Mybatis初识+CRUD

什么是mybatis

MyBatis 是一款优秀的持久层框架,它支持自定义 SQL、存储过程以及高级映射。MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

简单理解持久化

持久化是将程序数据在持久状态和瞬时状态间转换的机制。通俗的讲，就是瞬时数据（比如内存中的数据，是不能永久保存的）持久化为持久数据（比如持久化至数据库中，能够长久保存）。

初识Mybatis

使用前先导入mybatis的依赖

<!--        mybatis-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.7</version>
        </dependency>

允许静态资源导出

    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为核心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或一个预先配置的 Configuration 实例来构建出 SqlSessionFactory 实例。

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<!--configuration核心配置文件-->
<configuration>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=false&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="xiaofan"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>

<!--    每一个Mapper.xml都需要在Mybatis核心配置文件中注册-->
    <mappers>
        <mapper resource="com/xiaofan/dao/UserMapper.xml"></mapper>
    </mappers>

</configuration>

自己手动创建一个MybatisUtil工具类，方便获取sqlSession执行对象

public class MybatisUtils {

    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            //获取sqlSessionFactory对象
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory= new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

//    既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例
//    SqlSession 提供了在数据库执行 SQL 命令所需的所有方法
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

创建一个User实体类，UserMapper接口

public class User {
    private int id;
    private String name;
    private String pwd;

    public User() {
    }

    public User(int id, String name, String pwd) {
        this.id = id;
        this.name = name;
        this.pwd = pwd;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

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

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

public interface UserMapper {
    //查询全部用户
    List<User> getUserList();

    User getUserById2(Map<String,Object> map);

    //insert
    int addUser(User user);
    //update
    int updateUser(User user);
    //delete
    int deleteUser(int id);
}

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">

<!--        namespace绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.xiaofan.dao.UserMapper">
<!--    id->方法名  -->
    <select id="getUserList" resultType="com.xiaofan.pojo.User">
        select * from mybatis.user
    </select>
<!--    parameterType,普通类型可以不写，特殊类型一定要写-->
    <select id="getUserById2" parameterType="map" resultType="com.xiaofan.pojo.User">
        select * from mybatis.user where id=#{helloId} and name=#{name}
    </select>

<!--    对象中的属性可以直接取出来-->
    <insert id="addUser" parameterType="com.xiaofan.pojo.User">
        insert into mybatis.user (id,name,pwd) values (#{id},#{name},#{pwd})
    </insert>

    <update id="updateUser" parameterType="com.xiaofan.pojo.User">
        update mybatis.user set name = #{name},pwd=#{pwd} where id=#{id}
    </update>

    <delete id="deleteUser" parameterType="int">
        delete from mybatis.user where id=#{id}
    </delete>
</mapper>

最后写一个测试类，测试一下

    @Test
    public void test(){

        //获得sqlSession对象
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = mapper.getUserList();

        for (User user:userList){
            System.out.println(user);
        }
        //关闭sqlSession
        sqlSession.close();
    }

    @Test
    public void addUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        int updateRows = mapper.addUser(new User(4, "jack", "121212"));
        if(updateRows>0){
            System.out.println("insert ok!");
        }

        //提交事务
        sqlSession.commit();//mybatis不用开启事务，因为默认关闭自动提交
        sqlSession.close();
    }

有一个小细节，就是在MybatisUtil中，

    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession(true);
    }

openSession中有一个方法SqlSession openSession(boolean autoCommit);
将参数设置为true,开启自动提交，写增删改时可以不用手动提交。