Mybatis（一）：入门

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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：

这里写图片描述

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);
    }

}