一， 前一篇博客中，介绍了一下Mybatis和hibernate的对比，在这一篇博客说说mybatis的逆向工程，展示一下，只要有一个数据库，你的持久层，你的D层从此不用你自己手写了。
二、什么是逆向工程？
mybaits需要程序员自己编写sql语句，mybatis官方提供逆向工程 可以针对单表自动生成mybatis执行所需要的代码（mapper.java,mapper.xml、pojo），企业实际开发中，常用的逆向工程方式：由于数据库的表生成java代码。
三、Mybatis中逆向工程的使用
导入
- 导入已有的数据库，小编已经导入自己编写好的数据库，数据库结构如下：
把generatorSqlmapCustom JavaProject导入
修改配置文件的数据库连接以及数据库
指定数据库表
生成表对应的实体类
<javaModelGenerator targetPackage="com.dtt.mybatis.pojo"
targetProject=".\src">
<!-- enableSubPackages:是否让schema作为包的后缀 -->
<property name="enableSubPackages" value="false" />
<!-- 从数据库返回的值被清理前后的空格 -->
<property name="trimStrings" value="true" />
</javaModelGenerator>
mapper映射文件生成
<!-- targetProject:mapper映射文件生成的位置 -->
<sqlMapGenerator targetPackage="com.dtt.mybatis.mapper"
targetProject=".\src">
<!-- enableSubPackages:是否让schema作为包的后缀 -->
<property name="enableSubPackages" value="false" />
</sqlMapGenerator>
mapper接口生成
<!-- targetPackage：mapper接口生成的位置 -->
<javaClientGenerator type="XMLMAPPER"
targetPackage="com.dtt.mybatis.mapper"
targetProject=".\src">
<!-- enableSubPackages:是否让schema作为包的后缀 -->
<property name="enableSubPackages" value="false" />
</javaClientGenerator>
几个关键属性：
javaModelGenerator ：实体类生成；
sqlMapGenerator ：映射文件生成；
javaClientGenerator ：接口生成；
targetProject：生成文件的位置，本例中.\src，是指src下的目录；
targetPackage：生成包的名称；
修改完成后，运行GeneratorSqlmap.java中的main方法，就可以生成。
以User表为例，展示生成的代码：
- User.java
package com.dtt.mybatis.pojo;
import java.util.Date;
public class User {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
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 == null ? null : username.trim();
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex == null ? null : sex.trim();
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address == null ? null : address.trim();
}
}- UserMapper.java
这里生成了所有的增删改查的sql语句，调用的时候就直接使用对应的方法就可以了很方便吧。（重点学习一下自动生成的代码是如何封装的）
package com.dtt.mybatis.mapper;
import com.dtt.mybatis.pojo.User;
import com.dtt.mybatis.pojo.UserExample;
import java.util.List;
import org.apache.ibatis.annotations.Param;
public interface UserMapper {
int countByExample(UserExample example);
int deleteByExample(UserExample example);
int deleteByPrimaryKey(Integer id);
int insert(User record);
int insertSelective(User record);
List<User> selectByExample(UserExample example);
User selectByPrimaryKey(Integer id);
int updateByExampleSelective(@Param("record") User record, @Param("example") UserExample example);
int updateByExample(@Param("record") User record, @Param("example") UserExample example);
int updateByPrimaryKeySelective(User record);
int updateByPrimaryKey(User record);
}UserMapper.xml
<?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.dtt.mybatis.mapper.UserMapper" >
<resultMap id="BaseResultMap" type="com.dtt.mybatis.pojo.User" >
<id column="id" property="id" jdbcType="INTEGER" />
<result column="username" property="username" jdbcType="VARCHAR" />
<result column="birthday" property="birthday" jdbcType="DATE" />
<result column="sex" property="sex" jdbcType="CHAR" />
<result column="address" property="address" jdbcType="VARCHAR" />
</resultMap>
<sql id="Example_Where_Clause" >
<where >
<foreach collection="oredCriteria" item="criteria" separator="or" >
<if test="criteria.valid" >
<trim prefix="(" suffix=")" prefixOverrides="and" >
<foreach collection="criteria.criteria" item="criterion" >
<choose >
<when test="criterion.noValue" >
and ${criterion.condition}
</when>
<when test="criterion.singleValue" >
and ${criterion.condition} #{criterion.value}
</when>
<when test="criterion.betweenValue" >
and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
</when>
<when test="criterion.listValue" >
and ${criterion.condition}
<foreach collection="criterion.value" item="listItem" open="(" close=")" separator="," >
#{listItem}
</foreach>
</when>
</choose>
</foreach>
</trim>
</if>
</foreach>
</where>
</sql>
<sql id="Update_By_Example_Where_Clause" >
<where >
<foreach collection="example.oredCriteria" item="criteria" separator="or" >
<if test="criteria.valid" >
<trim prefix="(" suffix=")" prefixOverrides="and" >
<foreach collection="criteria.criteria" item="criterion" >
<choose >
<when test="criterion.noValue" >
and ${criterion.condition}
</when>
<when test="criterion.singleValue" >
and ${criterion.condition} #{criterion.value}
</when>
<when test="criterion.betweenValue" >
and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
</when>
<when test="criterion.listValue" >
and ${criterion.condition}
<foreach collection="criterion.value" item="listItem" open="(" close=")" separator="," >
#{listItem}
</foreach>
</when>
</choose>
</foreach>
</trim>
</if>
</foreach>
</where>
</sql>
<sql id="Base_Column_List" >
id, username, birthday, sex, address
</sql>
<select id="selectByExample" resultMap="BaseResultMap" parameterType="com.dtt.mybatis.pojo.UserExample" >
select
<if test="distinct" >
distinct
</if>
<include refid="Base_Column_List" />
from user
<if test="_parameter != null" >
<include refid="Example_Where_Clause" />
</if>
<if test="orderByClause != null" >
order by ${orderByClause}
</if>
</select>
<select id="selectByPrimaryKey" resultMap="BaseResultMap" parameterType="java.lang.Integer" >
select
<include refid="Base_Column_List" />
from user
where id = #{id,jdbcType=INTEGER}
</select>
<delete id="deleteByPrimaryKey" parameterType="java.lang.Integer" >
delete from user
where id = #{id,jdbcType=INTEGER}
</delete>
<delete id="deleteByExample" parameterType="com.dtt.mybatis.pojo.UserExample" >
delete from user
<if test="_parameter != null" >
<include refid="Example_Where_Clause" />
</if>
</delete>
<insert id="insert" parameterType="com.dtt.mybatis.pojo.User" >
insert into user (id, username, birthday,
sex, address)
values (#{id,jdbcType=INTEGER}, #{username,jdbcType=VARCHAR}, #{birthday,jdbcType=DATE},
#{sex,jdbcType=CHAR}, #{address,jdbcType=VARCHAR})
</insert>
<insert id="insertSelective" parameterType="com.dtt.mybatis.pojo.User" >
insert into user
<trim prefix="(" suffix=")" suffixOverrides="," >
<if test="id != null" >
id,
</if>
<if test="username != null" >
username,
</if>
<if test="birthday != null" >
birthday,
</if>
<if test="sex != null" >
sex,
</if>
<if test="address != null" >
address,
</if>
</trim>
<trim prefix="values (" suffix=")" suffixOverrides="," >
<if test="id != null" >
#{id,jdbcType=INTEGER},
</if>
<if test="username != null" >
#{username,jdbcType=VARCHAR},
</if>
<if test="birthday != null" >
#{birthday,jdbcType=DATE},
</if>
<if test="sex != null" >
#{sex,jdbcType=CHAR},
</if>
<if test="address != null" >
#{address,jdbcType=VARCHAR},
</if>
</trim>
</insert>
<select id="countByExample" parameterType="com.dtt.mybatis.pojo.UserExample" resultType="java.lang.Integer" >
select count(*) from user
<if test="_parameter != null" >
<include refid="Example_Where_Clause" />
</if>
</select>
<update id="updateByExampleSelective" parameterType="map" >
update user
<set >
<if test="record.id != null" >
id = #{record.id,jdbcType=INTEGER},
</if>
<if test="record.username != null" >
username = #{record.username,jdbcType=VARCHAR},
</if>
<if test="record.birthday != null" >
birthday = #{record.birthday,jdbcType=DATE},
</if>
<if test="record.sex != null" >
sex = #{record.sex,jdbcType=CHAR},
</if>
<if test="record.address != null" >
address = #{record.address,jdbcType=VARCHAR},
</if>
</set>
<if test="_parameter != null" >
<include refid="Update_By_Example_Where_Clause" />
</if>
</update>
<update id="updateByExample" parameterType="map" >
update user
set id = #{record.id,jdbcType=INTEGER},
username = #{record.username,jdbcType=VARCHAR},
birthday = #{record.birthday,jdbcType=DATE},
sex = #{record.sex,jdbcType=CHAR},
address = #{record.address,jdbcType=VARCHAR}
<if test="_parameter != null" >
<include refid="Update_By_Example_Where_Clause" />
</if>
</update>
<update id="updateByPrimaryKeySelective" parameterType="com.dtt.mybatis.pojo.User" >
update user
<set >
<if test="username != null" >
username = #{username,jdbcType=VARCHAR},
</if>
<if test="birthday != null" >
birthday = #{birthday,jdbcType=DATE},
</if>
<if test="sex != null" >
sex = #{sex,jdbcType=CHAR},
</if>
<if test="address != null" >
address = #{address,jdbcType=VARCHAR},
</if>
</set>
where id = #{id,jdbcType=INTEGER}
</update>
<update id="updateByPrimaryKey" parameterType="com.dtt.mybatis.pojo.User" >
update user
set username = #{username,jdbcType=VARCHAR},
birthday = #{birthday,jdbcType=DATE},
sex = #{sex,jdbcType=CHAR},
address = #{address,jdbcType=VARCHAR}
where id = #{id,jdbcType=INTEGER}
</update>
</mapper>小结
逆向工程在很多软件里面都有，比如EA的逆向工程，PB的逆向工程。还有很多，总之，这个就是其中的一些很有价值的东西，在企业运用的时候很方便，而且这些生成好的类，尽量不要去修改，如果非要修改的话，可以用pv类来实现继承拓展。这是很好的选择。
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