7.MyBatis

数据库JDBC封装框架
持久层框架
https://mybatis.org/mybatis-3/zh/index.html

1. 安装，导入依赖

2. 进行代理设置

   Mapper代理

3. 提前定义好业务逻辑所需的对象，例如User

4. 定义针对该对象的接口 ```java package cn.prelu.mapper;

import cn.prelu.pojo.User; import org.apache.ibatis.annotations.Param; import org.apache.ibatis.annotations.Select;

public interface UserMapper {

@Select( "SELECT * FROM tb_user Where username = #{username} and password = #{password};")
User select(@Param("username") String username, @Param("password") String password);

}

定义的Interface会被Mybatis继承使用<br />写一行执行逻辑
3. 将sql的执行逻辑和参数写入接口的方法里
3. 在同级目录下完成configxml的编写
```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>
    <!--起别名-->
    <typeAliases>
        <package name="cn.prelu.pojo"/>
    </typeAliases>
    <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:///db1?useSSL=false&amp;useServerPrepStmts=true&amp;characterEncoding=utf-8"/>
                <property name="username" value="root"/>
                <property name="password" value="asd"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--扫描mapper-->
        <package name="cn.prelu.mapper"/>
    </mappers>
</configuration>

注：如果在idea中可以使用mybatisX来简化撰写流程

1. 设置sql的映射文件的namespace属性为Mapper接口全限定名
2. 在Mapper里面保持参数和类型一致

3. 编码撰写业务逻辑

   调用MyBatis

4. 注册配置文件

   String resource = "mybatis-config.xml";
   InputStream inputStream = Resources.getResourceAsStream(resource);
   SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

   使用sqlsession资源读取配置文件进行调用
   其中需要指定流的目标是配置文件，使用工厂案例即可获取资源

5. 打开事务

   SqlSession sqlSession = sqlSessionFactory.openSession();

6. 接口对象获取事务的sql并执行

        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

7. 注册对象资源并传参

        User user = userMapper.select(username,password);
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();

8. 进行业务逻辑

        if(user != null){
            writer.write("LoginSuccess");
        }else{
            writer.write("登录失败");
        }

   MyBatisX

9. Idea插件可以在Mapper配置文件和接口中进行跳转

10. 快速编写sql片段

例如下面的namespace指定了代理所需要的sql语句，但是sql通过注解完成

<?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.prelu.mapper.UserMapper">
</mapper>

跳转接口定义如下：

public interface UserMapper {
    @Select( "SELECT * FROM tb_user Where username = #{username} and password = #{password};")
    User select(@Param("username") String username, @Param("password") String password);
}

完整的业务逻辑

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException ,NullPointerException{
        req.setCharacterEncoding("utf-8");
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username);
        System.out.println(password);
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.select(username,password);
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        if(user != null){
            writer.write("LoginSuccess");
        }else{
            writer.write("登录失败");
        }
    }

线程池优化

若每次调用都使用SQLSessionFactroy进行创建对象将会增加大量无用线程池的内存开销
解决方案：

1. 重复代码使用单例模式的静态方法进行工具类封装
2. 工厂模式创建线程池对象使用静态代码块调用，各个线程和模块之间使用同一内存区域的代码

   工具类的撰写：

   ```java package cn.prelu.util;

import org.apache.ibatis.io.Resources; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException; import java.io.InputStream;

public class SqlSessionFactoryUtil {

private static SqlSessionFactory sqlSessionFactory;
static {
    String resource = "mybatis-config.xml";
    InputStream inputStream = null;
    try {
        inputStream = Resources.getResourceAsStream(resource);
    } catch (IOException e) {
        e.printStackTrace();
    }
    sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
}
public static SqlSessionFactory getSqlSessionFactory(){
    return  sqlSessionFactory;
}

}

实现方法代码：
```java
       SqlSessionFactory sqlSessionFactory = SqlSessionFactoryUtil.getSqlSessionFactory();