Mybatis（一）：入门

Mybatis应该对于任何用Java的人来说都不陌生，以前对于知识有些没有系统性的整理和学习，所以这里把自己当做小白，再次学习下Mybatis。

在此，多说点没用的。学习新东西的最好的途径就是官网，因为以后要学习最新的东西的时候，没有那么多的教学视频和博客给你，甚至需要Google国外的网站。

MyBatis官方文档，http://www.mybatis.org/mybatis-3/zh/index.html

这是我见过的写的比较简单明了的文档之一了，小白完全可以根据文档学习。

一、什么是Mybatis

引用官网的话：

MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。

二、增删改查演示

需要了解的几个重要的类

SqlSessionFactoryBuilder 用于生成SqlSessionFactory

SqlSessionFactory 用户生成SqlSession

SqlSession 数据库操作核心类

工程结构：



Maven依赖

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd"> <modelVersion>4.0.0</modelVersion>
<groupId>cn.saytime</groupId>
<artifactId>mybatis_demo</artifactId>
<packaging>war</packaging>
<version>1.0</version>
<name>mybatis_demo Maven Webapp</name>
<url>http://maven.apache.org</url>

<dependencies>
<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.5</version>
</dependency>

<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.34</version>
</dependency>

<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
</dependencies>

<build>
<finalName>mybatis_demo</finalName>
</build>
</project>

实体类User

package cn.saytime.domain;

/**
* Created by L on 2018/1/3.
*/
public class User {

private Long id;
private String username;
private Integer age;

public User(){

}

public User(String username, Integer age) {
this.username = username;
this.age = age;
}

public Long getId() {
return id;
}

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

public String getUsername() {
return username;
}

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

public Integer getAge() {
return age;
}

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

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

UserMapper.java

package cn.saytime.mapper;

import cn.saytime.domain.User;
import java.util.List;

/**
* @author L
* @ClassName cn.saytime.mapper.UserMapper
* @Description
*/
public interface UserMapper {

/**
* 保存用户
* @param user
* @return
*/
boolean save(User user);

/**
* 删除用户
* @param id
* @return
*/
boolean delete(Long id);

/**
* 更新用户
* @param user
* @return
*/
boolean update(User user);

/**
* 根据ID查找用户信息
* @param id
* @return
*/
User selectById(Long id);

/**
* 用户总数量
* @return
*/
int selectCount();

/**
* 查找所有用户列表
* @return
*/
List<User> selectAll();

}

jdbc配置文件 jdbc.properties

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://127.0.0.1:3306/test
jdbc.username=root
jdbc.password=root

Mybatis配置文件 mybatis-config.xml

<?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>
<properties resource="jdbc.properties"/>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="mappers/UserMapper.xml"/>
</mappers>
</configuration>

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="cn.saytime.mapper.UserMapper" >

<resultMap type="cn.saytime.domain.User" id="userMap">
<id property="id" javaType="java.lang.Long" column="id" />
<result property="username" javaType="java.lang.String" column="username" />
<result property="age" javaType="java.lang.Integer" column="age" />
</resultMap>

<!-- 增 -->
<insert id="save" parameterType="cn.saytime.domain.User">
INSERT INTO `test`.`tb_user`(`username`, age) VALUES(#{username}, #{age})
</insert>

<!-- 删 -->
<delete id="delete" parameterType="java.lang.Long">
DELETE FROM `test`.`tb_user` WHERE id = #{id}
</delete>

<!-- 改 -->
<update id="update" parameterType="cn.saytime.domain.User">
UPDATE `test`.`tb_user` SET username = #{username}, age = #{age} WHERE id = #{id}
</update>

<!-- 根据ID查询 -->
<select id="selectById" parameterType="java.lang.Long" resultMap="userMap">
SELECT * FROM `test`.`tb_user` WHERE id = #{id}
</select>

<!-- 总数 -->
<select id="count" resultType="java.lang.Integer">
SELECT COUNT(1) AS c FROM `test`.`tb_user`
</select>

<!-- 查询所有 -->
<select id="selectAll" resultMap="userMap">
SELECT * FROM `test`.`tb_user`
</select>

</mapper>

三、测试

import cn.saytime.domain.User;
import cn.saytime.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.After;
import org.junit.Before;

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

/**
* Created by L on 2018/1/3.
*/
public class TestUserMapper {

SqlSessionFactory sqlSessionFactory = null;
SqlSession sqlSession = null;
UserMapper userMapper = null;

@Before
public void before(){
// mybatis 配置文件地址
String resource = "mybatis-config.xml";
InputStream inputStream = null;
try {
inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
e.printStackTrace();
}
// 加载配置文件，并构建sqlSessionFactory
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// 获取sqlSession对象
sqlSession = sqlSessionFactory.openSession();
userMapper = sqlSession.getMapper(UserMapper.class);
}

@After
public void after(){
if(sqlSession != null){
// 注意这里的commit，否则增删改操作提交不成功
sqlSession.commit();
sqlSession.close();
}
}

/**
* 查找所有用户
*/
@org.junit.Test
public void testSelectAll(){
// 第一种方式：动态代理获取UserMapper的代理类
List<User> users = userMapper.selectAll();
if(users != null && users.size() > 0){
for (User user : users) {
System.out.println(user);
}
}

// 第二种方式
User user = (User) sqlSession.selectOne("cn.saytime.mapper.UserMapper.selectById", 2L);
System.out.println(user);
}

}

本章以及未来几章的代码：

https://gitee.com/saytime/mybatis_demo