Mybatis

環境：jdk mysql maven idea

SSM框架、組態檔的，

一、簡介

1.1、什么是Mybatis？

Mysbatis是一款優秀的持久化框架

它支持制定化SQL，存盤過以及高級映射

Mybatis避免了幾乎所有的JDBC代碼和手動設定引數以及獲取結果集

Mybatis可以使用簡單的XML或注解來配置和映射原生型別，介面和POJO為資料庫中的記錄

Mybatis原本是apache的一個開源專案Batis2010年這個專案由apache software foundation遷移到了Google code，并改名為Mybati

2013年11月遷移到Github如何獲得mybatis？

Maven廠庫：

Maven下載網址：https://www.mybatis.org/mybatis-3Maven

 <dependency> 
?
?    <groupId>org.mybatis</groupId>
?
?     <artifactId>mybatis</artifactId> 
?
?     <version>3.5.11</version>
?
  </dependency>

Github網址: https://github.com/mybatis/mybatis-3中午網：https://mybatis.net.cn/

1.2、持久化

資料持久化

持久化就是將程式的資料在持久狀態和瞬時狀態轉化的程序

記憶體斷電即失資料庫（jdbc）、io檔案持久化

為什么需要持久化？

有一些物件，不能丟記憶體太貴了

13、持久層

Dao層、Service層、Controller層

完成持久化作業的代碼塊

層界十分明顯

1.4、為什么需要Mybatis？

幫助程式員將資料存入資料庫中

方便

傳統的jdbc太復雜了、簡化、框架、自動化

不用mybatis也可以，更容易上手優點：

簡單易學

靈活

Sql和代碼的分離，提高了可維護性

提供映射標簽，支持物件與資料庫的orm欄位關系映射

提供物件關系標簽，支持物件關系組建維護

提供xml標簽，支持撰寫動態sql

*使用的人多

二、創建一個Mybatis專案

1、創建maven專案

2、匯入依賴包

<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.47</version>
</dependency>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.5.10</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.13.2</version>
</dependency>
</dependencies>

這里的mybatis就是mybatis的依賴

3、創建工具類

package com.zeng.mybatis.utils;
?
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.IOException;
import java.io.InputStream;
?
public class MybatisUtils {
public static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSqlSession(){
return sqlSessionFactory.openSession();
}
}

4、創建物體類

package com.zeng.mybatis.pojo;
?
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 int getId() {
return id;
}
?
public void setId(int id) {
this.id = id;
}
?
public String getName() {
return name;
}
?
public void setName(String name) {
this.name = name;
}
?
public String getPwd() {
return pwd;
}
?
public void setPwd(String pwd) {
this.pwd = pwd;
}
?
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", pwd='" + pwd + '\'' +
'}';
}
}

5、創建Mapper介面

package com.zeng.mybatis.dao;
?
import com.zeng.mybatis.pojo.User;
?
import java.util.List;
?
public interface UserDao {
List<User> getUserList();
User getUserId(int id);
int addUser(User user);
int updateUser(User user);
int deleteUser(int id);
}

6、創建Mapper介面對應的xml檔案

<?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="com.zeng.mybatis.dao.UserDao">
<select id="getUserList" resultType="com.zeng.mybatis.pojo.User">
select * from User
</select>
</mapper>

7、在resources包下創建連接資料庫的對應資訊.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>
<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?characterEncoding=utf-8&amp;serverTimezone=UTC&amp;useSSL=false"/>
<property name="username" value="root"/>
<property name="password" value="12345678"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/zeng/mybatis/dao/UserMapper.xml"/>
</mappers>
</configuration>

三、CRUD

1、namespace

namespace中的包名要和Dao/mapper介面中的包名一致

2、select

選擇、查詢陳述句

id:就是對應的namespace中的方法名

resultType:Sql陳述句執行的回傳值！

parameterType:引數型別

2.1、對應例子

Mapper.xml對應標簽

<select id="getUserId" resultType="com.zeng.mybatis.pojo.User" parameterType="int">
select * from User where id = #{id}
</select>

@Text測驗方法

@Test
public void getUserIdTest(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserDao mapper = sqlSession.getMapper(UserDao.class);
User user = mapper.getUserId(1);
System.out.println(user);
}

3、insert增加標簽

Mapper.xml對應標簽

<insert id="addUser" parameterType="com.zeng.mybatis.pojo.User">
insert into User (id,name,pwd)
values (#{id},#{name},#{pwd});
</insert>

@Text測驗方

@Test
public void addUserTest(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserDao mapper = sqlSession.getMapper(UserDao.class);
int row = mapper.addUser(new User(4, "丁鈺", "85200"));
sqlSession.commit();
if (row>0){
System.out.println("插入成功");
}
sqlSession.close();
}

4、update修改標簽

Mapper.xml對應標簽

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

@Text測驗方法

@Test
public void updateUserTest(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserDao mapper = sqlSession.getMapper(UserDao.class);
int row = mapper.updateUser(new User(4, "丁鈺兒", "7894"));
sqlSession.commit();
if (row>0){
System.out.println("修改成功");
}
sqlSession.close();
}

5、delete洗掉標簽

Mapper.xml對應標簽

<delete id="deleteUser" parameterType="int">
delete
from User
where id = #{id};
</delete>

@Text測驗方法

@Test
public void deleteUserTest(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserDao mapper = sqlSession.getMapper(UserDao.class);
int row = mapper.deleteUser(4);
sqlSession.commit();
if (row>0){
System.out.println("洗掉成功");
}
sqlSession.close();
}

一定要記住增刪改的時候一定要增加事務

四、配置設定

configuration（配置）

properties（屬性）（重點）

settings（設定）（重點）

typeAliases（型別別名）（重點）

typeHandlers（型別處理器）

objectFactory（物件工廠）

plugins（插件）environments（環境配置）

environment（環境變數）

transactionManager（事務管理器）

dataSource（資料源）

databaseIdProvider（資料庫廠商標識）

mappers（映射器）（重點）

1、properties：

這些屬性可以在外部進行配置，并可以進行動態替換，你既可以在典型的 Java 屬性檔案中配置這些屬性，也可以在 properties 元素的子元素中設定，

例如：

<properties resource="org/mybatis/example/config.properties">
<property name="username" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/dev_user"/>
<property name="password" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/F2Fa3!33TYyg"/>
</properties>

設定好的屬性可以在整個組態檔中用來替換需要動態配置的屬性值，比如:

<dataSource type="POOLED">
<property name="driver" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${driver}"/>
<property name="url" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${url}"/>
<property name="username" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${username}"/>
<property name="password" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${password}"/>
</dataSource>

2、setting

cacheEnabled:全域性地開啟或關閉所有映射器組態檔中已配置的任何快取，true | falsetrue
lazyLoadingEnabled:延遲加載的全域開關，當開啟時，所有關聯物件都會延遲加載， 特定關聯關系中可通過設定 fetchType 屬性來覆寫該項的開關狀態，
logImpl:指定 MyBatis 所用日志的具體實作，未指定時將自動查找，SLF4J | LOG4J | LOG4J2 | JDK_LOGGING | COMMONS_LOGGING | STDOUT_LOGGING | NO_LOGGING

3、typeAliases:

型別別名可為 Java 型別設定一個縮寫名字，它僅用于 XML 配置，意在降低冗余的全限定類名書寫，例如：

<typeAliases>
<typeAlias alias="Author" type="domain.blog.Author"/>
<typeAlias alias="Blog" type="domain.blog.Blog"/>
<typeAlias alias="Comment" type="domain.blog.Comment"/>
<typeAlias alias="Post" type="domain.blog.Post"/>
<typeAlias alias="Section" type="domain.blog.Section"/>
<typeAlias alias="Tag" type="domain.blog.Tag"/>
</typeAliases>

當這樣配置時，Blog 可以用在任何使用 domain.blog.Blog 的地方，

也可以指定一個包名，MyBatis 會在包名下面搜索需要的 Java Bean，比如：

<typeAliases>
<package name="domain.blog"/>
</typeAliases>每一個在包 domain.blog 中的 Java Bean，在沒有注解的情況下，會使用 Bean 的首字母小寫的非限定類名來作為它的別名， 比如 domain.blog.Author 的別名為 author；若有注解，則別名為其注解值，見下面的例子：


@Alias("author")
public class Author {
...
}

4、mappers：

例如：用戶查找資源（包括 file:/// 形式的 URL）

<!-- 使用相對于類路徑的資源參考 -->
<mappers>
<mapper resource="org/mybatis/builder/AuthorMapper.xml"/>
<mapper resource="org/mybatis/builder/BlogMapper.xml"/>
<mapper resource="org/mybatis/builder/PostMapper.xml"/>
</mappers>

<!-- 使用完全限定資源定位符（URL） -->
<mappers>
  <mapper url="file:///var/mappers/AuthorMapper.xml"/>
  <mapper url="file:///var/mappers/BlogMapper.xml"/>
  <mapper url="file:///var/mappers/PostMapper.xml"/>
</mappers>

<!-- 使用映射器介面實作類的完全限定類名 -->
<mappers>
  <mapper />
  <mapper />
  <mapper />
</mappers>

<!-- 將包內的映射器介面實作全部注冊為映射器 -->
<mappers>
  <package name="org.mybatis.builder"/>
</mappers>

5、生命周期和作用域

生命周期，和作用域，之至關重要的，因為錯誤的使用會導致非常嚴重的并發問題

SqlSessionFactoryBuilder:

一旦創建了SqlSessionFactory，就不再需要它了

定義區域變數、

SqlSessionFactory:

說白了就是可以想象為：資料庫連接池

一旦被創建就應該在應用的運行期間一直存在，沒有任何理由丟棄它或重新創建另一個實體，

因此SqlSessionFactory的最佳作用域是應用作用域，

最簡單的就是使用單例模式或者靜態單例模式，

SqlSession:

連接到連接池的一個請求！

SqlSession的實體不是執行緒安全的，因此是不能被共享的，所以他的最佳作用域是請求或方法作用域，

用完之后需要趕緊關閉，否則資源被占用！

五、解決欄位名和屬性名不一致問題

package com.zeng.mybatis.pojo;

import org.apache.ibatis.type.Alias;

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

    public User() {
    }

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

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getPassword() {
        return password;
    }

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

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

這里可以看到資料庫的欄位名為pwd而物體類的屬性名為password

查詢結果：

解決方法：

1、更改sql陳述句

沒改之前

<select id="getUserId" resultType="com.zeng.mybatis.pojo.User" parameterType="int">
        select * from User where id = #{id}
    </select>

改之后

<select id="getUserId" resultType="com.zeng.mybatis.pojo.User" parameterType="int">
        select id,name,pwd as password from User where id = #{id}
    </select>

這里我們通過給欄位名取別名可以解決這個問題

2、使用resultMap

<?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="com.zeng.mybatis.dao.UserDao">
    <resultMap id="userMap" type="com.zeng.mybatis.pojo.User">
        <result property="id" column="id" />
        <result property="name" column="name"/>
        <result property="password" column="pwd"/>
    </resultMap>
    
    <select id="getUserId" resultMap="userMap">
        select * from User where id = #{id}
    </select>
</mapper>

這里我們使用resultMap也能查詢出來

resultMap 元素是 MyBatis 中最重要最強大的元素，

六、日志工廠

logImpl	指定 MyBatis 所用日志的具體實作，未指定時將自動查找，	SLF4J \| LOG4J \| LOG4J2 \| JDK_LOGGING \| COMMONS_LOGGING \| STDOUT_LOGGING \| NO_LOGGING

1、STDOUT_LOGGING

只需要配置一些檔案即可：

<settings>
        <setting name="logImpl" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/STDOUT_LOGGING"/>
    </settings>

運行方法進行測驗：

2、LOG4J

1、導包：

<!-- https://mvnrepository.com/artifact/log4j/log4j -->
<dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
</dependency>

2、創建組態檔log4j.properties

#將等級為DEBUG的日志資訊輸出到console和file這兩個目的地，console和file的定義在下面的代碼 log4j.rootLogger=DEBUG,console,file

#控制臺輸出的相關設定 log4j.appender.console = org.apache.log4j.ConsoleAppender log4j.appender.console.Target = System.out log4j.appender.console.Threshold=DEBUG log4j.appender.console.layout = org.apache.log4j.PatternLayout log4j.appender.console.layout.ConversionPattern=[%c]-%m%n

#檔案輸出的相關設定 log4j.appender.file = org.apache.log4j.RollingFileAppender log4j.appender.file.File=./log/logFile.log log4j.appender.file.MaxFileSize=10mb log4j.appender.file.Threshold=DEBUG log4j.appender.file.layout=org.apache.log4j.PatternLayout log4j.appender.file.layout.ConversionPattern=%p[%c]%m%n

#日志輸出級別 log4j.logger.org.mybatis=DEBUG log4j.logger.java.sql=DEBUG log4j.logger.java.sql.Statement=DEBUG log4j.logger.java.sql.ResultSet=DEBUG log4j.logger.java.sql.PreparedStatement=DEBUG

3、設定資源檔案

<settings>
    <setting name="logImpl" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/LOG4J"/>
</settings>

4、使用Logger類

@Test
    public void log4jTest(){
        Logger logger = Logger.getLogger(daoTest.class);
        logger.info("info:進入了log4jTest");
        logger.error("error:進入了log4jTest");
        logger.debug("debug:進入了log4jTest");
    }

七、分頁

1、limit

1、介面

List<User> getUserLimit(Map<String, Integer> map);

2、介面對應的mapper

    <select id="getUserLimit" parameterType="map" resultMap="userMap">
        select * from User limit #{start},#{end}
    </select>

3、測驗類

@Test
    public void getUserLimit(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        Map<String, Integer> limit = new HashMap<>();
        limit.put("start",1);
        limit.put("end",2);
        List<User> userLimit = mapper.getUserLimit(limit);
        for (User user : userLimit) {
            System.out.println(user);
        }
        sqlSession.close();
    }

4、結果

2、RowBounds

1、介面

List<User> getUserRowBounds();

2、實作類

    <select id="getUserRowBounds" resultMap="userMap">
        select * from User
    </select>

3、測驗

@Test
    public void getUserRowBounds(){
        RowBounds rowBounds = new RowBounds(1,2);
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        List<User> userList = sqlSession.selectList("com.zeng.mybatis.dao.UserDao.getUserRowBounds", null, rowBounds);
        for (User user : userList) {
            System.out.println(user);
        }
        sqlSession.close();
    }

3、分頁工具

網址：https://pagehelper.github.io/

八、使用注解開發

1、select查詢批量資料

1、介面

public interface UserDao {
    @Select("select * from User")
    List<User> getUserList();
}

2、核心xml配置

（使用注解的核心配置都在這里）

<mappers>
        <mapper />
    </mappers>

3、測驗

public class daoTest {
    @Test
    public void getUserList(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        List<User> userList = mapper.getUserList();
        for (User user : userList) {
            System.out.println(user);
        }
        sqlSession.close();
    }
}

2、select根據id查詢

1、介面

@Select("select * from User where id = #{uid}")
    User getUserById(@Param("uid")int id);

這里的@Param注解是用與有多個基本型別的引數時加上，但是只要時基本資料型別建議都加上

2、測驗

@Test
    public void getUserById(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        User userById = mapper.getUserById(1);
        System.out.println(userById);
        sqlSession.close();
    }

3、insert

1、介面

@Insert("insert into User (id,name,pwd) values(#{id},#{name},#{pwd})")
    int addUser(User user);

2、測驗

@Test
    public void addUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        mapper.addUser(new User(6, "趙云", "8520"));
        sqlSession.close();
    }

4、update

1、介面

@Update("update User set name=#{name},pwd=#{pwd} where id=#{id}")
    int updataUser(User user);

2、測驗

@Test
    public void updataUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        mapper.updataUser(new User(5,"葉凡","987654321"));
        sqlSession.close();
    }

5、delete

1、介面

@Delete("delete from User where id=#{id}")
    int delete(@Param("id")int id);

2、測驗

@Test
    public void deleteUser(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserDao mapper = sqlSession.getMapper(UserDao.class);
        mapper.delete(6);
        sqlSession.close();
    }

九、Lombok

官方介紹：
Project Lombok is a java library that automatically plugs into your editor and build tools, spicing up your java.Never write another getter or equals method again, with one annotation your class has a fully featured builder, Automate your logging variables, and much more.

Project Lombok是一個java庫，可以自動插入編輯器和構建工具，為您的java增添趣味，永遠不要再寫另一個getter或equals方法，一個注釋你的類就有了一個功能齊全的構建器，自動化你的日志變數，等等，

1、IDEA中安裝Lombok插件

里面的注解有：

@Getter and @Setter
@FieldNameConstants
@ToString
@EqualsAndHashCode
@AllArgsConstructor, @RequiredArgsConstructor and @NoArgsConstructor
@Log, @Log4j, @Log4j2, @Slf4j, @XSlf4j, @CommonsLog, @JBossLog, @Flogger, @CustomLog
@Data
@Builder
@SuperBuilder
@Singular
@Delegate
@Value
@Accessors
@Wither
@With
@SneakyThrows
@StandardException
@val
@var
experimental @var
@UtilityClass

2、添加Maven依賴

<!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.24</version>
    <scope>provided</scope>
</dependency>

3、使用

package com.zeng.mybatis.pojo;

import lombok.*;

@EqualsAndHashCode  //equals和hashcode方法
@ToString           //toString方法
@AllArgsConstructor //所有屬性的有參構造方法
@NoArgsConstructor  //無參構造方法
@Data               //注解在類，生成setter/getter、equals、canEqual、hashCode、toString方法，如為final屬性，則不會為該屬性生成setter方法
@Getter             //加在類上面就是所有屬性的get方法加在屬性上面可以生成單個get方法
@Setter             //加在類上面就是所有屬性的set方法加在屬性上面可以生成單個set方法
public class User {
    private int id;
    private String name;
    private String pwd;

}

上面的代碼等于下面的代碼

package com.zeng.mybatis.pojo;

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

    public User() {
    }

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

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getPassword() {
        return password;
    }

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

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

十、多表查詢

1、多對一

1、第一種方式

1、物體類

package com.zeng.mybatis.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private Teacher teacher;
}

package com.zeng.mybatis.pojo;

import lombok.Data;

@Data
public class Teacher {
    private int id;
    private String name;
}

2、介面

package com.zeng.mybatis.dao;

import com.zeng.mybatis.pojo.Student;

import java.util.List;

public interface studentMapper {
    public List<Student> getStudent();
}

package com.zeng.mybatis.dao;

import com.zeng.mybatis.pojo.Teacher;

public interface teacherMapper {
    public Teacher getTeacher(int id);
}

3、核心組態檔

<?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="db.properties"></properties>
    <!--    <typeAliases>
            &lt;!&ndash;<typeAlias type="com.zeng.mybatis.pojo.User" alias="user"></typeAlias>&ndash;&gt;
            <package name="com.zeng.mybatis.pojo"/>
        </typeAliases>-->
    <settings>
        <setting name="logImpl" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/LOG4J"/>
    </settings>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${driver}"/>
                <property name="url" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${jdbc.url}"/>
                <property name="username" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${username}"/>
                <property name="password" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper />
        <mapper />
    </mappers>
</configuration>

特別注意

mapper是映射介面的時候和mapper映射檔案時的區別

介面映射時：

    <mappers>
        <mapper />
        <mapper />
    </mappers>

mapper映射檔案

    <mappers>
        <mapper />
        <mapper />
    </mappers>

4、介面映射檔案

<?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="com.zeng.mybatis.dao.studentMapper">

    <select id="getStudent" resultMap="StudentTeacher">
        select * from Student
    </select>
    <resultMap id="StudentTeacher" type="com.zeng.mybatis.pojo.Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <association property="teacher" column="tid" javaType="com.zeng.mybatis.pojo.Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="com.zeng.mybatis.pojo.Teacher">
        select * from teacher where id = #{id}
    </select>
</mapper>

5、測驗類

@Test
    public void getStudentList(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        studentMapper mapper = sqlSession.getMapper(studentMapper.class);
        List<Student> student = mapper.getStudent();
        for (Student s : student) {
            System.out.println(s);
        }
        sqlSession.close();
    }

2、按照結果嵌套查詢

1、介面

public List<Student> getStudent2();

2、介面映射檔案

<select id="getStudent2" resultMap="student2">
        select s.id sid,s.name sname,t.name tname from student s,teacher t where s.tid = t.id
    </select>
    <resultMap id="student2" type="com.zeng.mybatis.pojo.Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="com.zeng.mybatis.pojo.Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

3、測驗類

@Test
    public void getStudentList2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        studentMapper mapper = sqlSession.getMapper(studentMapper.class);
        List<Student> studentList = mapper.getStudent2();
        for (Student student : studentList) {
            System.out.println(student);
        }
        sqlSession.close();
    }

1、一對多

1、聯合查詢ResultMap映射

1、物體類

教師類

package com.mybatis.pojo;

import java.util.List;
public class teacher {
    private int id;
    private String name;
    private List<student> students;

    public teacher() {
    }

    public teacher(int id, String name, List<student> students) {
        this.id = id;
        this.name = name;
        this.students = students;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public List<student> getStudents() {
        return students;
    }

    public void setStudents(List<student> students) {
        this.students = students;
    }

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

學生類

package com.mybatis.pojo;

public class student {
    private int id;
    private String name;
    private int tid;

    public student() {
    }

    public student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = tid;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public int getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

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

2、介面

教師介面

package com.mybatis.dao;

import com.mybatis.pojo.teacher;
import org.apache.ibatis.annotations.Param;

import java.util.List;

public interface teacherMapper {
    public List<teacher> getAllTS(@Param("stid") int sid);
}

學生介面

package com.mybatis.dao;

import com.mybatis.pojo.student;

import java.util.List;

public interface studentMapper {
    public List<student> getStudent();
}

3、核心組態檔

<?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="db.properties"></properties>
    <settings>
        <setting name="logImpl" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/LOG4J"/>
    </settings>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${driver}"/>
                <property name="url" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${jdbc.url}"/>
                <property name="username" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${username}"/>
                <property name="password" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/mybatis/dao/studentMapper.xml"/>
        <mapper resource="com/mybatis/dao/teacherMapper.xml"/>
    </mappers>
</configuration>

介面映射檔案

teacherMapper

<?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="com.mybatis.dao.teacherMapper">
    <resultMap id="teacherMap" type="com.mybatis.pojo.teacher">
        <result property="id" column="tid" />
        <result property="name" column="tname"/>
        <collection property="students" ofType="com.mybatis.pojo.student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="stid"/>
        </collection>
    </resultMap>
    <select id="getAllTS" resultMap="teacherMap">
        select t.id tis,t.name tname,s.id sid,s.name sname from teacher t,student s where t.id = s.tid and t.id = #{stid}
    </select>
</mapper>

4、測驗類

@org.junit.Test
    public void getAllTS(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        teacherMapper mapper = sqlSession.getMapper(teacherMapper.class);
        List<teacher> allTS = mapper.getAllTS(1);
        for (teacher teacher : allTS) {
            System.out.println(teacher);
        }
        sqlSession.close();
    }

2、子查詢映射

1、介面

List<teacher> getTeacherS(@Param("tid") int tid);

2、介面映射檔案

<select id="getTeacherS" resultMap="teacherMaps">
        select * from teacher where id=#{tid}
    </select>
    <resultMap id="teacherMaps" type="com.mybatis.pojo.teacher">
        <collection property="students" javaType="ArrayList" ofType="com.mybatis.pojo.student" select="getStudents" column="id"/>
    </resultMap>
    <select id="getStudents" resultType="com.mybatis.pojo.student">
        select * from student where tid=#{tid}
    </select>

3、測驗類

@org.junit.Test
    public void getTeacherS(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        teacherMapper mapper = sqlSession.getMapper(teacherMapper.class);
        List<teacher> teachers = mapper.getTeacherS(1);
        for (teacher teacher : teachers) {
            System.out.println(teacher);
        }
        sqlSession.close();
    }

十一、動態SQL

1、動態SQL環境搭建

1、創建資料庫

CREATE TABLE `blog` (
`id` VARCHAR(50) NOT NULL COMMENT '博客id',
`title` VARCHAR(100) NOT NULL COMMENT '博客標題',
`author` VARCHAR(30) NOT NULL COMMENT '博客作者',
`create_time` DATETIME NOT NULL COMMENT '創建時間',
`views` INT(30) NOT NULL COMMENT '瀏覽量'
)ENGINE=INNODB DEFAULT CHARSET=utf8;`blog`

2、創建物體類

package com.zeng.mybatis.pojo;

import lombok.Data;

import java.util.Date;
@Data
public class Blog {
    private String id;
    private String title;
    private String author;
    private Date create_time;
    private int views;
}

3、db.properties

driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf-8&serverTimezone=UTC&useSSL=false
username=root
password=12345678

4、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="db.properties"></properties>
    <settings>
        <setting name="logImpl" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/LOG4J"/>
        <setting name="mapUnderscoreToCamelCase" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/true"/>
    </settings>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${driver}"/>
                <property name="url" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${jdbc.url}"/>
                <property name="username" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${username}"/>
                <property name="password" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper />
    </mappers>
</configuration>

5、工具類MybatisUtil

package com.zeng.mybatis.utils;

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.IOException;
import java.io.InputStream;

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


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

6、生成ID工具類IDUtil

package com.zeng.mybatis.utils;

import java.util.UUID;

public class IDUtil {
    public static String getId(){
        return UUID.randomUUID().toString().replaceAll("-","");
    }
}

7、介面

package com.zeng.mybatis.dao;

import com.zeng.mybatis.pojo.Blog;

import java.util.List;

public interface BlogMapper {
    int addBlog(Blog blog);

}

8、介面映射

<?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="com.zeng.mybatis.dao.BlogMapper">
    <insert id="addBlog" parameterType="com.zeng.mybatis.pojo.Blog">
        insert into blog (id,title,author,create_time,views) values (#{id},#{title},#{author},#{create_time},#{views})
    </insert>
</mapper>

9、生成資料

package com.zeng.mybatis.dao;

import com.zeng.mybatis.pojo.Blog;
import com.zeng.mybatis.utils.IDUtil;
import com.zeng.mybatis.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;

import java.util.Date;

public class Test {
    @org.junit.Test
    public void addtest(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
        Blog blog = new Blog();
        blog.setId(IDUtil.getId());
        blog.setTitle("Mybatis學習");
        blog.setAuthor("曾老師");
        blog.setCreate_time(new Date());
        blog.setViews(9000);
        mapper.addBlog(blog);

        Blog blog1 = new Blog();
        blog1.setId(IDUtil.getId());
        blog1.setTitle("java學習");
        blog1.setAuthor("曾老師");
        blog1.setCreate_time(new Date());
        blog1.setViews(8000);
        mapper.addBlog(blog1);

        Blog blog2 = new Blog();
        blog2.setId(IDUtil.getId());
        blog2.setTitle("javaWeb學習");
        blog2.setAuthor("曾老師");
        blog2.setCreate_time(new Date());
        blog2.setViews(100000);
        mapper.addBlog(blog2);

        Blog blog3 = new Blog();
        blog3.setId(IDUtil.getId());
        blog3.setTitle("單片機");
        blog3.setAuthor("曾老師");
        blog3.setCreate_time(new Date());
        blog3.setViews(4000);
        mapper.addBlog(blog3);

        Blog blog4 = new Blog();
        blog4.setId(IDUtil.getId());
        blog4.setTitle("人工智能學習");
        blog4.setAuthor("曾老師");
        blog4.setCreate_time(new Date());
        blog4.setViews(500);
        mapper.addBlog(blog4);

        Blog blog5 = new Blog();
        blog5.setId(IDUtil.getId());
        blog5.setTitle("大資料");
        blog5.setAuthor("曾老師");
        blog5.setCreate_time(new Date());
        blog5.setViews(1500);
        mapper.addBlog(blog5);
        sqlSession.close();
    }
}

2、IF標簽

1、介面

List<Blog> getBlogIF(Map map);

2、介面映射

<select id="getBlogIF" parameterType="map" resultType="com.zeng.mybatis.pojo.Blog">
        select * from blog where 1=1
        <if test="title != null">
            and title = #{title}
        </if>
    </select>

3、測驗類

@org.junit.Test
    public void getBlogIF(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
        HashMap map = new HashMap<>();
        map.put("title","單片機");
        List<Blog> blogs = mapper.getBlogIF(map);
        for (Blog blog : blogs) {
            System.out.println(blog);
        }
        sqlSession.close();
    }

3、choose（when、otherwise）

1、介面

List<Blog> getBlogChoose(Map map);

2、介面映射

<select id="getBlogChoose" parameterType="map" resultType="com.zeng.mybatis.pojo.Blog">
        select * from blog where 1=1
    <choose>
        <when test="title != null">
            and title = #{title}
        </when>
        <when test="author != null">
            and author = #{author}
        </when>
        <otherwise>
            and views >= #{views}
        </otherwise>
    </choose>
    </select>

3、測驗類

@org.junit.Test
    public void getBlogChoose(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
        HashMap Map = new HashMap<>();
        Map.put("author","曾老師");
        Map.put("views",8000);
        List<Blog> blogs = mapper.getBlogChoose(Map);
        for (Blog blog : blogs) {
            System.out.println(blog);
        }
        sqlSession.close();
    }

4、trim（where、set）

1、where

1、介面

List<Blog> getBlogWhere(Map map);

2、介面映射

<select id="getBlogWhere" resultType="com.zeng.mybatis.pojo.Blog" parameterType="map">
        select * from blog
        <where>
            <choose>
                <when test="title != null">
                    title = #{title}
                </when>
                <when test="author != null">
                    and author = #{author}
                </when>
            </choose>
        </where>
    </select>

3、測驗類

@org.junit.Test
    public void getBlogWhere(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
        HashMap map = new HashMap<>();
        map.put("title","人工智能學習");
        List<Blog> blogs = mapper.getBlogWhere(map);
        for (Blog blog : blogs) {
            System.out.println(blog);
        }
        sqlSession.close();
    }

2、set

1、介面

int modifyBlogSet(Map map);

2、介面映射

<update id="modifyBlogSet" parameterType="map">
        update blog
    <set>
        <if test="title != null">
            title = #{title},
        </if>
        <if test="author != null">
            author = #{author}
        </if>
    </set>
    <where>
        <if test="id != null">
            id = #{id}
        </if>
    </where>
    </update>

3、測驗類

@org.junit.Test
    public void modifyBlog(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
        HashMap map = new HashMap<>();
        map.put("id","49aa2e7503434b86a33c527cf520e6a9");
        map.put("title","單片機1");
        map.put("author","超級大帥哥");
        mapper.modifyBlogSet(map);
        sqlSession.close();
    }

3、trim

1、介面

List<Blog> getBlogTrim(Map map);

2、介面映射

<select id="getBlogTrim" parameterType="map" resultType="com.zeng.mybatis.pojo.Blog">
        select * from blog
        <trim prefix="where" suffixOverrides="and | or">
            <if test="title != null">
                title = #{title}
            </if>
            <if test="author != null">
                and author = #{author}
            </if>
        </trim>
    </select>

3、測驗

@org.junit.Test
    public void getBlogTrim(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
        HashMap map = new HashMap<>();
        map.put("title","單片機1");
        map.put("author","超級大帥哥");
        List<Blog> blogs = mapper.getBlogTrim(map);
        for (Blog blog : blogs) {
            System.out.println(blog);
        }
        sqlSession.close();
    }

十二、快取

1、快取簡介

快取（cache），原始意義是指訪問速度比一般隨機存取存盤器（RAM）快的一種高速存盤器，通常它不像系統主存那樣使用DRAM技術，而使用昂貴但較快速的SRAM技術，快取的設定是所有現代計算機系統發揮高性能的重要因素之一，

1、什么是快取？

存在記憶體中的臨時資料

將用戶經常查詢的資料放在快取（記憶體）中，用戶去查詢資料就不用從磁盤上（關系型資料庫資料檔案）查詢，從快取中查詢，從而提高查詢效率，解決了高并發系統的性能問題

2、為什么使用快取？

減少和資料庫的互動次數，減少系統開銷，提高系統效率

3、什么樣的資料能使用快取？

經常查詢并且不經常改變的資料

2、一級快取

一級快取也叫本地快取：Sqlsession

與資料庫同一次會話期間查詢到的資料會放在本地快取中

以后如果需要獲取相同的資料，直接從快取中拿，沒必要再去查詢資料庫

測驗步驟：

1、開啟日志！

匯入log4j依賴

<dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
      <version>1.2.12</version>
      <scope>test</scope>
    </dependency>

配置日志檔案


log4j.rootLogger=DEBUG,console,file


log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.Target = System.out
log4j.appender.console.Threshold=DEBUG
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%c]-%m%n


log4j.appender.file = org.apache.log4j.RollingFileAppender
log4j.appender.file.File=./log/logFile.log
log4j.appender.file.MaxFileSize=10mb
log4j.appender.file.Threshold=DEBUG
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=[%p][%d{yy-MM-dd}][%c]%m%n


log4j.logger.org.mybatis=DEBUG
log4j.logger.java.sql=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.ResultSet=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG

2、測驗在一個Session中查詢兩次相同記錄

@Test
public void getUserById(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    userMapper mapper = sqlSession.getMapper(userMapper.class);
    user user = mapper.getUserById(1);
    System.out.println(user);
    System.out.println("<==========================================================>");
    user user2 = mapper.getUserById(1);
    System.out.println(user2);
    sqlSession.close();
}

3、查看日志輸出

可以清楚的看見只運行了一次sql陳述句

快取失效的情況：

1、查詢不同的東西

這里分別查詢id為1和2的兩條記錄

@Test
    public void getUserById(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        userMapper mapper = sqlSession.getMapper(userMapper.class);
        user user = mapper.getUserById(1);
        System.out.println(user);
        System.out.println("<==========================================================>");
        user user2 = mapper.getUserById(2);
        System.out.println(user2);
        sqlSession.close();
    }

這里可以看見運行了兩次sql陳述句

2、增刪改操作，可能會改變原來的資料，所以必定會重繪快取！

這里我們更改了第一條記錄，并且對id為1的記錄查詢兩次

@Test
public void getUserById(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    userMapper mapper = sqlSession.getMapper(userMapper.class);
    user user = mapper.getUserById(1);
    System.out.println(user);
    System.out.println("<==========================================================>");
    user setUser = new user();
    setUser.setId(1);
    setUser.setName("張三（改）");
    setUser.setPwd("8520");
    mapper.updateUser(setUser);
    System.out.println("<==========================================================>");
    user user1 = mapper.getUserById(1);
    System.out.println(user1);
    sqlSession.close();
}

這里可以清楚的看見在第一次查詢過后，對記錄進行更改，然后再次查詢也會再執行一次sql陳述句，所以我們在執行增刪改的時候會更新資料，因此會重新連接查詢

3、查詢不同的Mapper.xml

4、手動清除快取

這里我們使用clearCache()方法來手動清除快取

@Test
public void getUserById(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    userMapper mapper = sqlSession.getMapper(userMapper.class);
    user user = mapper.getUserById(1);
    System.out.println(user);
    System.out.println("<==========================================================>");
    sqlSession.clearCache();//手動清楚快取
    user user1 = mapper.getUserById(1);
    System.out.println(user1);
    sqlSession.close();
}

這里我們可以看到在使用clearCache()方法，手動清除快取后就會對資料就行第二次連接查詢

3、二級快取

二級快取也叫全域快取，一級快取作用域太低，所以誕生了二級快取

基于namespace級別的快取，一個名稱空間，對應一個二級快取

作業機制

一個會話查詢一條資料，這個資料就會被放在當前會話的一級快取中；

如果當前會話關閉了，這個會話對應的一級快取就沒了；但是我們想要的是，會話關閉了，一級快取中的資料會被保存到二級快取中；

新的會話查詢資訊，就可以從二級快取中獲取內容；

不同的mapper查出的資料會放在自己對應的快取(map)中

步驟：

1、開啟全域快取

<settings>
        <!--開啟二級快取-->
        <setting name="cacheEnabled" value="https://www.cnblogs.com/littlenoob/archive/2023/03/27/true"/>
    </settings>

2、在要使用二級快取的Mapper中開啟

<cache/>

直接加一個<cache/>標簽就可以開啟二級快取了

<cache
            eviction="FIFO"
            flushInterval="60000"
            size="512"
            readOnly="true"/>

當然里面可以配置各種引數，引數可以自行配置

3、測驗

@Test
    public void getUserById(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        SqlSession sqlSession2 = MybatisUtils.getSqlSession();

        userMapper mapper = sqlSession.getMapper(userMapper.class);
        user user = mapper.getUserById(2);
        System.out.println(user);

        sqlSession.close();
        System.out.println("<======================================================================>");

        userMapper mapper2 = sqlSession2.getMapper(userMapper.class);
        user user1 = mapper2.getUserById(2);
        System.out.println(user1);


        sqlSession2.close();
    }

問題

org.apache.ibatis.cache.CacheException: Error serializing object.  Cause: java.io.NotSerializableException: com.mybatis.pojo.user

沒有引數就要序列化物體類

可以看出在會話一關閉后，資料就存入了二級快取，所有第二次查詢相同資料的時候沒有走資料庫，而是直接在二級快取中取出

只要開啟了二級快取，在同一個Mapper下就有效

所有的資料都會先放在一級快取中；

只有當會話提交，或者關閉的時候，才會提交到二級緩沖中！

4、快取原理

一級快取是SqlSession級別的快取，默認開啟，不同的sqlSession之間的快取資料區域（HashMap）是互相不影響的，

二級快取是mapper級別的快取，基于mapper檔案的namespace，多個SqlSession去操作同一個Mapper的sql陳述句，多個SqlSession可以共用二級快取，二級快取是跨SqlSession的，

一級快取先在資料庫中查詢資料，然后存入二級快取中，如果二級快取中沒有再從資料庫中查找

5、Ehcahe

什么是EhCache？

EhCache 是一個純Java的行程內快取框架，具有快速、精干等特點，是Hibernate中默認的CacheProvider，他和mybatis當中的二級快取唯一的區別就是，他是存盤于硬碟當中的，也就是他是可以用來做分布式快取的，

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