优化快速开始demo

在快速开始中，获取SqlSession对象的操作比较复杂，可以考虑将获取SqlSession对象的操作封装成一个方法。SqlSession对象是由SqlSessionFactory对象创建的，SqlSessionFactory是线程安全的，所以可以用单例模式来创建SqlSessionFactory对象。

package org.example.util;

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 java.io.InputStream;

public class MyBatisUtil {
    private static volatile SqlSessionFactory sqlSessionFactory;

    public static SqlSession getSqlSession() {
        try {
            if (sqlSessionFactory == null) {
                InputStream stream = Resources.getResourceAsStream("mybatis-config.xml");
                synchronized (MyBatisUtil.class) {
                    if (sqlSessionFactory == null) {
                        sqlSessionFactory = new SqlSessionFactoryBuilder().build(stream);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return sqlSessionFactory.openSession();
    }
}

修改UserDaoImpl实现类

SqlSession继承了AutoCloseable接口，所以可以将其放到try后面自动关闭。

改造前

package org.example.dao.impl;

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.example.dao.UserDao;
import org.example.pojo.User;

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

public class UserDaoImpl implements UserDao {
    private SqlSession sqlSession;

    @Override
    public User selectUser(int id) {
        try {
            // 读取主配置文件
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
            // 创建SqlSessionFactory对象
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            // 获取SqlSession对象
            sqlSession = sqlSessionFactory.openSession();
            // 查询操作
            return sqlSession.selectOne("test.selectUser", 1);
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    @Override
    public Integer insertUser(User user) {
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            sqlSession = sqlSessionFactory.openSession();
            Integer affected = sqlSession.insert("test.insertUser", user);
            sqlSession.commit();
            return affected;
        } catch (Exception e) {
            e.printStackTrace();
            return 0;
        }
    }

    @Override
    public List<User> selectAllUsers() {
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            sqlSession = sqlSessionFactory.openSession();
            return sqlSession.selectList("test.selectAllUsers");
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}

改造后

package org.example.dao.impl;

import org.apache.ibatis.session.SqlSession;
import org.example.dao.UserDao;
import org.example.pojo.User;
import org.example.util.MyBatisUtil;

import java.util.List;

public class UserDaoImpl implements UserDao {

    @Override
    public User selectUser(int id) {
        try (SqlSession sqlSession = MyBatisUtil.getSqlSession();) {
            return sqlSession.selectOne("test.selectUser", id);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    public Integer insertUser(User user) {
        try (SqlSession sqlSession = MyBatisUtil.getSqlSession();) {
            Integer affected = sqlSession.insert("test.insertUser", user);
            sqlSession.commit();
            return affected;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return 0;
    }

    @Override
    public List<User> selectAllUsers() {
        try {
            SqlSession sqlSession = MyBatisUtil.getSqlSession();
            return sqlSession.selectList("test.selectAllUsers");
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

从配置中读取数据库连接信息

一般情况下，为了方便管理，会将数据库配置信息放到配置文件中，mybatis-config.xml配置会从该配置文件中读取相关信息。

在resources目录下创建db.properties配置文件，里面填写数据库的连接信息：

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://coderlzw.cn/test
jdbc.user=root
jdbc.password=root

修改mybatis-config.xml配置，使其从配置中加载连接信息。

<?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>
    <!-- 注册配置文件 -->
    <properties resource="db.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="${jdbc.user}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!-- sql 映射 -->
        <mapper resource="userMapper.xml"/>
    </mappers>
</configuration>