从零再学mybatis

一时失言乱红尘 2022-02-18 15:51 219阅读 0赞

从零再学mybatis  
直观的来说mybatis有以下特点

1.  使用灵活,sql高度可定制
2.  性能优化易操作
3.  入门门槛低,只需要掌握sql语法即可
4.  实现与接口相分离
5.  orm 框架(半自动,需要手动编写sql)

mybatis如此优秀,易上手,性能可以有效优化,使得其成为非常流行的数据库后端框架之一.

现在针对mybatis提出以下问题:

1.  mybatis 如何将.xml 文件mapper接口绑定到一起产生对象的?
2.  mybatis的缓存是如何管理的

# mybatis 基本环境搭建: #

## 需要的文件 ##

1.  数据库脚本
2.  pojo 类 User
3.  UserMapper 接口,UserMapper.xml
4.  mybatis.jar 包
5.  mybatis-config.xml
6.  db.properties

## 具体步骤 ##

1.  sql 脚本

DROP TABLE IF EXISTS `user`;
    CREATE TABLE `user` (
      `id` int(11) NOT NULL AUTO_INCREMENT,
      `name` varchar(20) CHARACTER SET utf8mb4 DEFAULT NULL,
      `password` varchar(32) CHARACTER SET utf8mb4 DEFAULT NULL,
      `sex` varchar(10) CHARACTER SET utf8mb4 DEFAULT NULL,
      `description` varchar(255) CHARACTER SET utf8mb4 DEFAULT '',
      PRIMARY KEY (`id`)
    ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1;

1.  User 类

public class User { 
        private int id;
        private String name;
        private String sex;
        private String description;
        private String password;
    
       
        public User() { 
        }
    
        public User(String name, String sex, String description) { 
            this.name = name;
            this.sex = sex;
            this.description = description;
        }
    
        @Override
        public String toString() { 
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", sex='" + sex + '\'' +
                    ", description='" + description + '\'' +
                    '}';
        }
        ... 省略getter/setter
        }

1.  UserMapper.java 接口 以及UserMapper.xml 文件

public interface UserMapper { 
        void insert(User user);
        User getById(int id);
    }

<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    
    <mapper namespace="top.forethought.framework.mybatis.dao.UserMapper">
        <resultMap type="top.forethought.framework.jdbctemplate.User" id="usermap">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
            <result column="password" property="password"/>
            <result column="sex" property="sex"/>
            <result column="description" property="description"/>
        </resultMap>
    
        <select id="getById" parameterType="int" resultMap="usermap">
            select * from user where id=#{id}
        </select>
        <insert id="insert">
            insert into user ( name,password, sex,description) values (
           #{name}, #{password},#{sex}, #{description}
            )
        </insert>
    
    
    
    </mapper>

1.  pom.xml 依赖

<dependency>
                <groupId>org.mybatis</groupId>
                <artifactId>mybatis</artifactId>
                <version>${mybatis.version}3.2.8</version>
            </dependency>
             <dependency>
                <groupId>org.mybatis</groupId>
                <artifactId>mybatis-spring</artifactId>
                <version>${mybatis.spring.version}</version>
            </dependency>

1.  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="top/forethought/framework/configs/db.properties">
    
    
        </properties>
    
        <environments default="development">
            <environment id="development">
                <transactionManager type="JDBC"/>
                <dataSource type="POOLED">
                    <property name="driver" value="${driver}"/>
                    <property name="url" value="${jdbcUrl}"/>
                    <property name="username" value="${username}"/>
                    <property name="password" value="${password}"/>
                </dataSource>
            </environment>
        </environments>
    
    
        <mappers>
            <mapper resource="top/forethought/framework/mybatis/mapper/UserMapper.xml"/>
        </mappers>
    </configuration>

1.  db.properties ,配置具体的数据库参数,供xml文件中引用

username=noneroot
    password=18142513872
    jdbcUrl=jdbc:mysql://localhost:3306/test_index?useUnicode=true&characterEncoding=utf8&serverTimezone=GMT
    driver=com.mysql.jdbc.Driver

1.  测试类 TestMapper.java

public class TestMapper { 
        @Test
        public void testMybatis() throws IOException { 
    
            String resource = "top/forethought/framework/configs/mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory =
                    new SqlSessionFactoryBuilder().build(inputStream);
    
            SqlSession sqlSession=sqlSessionFactory.openSession();
            UserMapper mapper=  sqlSession.getMapper(UserMapper.class);
        User user=new User();
        user.setName("123");
        user.setSex("男");
        user.setDescription("这是人物描述"+System.currentTimeMillis());
        mapper.insert(user);
        System.out.println("查询来自数据库:"+mapper.getAll().toString());
        sqlSession.commit();// 如果不commit 是不能提交事务到数据库的
        sqlSession.close();
        }
    }

1.  测试效果

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70]

# mybatis如何产生实实在在的mapper对象 #

## 先看最终结果 ##

![在这里插入图片描述][20190417213441773.png]显然最终得到的是实现了UserMapper 中接口方法的代理类

## 再看debug 过程 ##

以下截图主要是为了记录执行流程  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 1]**MapperRegistry**:看名字可以知道这是mapper注册器,所有mapper接口与相应实现xml文件绑定成功成功后会保存一条记录到knownMappers 之中  
knownNappers ,是一个Map ,以接口名为key(比如这里是UsernMapper),  
MapperProxyFactory 为val  
即:记录了接口与mapper代理工厂之间的映射关系  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 2]**MapperProxyFactory**: 有一个Map 来保存 接口中的方法到mapper.xml 中的具体实现(MapperMethod)的映射关系  
![在这里插入图片描述][20190417215302565.png]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 3]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 4]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 5]MapperMethod:  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 6]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 7]![在这里插入图片描述][201904172200047.png]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 8]

## 大体流程如下: ##

MapperRegistry 的getMapper 方法中,调用了MapperProxyFactory的newInstance 方法获取到  
根据接口生成的代理对象

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 9]生成的代理对象应该使用的是jdk的动态代理  
调用mapper.insert() 方法时,可以发现  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 10]

## 结论 ##

1.  mapper 与xml的绑定,使用了工厂模式产生代理对象(实现了接口)
2.  采用了jdk动态代理(InvocationHandler)
3.  使用反射创建代理对象
4.  主要参与的类有MapperRegistry(根据接口找代理MapperProxyFactory),MapperProxyFactory(造代理对象)

# mybatis 如何配置或者是管理缓存 #

可以查看打印输出的sql语句来判断是否使用了缓存

1.  配置sql输出

在mybatis-config.xml 的中添加以下内容(注意位置,会有提示,按照提示处理标签顺序即可,这是由dtd来约束的)

1.  修改测试类代码

public class TestMapper { 
        @Test
        public void testMybatis() throws IOException { 
    
            String resource = "top/forethought/framework/configs/mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory =
                    new SqlSessionFactoryBuilder().build(inputStream);
    
            SqlSession sqlSession=sqlSessionFactory.openSession();
            UserMapper mapper=  sqlSession.getMapper(UserMapper.class);
    // User user=new User();
    // user.setName("123");
    // user.setSex("男");
    // user.setDescription("这是人物描述"+System.currentTimeMillis());
    // mapper.insert(user);
            // 连续在相同条件下查询两次数据库,发现输出的sql只有一句
            System.out.println("查询来自数据库:"+mapper.getAll().toString());
            System.out.println("查询来自数据库:"+mapper.getAll().toString());
       // sqlSession.commit();// 如果不commit 是不能提交事务到数据库的
        sqlSession.close();
        }
    }

输出:  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 11]尝试关闭mybatis缓存  
mybatis-config.xml 的settings添加

**测试效果:**  
发现还是只是输出了一句,难道是打开方式不对!!!  
cacheEnabled:是缓存的全局配置,如果关闭,那么其他有关缓存的配置都将是无效的

> Globally enables or disables any caches configured in any mapper under this configuration.

参数配置错了,应该配置 **localCacheScope**属性,可以配置cache范围是SESSION还是STATEMENT  
**SESSION**:表示同一个session,所有的查询都会被缓存,那么相同的查询执行第二遍,那么就是去缓存取数据了,而不是再次去数据库查询  
**STATEMENT**:表示每一个语句都是去数据库查询

> MyBatis uses local cache to prevent circular references and speed up repeated nested queries. By default (SESSION) all queries executed during a session are cached. If localCacheScope=STATEMENT local session will be used just for statement execution, no data will be shared between two different calls to the same SqlSession

效果:

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 12]

### 疑问: ###

可能对SESSION会有疑问了,如果是设置为SESSION,同一个session内指向两次相同查询的之间再执行一条插入语句,那么第二次查询还是会从缓存取吗?  
测试结果发现,这样是执行了两次数据库查询  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 13]

更多的settings 相关配置,[传送][Link 1]

## 其他小问题: ##

### Cause: java.sql.SQLException: Incorrect string value: ‘\\xE7\\x94\\xB7’ for column ‘sex’ at row 1 ###

原因:数据库字符编码问题  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2OTIyOTI3_size_16_color_FFFFFF_t_70 14]

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[20190417213441773.png]: /images/20220218/f5bc7aa2821e42c3a43d2729e683f1a8.png
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[20190417215302565.png]: https://img-blog.csdnimg.cn/20190417215302565.png
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[201904172200047.png]: https://img-blog.csdnimg.cn/201904172200047.png
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[Link 1]: http://www.mybatis.org/mybatis-3/configuration.html#settings
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