MyBatis框架——动态SQL

　　MyBatis 作为⼀个“半⾃动化”的 ORM 框架，需要开发者⼿动定义 SQL 语句。

　　在业务需求⽐较复杂的情 况下，⼿动拼接 SQL 语句的⼯作量会⾮常⼤，为了适⽤于不同的业务需求，往往需要做很多重复性的⼯作，这种⼯作对于开发者来讲⽐较耗费时间，同时也容易出错。

　　此时可以使用MyBatis框架提供的动态拼接SQL，不同条件查询时，可以复用同一条SQL语句。

示例：定义一个方法，通过某些（部分）属性来查询User对象

1、定义实体类User

package com.sunjian.entity;

import java.util.List;

/**
 * @author sunjian
 * @date 2020/3/22 9:59
 */
public class User {
    private Integer id;
    private String username;
    private String password;
    private Integer age;
    private List<Integer> ids;

    public List<Integer> getIds() {
        return ids;
    }

    public void setIds(List<Integer> ids) {
        this.ids = ids;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + username + '\'' +
                ", password='" + password + '\'' +
                ", age=" + age +
                '}';
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }
}

2、创建UserRepositoryDynamicSQL接口

package com.sunjian.repository;

import com.sunjian.entity.User;

import java.util.List;

/**
 * @author sunjian
 * @date 2020/3/22 10:44
 */
public interface UserRepositoryDynamicSQL {
    public User findUserByUser(User user);
    public void updateUser(User user);
    public List<User> findAllByUser(User user);
}

3、创建UserRepositoryDynamicSQL.xml文件，编写查询SQL

<?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.sunjian.repository.UserRepositoryDynamicSQL">
    <!-- if -->
    <!--<select id="findUserByUser" parameterType="com.sunjian.entity.User" resultType="com.sunjian.entity.User">-->
    <!--select * from t_user-->
    <!--<where>-->
    <!--<if test="id != null">-->
    <!--id = #{id}-->
    <!--</if>-->
    <!--<if test="username != null">-->
    <!--and username = #{username}-->
    <!--</if>-->
    <!--<if test="password != null">-->
    <!--and password = #{password}-->
    <!--</if>-->
    <!--<if test="age != null">-->
    <!--and age = #{age}-->
    <!--</if>-->
    <!--</where>-->
    <!--</select>-->

    <!-- choose when -->
    <!--<select id="findUserByUser" parameterType="com.sunjian.entity.User" resultType="com.sunjian.entity.User">-->
    <!--select * from t_user-->
    <!--<where>-->
    <!--<choose>-->
    <!--<when test="id != null">-->
    <!--id = #{id}-->
    <!--</when>-->
    <!--<when test="username != null">-->
    <!--and username = #{username}-->
    <!--</when>-->
    <!--<when test="password != null">-->
    <!--and password = #{password}-->
    <!--</when>-->
    <!--<when test="age != null">-->
    <!--and age = #{age}-->
    <!--</when>-->
    <!--</choose>-->
    <!--</where>-->
    <!--</select>-->

    <!-- trim  -->
    <select id="findUserByUser" parameterType="com.sunjian.entity.User" resultType="com.sunjian.entity.User">
        select * from t_user
        <trim prefix="where" prefixOverrides="and">
            <if test="id != null">
                id = #{id}
            </if>
            <if test="username != null">
                and username = #{username}
            </if>
            <if test="password != null">
                and password = #{password}
            </if>
            <if test="age != null">
                and age = #{age}
            </if>
        </trim>
    </select>

    <!-- set -->
    <update id="updateUser" parameterType="com.sunjian.entity.User">
        update t_user
        <set>
            <if test="id != null">
                id = #{id},
            </if>
            <if test="username != null">
                username = #{username},
            </if>
            <if test="password != null">
                password = #{password},
            </if>
            <if test="age != null">
                age = #{age}
            </if>
        </set>
        where id = #{id}
    </update>

    <!-- foreach -->
    <select id="findAllByUser" parameterType="com.sunjian.entity.User" resultType="com.sunjian.entity.User">
        select * from t_user
        <where>
            <foreach collection="ids" item="id" open="id in (" close=")" separator=",">
                #{id}
            </foreach>
        </where>
    </select>

</mapper>

4、创建全局config.xml，注册UserRepository.xml文件

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

<configuration>
    <!-- 打印SQL语句 -->
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
        <!-- 开启延迟加载 -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!-- 开启二级缓存 -->
        <setting name="cacheEnabled" value="true"></setting>
    </settings>

    <!-- 配置 MyBatis 数据源 -->
    <environments default="development">
        <environment id="development">
            <!-- JDBC事务管理 -->
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 注册 -->
    <mappers>

        <mapper resource="com/sunjian/repository/UserRepositoryDynamicSQL.xml"></mapper>

    </mappers>

</configuration>

5、创建测试类，执行查询

package com.sunjian.test;

import com.sunjian.entity.Order;
import com.sunjian.entity.User;
import com.sunjian.repository.OrderRepository;
import com.sunjian.repository.UserRepositoryDynamicSQL;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;

/**
 * @author sunjian
 * @date 2020/3/22 10:20
 */
public class TestDynamicSQL {
    public static void main(String[] args) {

        // Mapper 代理实现自定义接口
        InputStream inputStream = User.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 获取接口的处理对象
        UserRepositoryDynamicSQL userRepositoryDynamicSQL = sqlSession.getMapper(UserRepositoryDynamicSQL.class);
        User user = new User();
        user.setUsername("张三");
        user.setPassword("321");
        User user1 = userRepositoryDynamicSQL.findUserByUser(user);
        System.out.println(user1);

        User u = new User();
        u.setId(2);
        u.setUsername("张三三");
        u.setPassword("123");
        userRepositoryDynamicSQL.updateUser(u);
        sqlSession.commit();
        User user2 = userRepositoryDynamicSQL.findUserByUser(u);
        System.out.println(user2);

        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        list.add(6);
        User u2 = new User();
        u2.setIds(list);
        List<User> users = userRepositoryDynamicSQL.findAllByUser(u2);
        for(User us:users){
            System.out.println(us);
        }
    }

}

Opening JDBC Connection
Created connection 1846406218.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@6e0dec4a]
==>  Preparing: select * from t_user where age = ?
==> Parameters: 33(Integer)
<==    Columns: id, username, password, age
<==        Row: 2, 张三三, 123, 33
<==      Total: 1
User{id=2, name='张三三', password='123', age=33}
==>  Preparing: update t_user SET id = ?, username = ?, password = ? where id = ?
==> Parameters: 2(Integer), 张三三(String), 123(String), 2(Integer)
<==    Updates: 1
Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@6e0dec4a]
==>  Preparing: select * from t_user where id = ? and username = ? and password = ?
==> Parameters: 2(Integer), 张三三(String), 123(String)
<==    Columns: id, username, password, age
<==        Row: 2, 张三三, 123, 33
<==      Total: 1
User{id=2, name='张三三', password='123', age=33}
==>  Preparing: select * from t_user WHERE id in ( ? , ? , ? )
==> Parameters: 1(Integer), 2(Integer), 6(Integer)
<==    Columns: id, username, password, age
<==        Row: 1, 张, 123, 88
<==        Row: 2, 张三三, 123, 33
<==        Row: 6, 张三, 123, 19
<==      Total: 3
User{id=1, name='张', password='123', age=88}
User{id=2, name='张三三', password='123', age=33}
User{id=6, name='张三', password='123', age=19}

OK.