MyBatis 实践
标签: Java与存储
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.
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.
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>
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>
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")));
}
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;
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.
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:
- user/order:
- user -> order: 一对多
- order -> user: 一对一
- order/orderdetail:
- order -> orderdetail:一对多
- orderdetail -> order:一对一
- orderdetail/item:
- orderdetail -> item:一对一
- item -> orderdetail:一对多
- user/order:
- 表内容
‘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,
address
FROM `order`, user
WHERE `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 {
@Autowired
private OrderDAO dao;
@Test
public void client() throws Exception {
Order order = dao.selectOrderWithUser(3);
System.out.println(order);
}
}
‘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_id
FROM `order`, user, orderdetail
WHERE `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;
‘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">
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>
- OrderDAO
List<User> selectUserItem() throws Exception;
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.
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 user
WHERE 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 {
@Autowired
private OrderDAO dao;
@Test
public 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
.