;// ... }

• PO: 新增Order, 將User組合到Order中:

public class Order implements Serializable {private Integer id;private Integer userId;private String number;private Date createTime;private String note;private User user;// ... }

• mapper

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <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"/></association></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></mapper>

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

public interface OrderDAO {Order selectOrderWithUser(Integer id) throws Exception; }

• Client

@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);} }

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‘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<OrderDetail> orderDetails訂單明細屬性:

public class OrderDetail implements Serializable {private Integer id;private Integer orderId;private Integer itemId;private Integer itemNumber;// ... }

• mapper

<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"/></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> 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

Order selectOrderWithDetail(Integer id) throws Exception;

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‘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<Order>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<OrderDetail>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

public class Item {private Integer id;private String name;private Float price;private String detail;private String pic;private Date createTime;//... }

• mapper

<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"/></association></collection></collection> </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>

• OrderDAO

List<User> selectUserItem() throws Exception;

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resultMap小結:

使用<association/>和<collection/>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.

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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.<resultMap/>的<association/>/<collection/>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:

<settings><setting name="cacheEnabled" value="true"/><setting name="lazyLoadingEnabled" value="true"/><setting name="aggressiveLazyLoading" value="false"/> </settings>

Mapper

• 只查詢訂單信息

<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"/> </resultMap><select id="selectOrderWithUser" resultMap="order_user_map">SELECT *FROM `order`; </select> 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)

<select id="selectUserById" parameterType="java.lang.Integer" resultType="com.fq.domain.User">SELECT *FROM userWHERE id = #{id}; </select>

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)

List<Order> selectOrderWithUser() throws Exception;

• Client

@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());}} }

debug上面Client, 觀察log信息,會發(fā)現只有當確實需要User信息時才會調用selectUserById.

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轉載于:https://www.cnblogs.com/itrena/p/5926903.html

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