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MyBatis实践
2025-07-22 09:57  点击:0

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
<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>

会自动处理第一个AND(MyBatis还提供了自定义行为的元素, 详细可参考MyBatis文档).

UserDAO 
List selectUser(User user) throws Exception;
Client 
@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);}

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提供了元素,他有点像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

用于动态更新语句的解决方案为,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 user                        name = #{name} ,                            password = #{password} ,                WHERE id = #{id};

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:

<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
collectionSQL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 selectUser(List ids) throws Exception;

批量插入用户案例

mapper 
    INSERT INTO user(name, password) VALUES                        (#{user.name}, #{user.password})
UserDAO 
void insertUserList(List users) throws Exception;
Client 
@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")));}

传入数组

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>

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

UserDAO 
List 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:

<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>

经验:最好基于单表定义SQL片段,而且在SQL片段中不要包含/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

‘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 
<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 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 
<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 
<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 
List selectUserItem() throws Exception;

resultMap小结:

使用和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.的/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:

<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

只查询订单信息 
<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) 
<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>

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 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 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.