MyBatis 实践

标签： Java与存储

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动态SQL

动态SQL提供了对SQL语句的灵活操作,通过表达式进行判断,对SQL进行拼接/组装.

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if

对查询条件进行判断,如果输入参数不为空才进行查询条件的拼接.

• 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/>会自动处理第一个AND(MyBatis还提供了自定义<where/>行为的<trim/>元素, 详细可参考MyBatis文档).

• 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与name为null, 因此这两个条件不会拼接在SQL中,这一点可以调试时日志中看出.

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choose/when/otherwise

有些时候,我们并不想用到所有的条件语句,而只想从中选择一二.针对这种情况,MyBatis提供了<choose/>元素,他有点像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

用于动态更新语句的解决方案为<set/>,set元素可以被用于动态包含需要更新的列, 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可以实现向SQL中传递数组或List:

传入List

查询多个id的用户信息, 可以由下面两种SQL实现:

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

因此其foreach的定义也有如下两种方案:

<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解析的参数名
index	循环下标
item	单个元素的名
open	循环开始输出
close	循环结束输出
separator	中间分隔输出

传递List作为parameterType时,SQL解析参数名固定为list.

• UserDAO

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

批量插入用户案例

• 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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传入数组

• 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>

与List类似,传递数组作为parameterType时,SQL解析参数名固定为array.

• UserDAO

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

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SQL片段

可以将一段公共的SQL语句抽取出来, 作为一个SQL片段, 供其他SQL调用:

<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>

经验:最好基于单表定义SQL片段,而且在SQL片段中不要包含<where>/<set>之类的标签,这样可以保证SQL片段重用度更高.

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关联查询

• 数据模型分析思路

  • 每张表的数据内容:分模块对每张表记录的内容进行熟悉,相当于学习系统需求/功能.
  • 每张表重要的字段:非空字段/外键字段等.
  • 表与表之间的数据库级别关系: 外键关系.
  • 表与表之间的业务关系:建立在某个业务的基础上去分析.

• 订单/商品数据模型
  

  • 表内容
    • user: 购买商品的用户信息
    • order: 用户创建的订单
    • orderdetail: 订单详细(购买商品信息)
    • item: 商品信息
  • 表与表之间的业务关系:
    1. user/order:
       • user -> order: 一对多
       • order -> user: 一对一
    2. order/orderdetail:
       • order -> orderdetail:一对多
       • orderdetail -> order:一对一
    3. orderdetail/item:
       • orderdetail -> item:一对一
       • item -> orderdetail:一对多

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‘一对一’查询

需求: 查询订单信息,关联查询(创建订单的)用户信息.

由以上分析可知主查询为order表,而order -> user关系为一对一, 因此使用resultMap将查询结果的订单信息映射到Order中,将用户信息映射到Order中的User属性.

• 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, 将User组合到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: 映射关联查询的单条记录(将关联查询信息映射到PO对象属性).

• 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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‘一对多’查询

需求: 查询订单及订单明细的信息(一对多).

• PO: 定义OrderDetail,并在Order中添加List<OrderDetail> orderDetails订单明细属性:

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	指定关联查询的结果集存储到的属性
ofType	指定关联查询结果集中的对象类型

• OrderDAO

Order selectOrderWithDetail(Integer id) throws Exception;

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‘多对多’查询

需求: 查询用户及用户购买商品信息.

由于User表与Item表没有直接关联,因此只能通过Order表与OrderDetail表进行关联.

• 思路:
  1) 将用户信息映射到User中.
  2) 在User中添加List<Order>订单列表属性,将用户创建的订单映射到orders.
  3) 在Order中添加List<OrderDetail>订单明细列表属性,将订单的明细映射到orderDetails.
  4) 在OrderDetail中添加Item属性,将订单明细所对应的商品映射到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小结:

使用<association/>和<collection/>可以完成一对一和一对多的高级映射.

• association: 将关联查询信息映射到一个PO对象中.
• collection: 将关联查询信息映射到一个集合中.

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延迟加载

关联查询时,使用MyBatis 延迟加载 特性可有效减轻数据库压力.首次查询只查询主表信息,等需要时再去查询关联表信息.<resultMap/>的<association/>/<collection/>具备延迟加载功能.

需求: 查询订单信息并关联查询用户信息.

延迟加载开关

• 在MyBatis核心配置文件(mybatis-configuration.xml)中配置:
  1) lazyLoadingEnabled : 设置是否懒加载.默认false,则所有关联查询都会被初始化加载.
  2) aggressiveLazyLoading : 设置是否积极加载. 默认true,所有关联属性被初始化加载.

• Settings配置:

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

Mapper

• 只查询订单信息

<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	指定关联查询Statement为com.fq.mybatis.UserDAO.selectUserById.
column	指定关联查询时将users_id值传入selectUserById.

• 关联查询用户信息(namespace为com.fq.mybatis.UserDAO)

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

将上面查询到的订单信息中的user_id传入selectUserById来关联查询用户信息.

• 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, 观察log信息,会发现只有当确实需要User信息时才会调用selectUserById.

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