Java中的XML映射文件主要用于将Java对象与XML文档之间进行转换。它通常用于处理数据交换和存储，例如将Java对象转换为XML格式以便在网络上传输或保存到文件中，或者将XML文档解析为Java对象以进行处理。这种转换可以通过Java的JAXB（Java Architecture for XML Binding）库来实现。

如何实现XML映射实现对数据库的增删改，以及动态查询操作：

实体类对象： 

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Emp {
    private Integer id; //ID
    private String username; //用户名
    private String password; //密码
    private String name; //姓名
    private Short gender; //性别, 1 男, 2 女
    private String image; //图像url
    private Short job; //职位, 说明: 1 班主任,2 讲师, 3 学工主管, 4 教研主管, 5 咨询师'
    private LocalDate entrydate; //入职日期
    private Integer deptId; //部门ID
    private LocalDateTime createTime; //创建时间
    private LocalDateTime updateTime; //修改时间
}

package com.itheima.mapper;

import com.itheima.pojo.Emp;
import org.apache.ibatis.annotations.*;
import org.yaml.snakeyaml.events.Event;

import java.time.LocalDate;
import java.util.List;

@Mapper
public interface EmpMapper {

    //根据ID删除数据
    @Delete("delete from emp where id = #{id}")
    public void delete(Integer id);
    //public int delete(Integer id);

    //新增员工
    @Options(useGeneratedKeys = true, keyProperty = "id")
    @Insert("insert into emp(username,name,gender,image,job,entrydate,dept_id,create_time,update_time) " +
            "values(#{username},#{name},#{gender},#{image},#{job},#{entrydate},#{deptId},#{createTime},#{updateTime})")
    public void insert(Emp emp);


    //更新数据
    @Update("update emp set username=#{username},gender=#{gender},name=#{name},image=#{image},job=#{job}" +
            ",entrydate=#{entrydate},dept_id=#{deptId},update_time=#{updateTime} where id = #{id} ")
    public void update(Emp emp);

    //根据Id查询数据
    @Select("select * from emp where id = ${id} ")
    public Emp getById(Integer id);

    //动态查询sql语句
    public List<Emp> list(String name, Short gender, LocalDate start , LocalDate end);
}

xml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.itheima.mapper.EmpMapper">

    <select id="list" resultType="com.itheima.pojo.Emp">
        select *
        from emp
        where
        <if test="name!=null">
            name like concat('%',#{name},'%')
        </if>
        <if test="gender != null">
            and gender = #{gender}
        </if>
        <if test="start!=null and end!=null">
            and entrydate between #{start} and #{end}
        </if>
        order by update_time desc
    </select>

</mapper>

设置启动类进行测试：

package com.itheima;

import com.itheima.mapper.EmpMapper;
import com.itheima.pojo.Emp;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.util.Arrays;
import java.util.List;

@SpringBootTest
class SpringbootMybatisCrudApplicationTests {

//    @Autowired
//    private EmpMapper empMapper;
//    @Test
//    public void testDelete(){
//        empMapper.delete(17);
//    }
    @Autowired
    private EmpMapper empMapper;

    //根据ID删除
    @Test
    public void testDelete(){
//        int delete = empMapper.delete(17);
//        System.out.println(delete);
        empMapper.delete(16);
    }

    @Test
    //增加员工
    public void testInsert(){
        //构造员工对象
        Emp emp = new Emp();
        emp.setUsername("Tom2");
        emp.setName("汤姆3");
        emp.setGender((short)1);
        emp.setImage("1.jpg");
        emp.setJob((short)1);
        emp.setEntrydate(LocalDate.of(2000, 1, 1));
        emp.setCreateTime(LocalDateTime.now());
        emp.setUpdateTime(LocalDateTime.now());
        emp.setDeptId(1);

        //执行添加员工的操作
        empMapper.insert(emp);
        System.out.println(emp.getId());
    }

//    更新员工信息
    @Test
    public void testUpdate(){
        //构造员工对象
        Emp emp = new Emp();
        emp.setId(18);
        emp.setUsername("Tom1");
        emp.setName("汤姆12");
        emp.setGender((short)1);
        emp.setImage("1.jpg");
        emp.setJob((short)1);
        emp.setEntrydate(LocalDate.of(2000, 1, 1));
        emp.setUpdateTime(LocalDateTime.now());
        emp.setDeptId(1);

        empMapper.update(emp);
        System.out.println(emp.getId());
    }

    //根据id查询员工信息
    @Test
    public void testGetById(){
        Emp emp = empMapper.getById(18);
        System.out.println(emp);
    }

    //根据条件查询员工信息
    @Test
    public void testList(){
        List<Emp> empList = empMapper.list("张", (short) 1, LocalDate.of(2010, 1, 1), LocalDate.of(2020, 1, 1));
//        List<Emp> empList = empMapper.list("张", null, null, null);
        System.out.println(empList);
    }
}

目录等级：

其实以上XML,文件还有一个小bug，如果查询的姓名为null，此时编译会报错：

 

观看控制台发现，确实SQL语句编译后多了，and，因此使用<where>标签可以有效的解决这个问题：（<set>标签可以删除多余的 ','）

<?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.itheima.mapper.EmpMapper">

    <select id="list" resultType="com.itheima.pojo.Emp">
        select *
        from emp
        <where>
            <if test="name!=null">
                name like concat('%',#{name},'%')
            </if>
            <if test="gender != null">
                and gender = #{gender}
            </if>
            <if test="start!=null and end!=null">
                and entrydate between #{start} and #{end}
            </if>
        </where>
        order by update_time desc
    </select>

</mapper>