第二章：MyBatis映射文件深入

OVERVIEW

第二章：MyBatis映射文件深入

一、Mybatis的Dao层实现

传统开发方式是手动对dao层进行实现，

而企业级开发的主流，采用的是Mybatis的代理开发方式实现dao层的开发。

Mapper接口开发方法只需要编写Mapper接口（类似于dao层接口），

由Mybatis框架根据接口定义创建接口的动态代理对象，代理对象的方法体同dao接口实现类的方法。

Mapper接口开发需要遵循以下规范：

Mapper.xml文件中的namespace与mapper接口的全限定名相同
Mapper接口方法名和Mapper.xml中定义的每个statement的id相同
Mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同
Mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同

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二、动态SQL

Mybaits的映射文件中，有时候业务逻辑复杂sql语句是动态变化的，就难以满足要求了：

1.环境搭建

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package com.itcast.domain;

public class User {
    private int id;
    private String username;
    private String password;

    public int getId() { return id; }

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

    public String getUsername() { return username; }

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

    public String getPassward() { return password; }

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

    /*方便测试与打印生成tostring方法*/
    @Override
    public java.lang.String toString() {
        return "User{" + "id=" + id + ", username=" + username + ", passward=" + password + '}';
    }
}

package com.itcast.mapper;

import com.itcast.domain.User;

import java.util.List;

public interface UserMapper {
    public List<User> findByCondition(User user);
}

<?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.itcast.mapper.UserMapper">
    <!--查询操作-->
    <select id="findByCondition" parameterType="com.itcast.domain.User" resultType="com.itcast.domain.User">
        select * from user where id = #{id} and username = #{username} and password = #{password}
    </select>
</mapper>

<?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>
    <!--配置数据源环境-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/test"/>
                <property name="username" value="root"/>
                <property name="password" value="20001201"/>
            </dataSource>
        </environment>
    </environments>
    <!--加载映射文件-->
    <mappers>
        <mapper resource="com.itcast.mapper/UserMapper.xml"></mapper>
    </mappers>
</configuration>

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### direct messages to file mylog.log ###
log4j.appender.file=org.apache.log4j.FileAppender
log4j.appender.file.File=c:/mylog.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### set log levels - for more verbose logging change 'info' to 'debug' ###

log4j.rootLogger=debug, stdout

package com.itcast.test;

import com.itcast.domain.User;
import com.itcast.mapper.UserMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

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

public class MapperTest {
    @Test
    public void test1() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        //mybatis实现dao层
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        //模拟user
        User condition = new User();
        condition.setId(1);
        condition.setUsername("zhangsan");
        condition.setPassward("123");

        //执行操作
        List<User> userList = mapper.findByCondition(condition);
        System.out.println(userList);
    }
}

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2.条件查询if

当部分查询条件缺失时，可以在UserMapper.xml文件中使用if标签拼接sql语句，修改文件如下：

<?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.itcast.mapper.UserMapper">
    <!--查询操作-->
    <select id="findByCondition" parameterType="com.itcast.domain.User" resultType="com.itcast.domain.User">
        select * from user
        <where>
            <if test="id != 0">
                and id = #{id}
            </if>
            <if test="username != null">
                and username = #{username}
            </if>
            <if test="password != null">
                and password = #{password}
            </if>
        </where>
    </select>
</mapper>

当MapperTest中给出的查询条件变化时，sql可以进行动态查询：

缺失所有查询条件，sql默认查询所有记录：

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给出id的情况下，则查询id为1的所有记录：

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3.复杂查询foreach

Mybatis提供了用于数组和集合循环遍历的<foreach>元素，实现复杂的查询操作，修改UserMapper.xml文件如下：

<?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.itcast.mapper.UserMapper">
    <!--查询操作-->
    <select id="findByCondition" parameterType="com.itcast.domain.User" resultType="com.itcast.domain.User">
        select * from user
        <where>
            <if test="id != 0">
                and id = #{id}
            </if>
            <if test="username != null">
                and username = #{username}
            </if>
            <if test="password != null">
                and password = #{password}
            </if>
        </where>
    </select>
    <select id="findByids" parameterType="java.util.List" resultType="com.itcast.domain.User">
        select * from user
        <where>
            <foreach collection="list" open="id in(" close=")" item="id" separator=",">
                #{id}
            </foreach>
        </where>
    </select>
</mapper>

package com.itcast.mapper;

import com.itcast.domain.User;

import java.util.List;

public interface UserMapper {
    public List<User> findByCondition(User user);
    public List<User> findByids(List<Integer> ids);
}

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4.Sql片段抽取

Sql中可以将重复的sql提取出来，使用时用include应用即可最终达到sql重用的目的：

<?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.itcast.mapper.UserMapper">
    <!--sql片段抽取-->
    <sql id="selectUser">select * from user</sql>
    <!--查询操作-->
    <select id="findByCondition" parameterType="com.itcast.domain.User" resultType="com.itcast.domain.User">
        <include refid="selectUser"></include>
        <where>
            <if test="id != 0">
                and id = #{id}
            </if>
            <if test="username != null">
                and username = #{username}
            </if>
            <if test="password != null">
                and password = #{password}
            </if>
        </where>
    </select>
    <select id="findByids" parameterType="java.util.List" resultType="com.itcast.domain.User">
        <include refid="selectUser"></include>
        <where>
            <foreach collection="list" open="id in(" close=")" item="id" separator=",">
                #{id}
            </foreach>
        </where>
    </select>
</mapper>