MyBatis持久层框架

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本文详细介绍了MyBatis持久层框架的使用，包括如何导入核心库、配置核心文件、创建SqlSessionFactory，以及如何创建实体类、映射文件和执行SQL。还探讨了解决属性名与列名不一致的问题、分页实现以及注解开发映射等高级用法，展示了多对一和一对多的关系映射处理，并讲解了动态SQL的应用。

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1、MyBatis是一个基于Java的持久层框架，而持久化是指数据从瞬时状态变为持久状态，持久层是完成持久化工作的代码块-Dao层。即，MyBatis帮助开发人员将数据存入数据库中和从数据库中取数据。通过框架可以减少重复代码，提高开发效率。MyBatis是一个半自动化的ORM框架。

2、MyBatis使用

>导包：

核心包：mybatis-3.2.7.jar

依赖包：

数据库驱动包：

>编写MyBatis的核心配置文件

<?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>





<environments default="development">
<environment id="development">

<transactionManager type="JDBC"/>

<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/shop?serverTimezone=UTC"/>
<property name="username" value="root"/>
<property name="password" value="MySQL1003150748"/>
</dataSource>
</environment>
</environments>
<mappers>
   
<mapper resource="com/myBatis/entity/tea.mapper.xml"/>
</mappers>
</configuration>

>创建sqlSessionFactory

import java.io.IOException;
import java.io.InputStream;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class MyBatisUtil {
   /*
   * 通过配置文件创建SqlSessionFactory 是一个SqlSession的工厂类
   */
   public static SqlSessionFactory getSqlSessionFactory() throws IOException {
       String resource = "mybatis.cfg.xml";
       InputStream inputStream = Resources.getResourceAsStream(resource);
       SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
       return sqlSessionFactory;
   }
   /*
   * SqlSession通过映射执行sql语句（通过id找到对应的sql语句，执行sql语句）
   */
   public static SqlSession getSession() throws IOException {
       SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
       return sqlSessionFactory.openSession();
   }
}

>创建实体类

package com.myBatis.entity;

public class Tea {
   private int id ; //饮品编号
   private String gname ; //饮品名称
   private double gprice ; //饮品价格

   public Tea() {}
   public int getId() {
       return id;
   }
   public void setId(int id) {
       this.id = id;
   }
   public String getGname() {
       return gname;
   }
   public void setGname(String gname) {
       this.gname = gname;
   }
   public double getGprice() {
       return gprice;
   }
   public void setGprice(double gprice) {
       this.gprice = gprice;
   }
   public Tea(int id, String gname, double gprice) {
       super();
       this.id = id;
       this.gname = gname;
       this.gprice = gprice;
   }
   @Override
   public String toString() {
       return "TeaBean [id=" + id + ", gname=" + gname + ", gprice=" + gprice + "]";
   }
}

>编写sql语句的映射文件

<?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.myBatis.entity.TeaMapper">
   
   <select id="selectTea" resultType="com.myBatis.entity.Tea">
   select * from goodstable where id = #{id}
   </select>
</mapper>

>测试类

package com.myBatis.test;

import java.io.IOException;
import org.apache.ibatis.session.SqlSession;
import com.myBatis.entity.Tea;
import com.myBatis.util.MyBatisUtil;

public class Test {
   public static void main(String[] args) throws IOException {
       SqlSession session = MyBatisUtil.getSession() ;
       Tea tea = session.selectOne("com.myBatis.entity.TeaMapper.selectTea",27) ;
       System.out.println(tea.getGname());
       session.close();
   }
}

3、解决实体类中属性名（password）与数据库列名（ pwd）不一致的问题

不一致出现的问题：不一致的属性查询的内容为null

不一致的原因在于：mabatis会根据查询的列名调用其setter方法进行设值（在mapper映射文件中定义）

解决办法一：为列名指定别名，别名和java实体类的属性名一致

<select id="selectTea" resultType="com.myBatis.entity.Tea">
select Name, pwd password from goodstable where id = #{id}
</select>

解决方法：

<select id="selectTea" resultMap="TeaMap">
select Name, pwd from goodstable where id = #{id}
</select>

<resultMap type="Tea" id="TeaMap">
<id column="id" property="id" />主键
<result column="pwd" property="password" />数据库中表的列名和java实体类中的属性名
</resultMap>

4、分页的实现

>limit约束实现

mapper映射中定义：

<select id="selectAll" parameterType="Map" resultType="com.myBatis.entity.Tea">
       select * from goodstable limit #{startIndex},#{pageSize}
   </select>

Dao层方法：

import java.io.IOException;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.apache.ibatis.session.SqlSession;
import com.myBatis.entity.Tea;
import com.myBatis.util.MyBatisUtil;

public class TeaDao {
   //分页查询
   public List<Tea> getAll(int currentPage,int pageSize) throws IOException {
       SqlSession session = MyBatisUtil.getSession() ;
       Map<String,Integer> map = new HashMap<String,Integer>();
       map.put("startIndex",(currentPage-1)*pageSize); //当前页第一条记录的下标
       map.put("pageSize", pageSize);
       List<Tea> list = session.selectList("com.myBatis.entity.TeaMapper.selectAll", map);
       session.close();
       return list;
   }
}

>通过RowBounds实现

在mapper映射中直接查询所有：

<select id="selectTea" resultType="com.myBatis.entity.Tea">
select * from goodstable
</select>

在Dao层方法中通过RowBounds实现查询指定条数的分页数据：

//参数index是下标，size是数据

RowBounds rowBounds = new RowBounds((currentPage-1)*pageSize,pageSize);
List<Tea> list = session.selectList("com.myBatis.entity.TeaMapper.selectTea", null, rowBounds)

5、使用注解（本质通过接口）开发映射mapper语句

//编写Dao接口
public interface UserDao {
   @Select("select * from user")
   public List<User> getList();
   @Insert("insert into user(name,pwd) value(#{name},#{pwd})")
   public int insert(User user);
}
//mapper映射
<mappers>
   <mapper class="com.dao.UserDao" />
</mappers>
//调用
UserDao userDao = session.getMapper(UserDao.class);
//调用userDao中方法……
会通过动态代理生成该接口的实现类，在测试类调用接口中定义的方法时，其实是调用其实现类中的方法
6、数据库表多对一处理

<mapper namespace="com.entity.student.mapper">
   
   
   <select id="getStudents" resultMap="StudentTeacher">
       select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname from student s,teacher t where s.tid=t.id
   </select>
   <resultMap type="Student" id="StudentTeacher">
       <id column="sid" property="id" />
       <result column="sname" property="name" />
       
       <association property="teacher" javaType="Teacher" >
           <id column="tid" property="id" />
           <result column="tname" property="name" />
       </association>
   </resultMap>
   ————————————————————————————————————————————————————————
   
   <select id="getStudents" resultMap="StudentTeacher">
       select * from student
   </select>
   <resultMap type="Student" id="StudentTeacher">
       
       <association property="teacher" column="tid" javaType="Teacher" select="getTeacher">
       </association>
   </resultMap>
   <selest id="getTeacher" resultType="Teacher">
       select * from teacher where id=#{id}
   </select>
</mapper>

7、表中一对多处理

<select id="getStudents" resultMap="TeacherStudent">
       select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname from student s,teacher t where s.tid=t.id
   </select>
   <resultMap type="Teacher" id="TeacherStudent">
       <id column="tid" property="id" />
       <result column="tname" property="name" />
       <collection property="students" ofType="Student" >
           <id column="sid" property="id" />
           <result column="sname" property="name" />
       </collection>
   </resultMap>

————————————————————————————————————————————————————————
   <select id="getTeacher" resultMap="TeacherStudent">
       select * from teacher where id=#{id}
   </select>
   <resultMap type="Teacher" id="TeacherStudent">
       <collection property="students" column="id" javaType="ArrayList" ofType="student" select="getStudentByTid">
       </association>
   </resultMap>
   <select id="getStudentByTid" resultType="Student">
       select * from student where tid=#{id}
   </select>

8、动态sql

动态sql指根据不同的查询条件，生成不同的sql语句
Mapper文件
<mapper namespace="">
   <select id="" parameterType="" resultType="">
       select * from user
       <where>
           <if test="name!=null">
               name like CONCAT('%',#{name},'%')
           </if>
       </where>
   <select>
</mapper>

注：mybatis中like查询要使用CONCAT字符连接