力扣MyBatis框架三万多字干货，来了解一下？

柔光的暖阳◎ 2023-10-05 08:55 10阅读 0赞

**本文原创首发CSDN，本文链接[https://blog.csdn.net/qq\_41464123/article/details/108172276][https_blog.csdn.net_qq_41464123_article_details_108172276]作者博客[https://blog.csdn.net/qq\_41464123][https_blog.csdn.net_qq_41464123]，转载请带上本链接，谢谢配合。**

--------------------

#### 文章目录 ####

*  一、前置工作
 *   *  1.1 MyBatis简介
     *  1.2 个人点评分析
     *  1.3 idea新建项目配置
     *  1.4 依赖、资源过滤
     *  1.5 创建数据库
     *  1.6 idea连接MySQL
 *  二、增删改查
 *   *  2.1 公用类
     *   *  数据库配置
         *  mybatis配置
         *  实体类
         *  SqlSession工具类
         *  随机ID工具类
     *  2.2 增加用户
     *  2.3 删除用户
     *  2.4 修改用户
     *  2.5 单个查询
     *  2.6 查询全部
 *  三、resultMap结果集映射
 *   *  需要实现的功能
     *  mapper接口
     *  XML具体实现
     *  测试
     *  日志输出
 *  四、分页
 *   *  4.1 XML版 limit
     *  4.2 Java注解版 limit
 *  五、多对一
 *   *  需求
     *  新实体类
     *  方法一
     *  方法二
 *  六、一对多
 *   *  需求
     *  实体类
     *  具体实现
 *  七、动态SQL
 *   *  简介
     *  新实体类
     *  IF
     *  Choose
     *  Foreach
 *  八、缓存
 *   *  8.1 一级缓存
     *  8.2 二级缓存
     *   *  关闭二级缓存
         *  开启二级缓存

--------------------

## 一、前置工作 ##

### 1.1 MyBatis简介 ###

*  **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为数据库中的记录。**
 *  **官方文档**：https://mybatis.org/mybatis-3/zh/index.html

### 1.2 个人点评分析 ###

*  MyBatis是一款优秀的持久层框架，通俗来说就是用来连接数据库。在没有JPA和MyBatis之前，原生JDBC式操作设置参数和获取结果集的工作非常臃肿枯燥。
 *  在MyBatis中可以通过简单的 XML或者Java注解实现JDBC大量代码才能完成的操作。
 *  MyBatis简单易学，不需要第三方依赖，只需要看官方文档就可以学习。
 *  MyBatis将SQL语句放到XML文件内，统一管理和优化。SQL作为数据库领域的霸主，基本可以实现我们所有的功能，而MyBatis可以很方便使用原生SQL。
 *  高内聚低耦合，SQL不放在Dao，使得系统设计清晰，而JPA没有做到这一点
 *  在最近参与的项目编写中，采用了Xboot框架，其中整合了MyBatis和JPA。从实际使用性来说，MyBatis的优势远远大于JPA。

### 1.3 idea新建项目配置 ###

*  jdk 11.0.6
 *  maven 3.6.2 阿里云
 *  idea 2019.3

### 1.4 依赖、资源过滤 ###

放在父 **pom.xml** 中即可

<properties>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>
    
    <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>3.5.2</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.0</version>
        </dependency>
    </dependencies>
    
    <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>

### 1.5 创建数据库 ###

**因为关联外键、异步等原因，必须分步执行，否则会出错**

CREATE DATABASE `test`;
    USE `test`
    
    CREATE TABLE `user`(
    	`id` VARCHAR(50) NOT NULL,
    	`name` VARCHAR(10) DEFAULT NULL,
    	`age` INT(10) DEFAULT NULL,
    	PRIMARY KEY (`id`)
    )ENGINE=INNODB DEFAULT CHARSET=UTF8;
    
    CREATE TABLE `teacher`(
    	`id` VARCHAR(50) NOT NULL,
    	`name` VARCHAR(10) DEFAULT NULL,
    	PRIMARY KEY (`id`)
    )ENGINE=INNODB DEFAULT CHARSET=utf8
    
    CREATE TABLE `student`(
    	`id` VARCHAR(50) NOT NULL,
    	`name` VARCHAR(10) DEFAULT NULL,
    	`teacher` VARCHAR(50) DEFAULT NULL ,
    	PRIMARY KEY (`id`),
    	FOREIGN KEY(`teacher`) REFERENCES teacher(id)
    )ENGINE=INNODB DEFAULT CHARSET=utf8
    
    create table `book`(
        `id` varchar(50) primary key,
        `name` varchar(20) default null,
        `money` double default 0.0,
        `buy_date` datetime
    )engine=InnoDB default charset=utf8
    
    INSERT INTO `user`(`id`,`name`,`age`) VALUES 
    (0,"ZWZ01","123456"),
    (1,"ZWZ02","123456"),
    (2,"ZWZ03","123456");
    
    INSERT INTO `teacher`(`id`,`name`) VALUES (1,'ZWZ');
    INSERT INTO `student`(`id`,`name`,`teacher`) VALUES (1,'ZWZ01',1);
    INSERT INTO `student`(`id`,`name`,`teacher`) VALUES (2,'ZWZ02',1);
    INSERT INTO `student`(`id`,`name`,`teacher`) VALUES (3,'ZWZ03',1);
    INSERT INTO `student`(`id`,`name`,`teacher`) VALUES (4,'ZWZ04',1);
    INSERT INTO `student`(`id`,`name`,`teacher`) VALUES (5,'ZWZ05',1);

### 1.6 idea连接MySQL ###

设置时区后，点击 Test Connection

<table> 
 <thead> 
  <tr> 
   <th>属性</th> 
   <th>值</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>serverTimezone</td> 
   <td>Asia/Shanghai</td> 
  </tr> 
 </tbody> 
</table>

## 二、增删改查 ##

### 2.1 公用类 ###

#### 数据库配置 ####

db.properties，用于连接数据库的重要信息，一般为驱动名、URL地址、用户名、密码

driver=com.mysql.jdbc.Driver
    url=jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&characterEncoding=utf-8&useSSL=false
    username=root
    password=123456

#### mybatis配置 ####

mybatis-config.xml，mybatis的配置文件

<?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"/>
    
        <settings>
            
            <setting name="logImpl" value="STDOUT_LOGGING"/>
            
            <setting name="mapUnderscoreToCamelCase" value="true"/>
        </settings>
        <typeAliases>
            <package name="zwz.pojo"/>
        </typeAliases>
    
        <environments default="zwz">
            <environment id="zwz">
                <transactionManager type="JDBC"></transactionManager>
                <dataSource type="POOLED">
                    <property name="driver" value="${driver}"/>
                    <property name="url" value="${url}"/>
                    <property name="username" value="${username}"/>
                    <property name="password" value="${password}"/>
                </dataSource>
            </environment>
        </environments>
    
        <mappers>
            <mapper class="zwz.mapper.UserMapper"/>
        </mappers>
    </configuration>

#### 实体类 ####

用户类User，包含id、name、age三个变量，其中Id由工具类 IdUtils 自动生成

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class User {
        
        private String id;
        private String name;
        private int age;
    }

#### SqlSession工具类 ####

*  用于获取session对象
 *  先从 SqlSessionFactoryBuilder 工厂构造器构建SqlSessionFactory
 *  再从SqlSessionFactory工厂生产SqlSession
 *  为了简化代码，封装成一个工具类

public class MybatisUtils {
        
        private 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();
        }
    }

#### 随机ID工具类 ####

该类用于随机生成ID，其中横线被替换为空，也可以替换成其他值

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

--------------------

### 2.2 增加用户 ###

**mapper层接口**

要实现增加用户，第一步先写一个接口，在用XML具体实现这个接口

public interface UserMapper {
        
        // 增加用户
        int addUser(User user);
    }

**XML具体实现**

*  namespace是命名空间，必须和接口的地址相对应
 *  id为接口方法名，必须对应
 *  parameterType为传入参数类型，因为已配置别名，一般使用首字母小写的类名
 *  \#\{\}中的字符串，要和实体类变量名对应，或用resultMap

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.UserMapper">
    
        <insert id="addUser" parameterType="user">
            insert into user(id, name, age) VALUES (#{id},#{name},#{age});
        </insert>
    </mapper>

Insert, Update, Delete 元素的属性如下

<table> 
 <thead> 
  <tr> 
   <th align="left">属性</th> 
   <th align="left">描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><code>id</code></td> 
   <td align="left">在命名空间中唯一的标识符，可以被用来引用这条语句。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>parameterType</code></td> 
   <td align="left">将会传入这条语句的参数的类全限定名或别名。这个属性是可选的，因为 MyBatis 可以通过类型处理器（TypeHandler）推断出具体传入语句的参数，默认值为未设置（unset）。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>parameterMap</code></td> 
   <td align="left">用于引用外部 parameterMap 的属性，目前已被废弃。请使用行内参数映射和 parameterType 属性。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>flushCache</code></td> 
   <td align="left">将其设置为 true 后，只要语句被调用，都会导致本地缓存和二级缓存被清空，默认值：（对 insert、update 和 delete 语句）true。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>timeout</code></td> 
   <td align="left">这个设置是在抛出异常之前，驱动程序等待数据库返回请求结果的秒数。默认值为未设置（unset）（依赖数据库驱动）。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>statementType</code></td> 
   <td align="left">可选 STATEMENT，PREPARED 或 CALLABLE。这会让 MyBatis 分别使用 Statement，PreparedStatement 或 CallableStatement，默认值：PREPARED。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>useGeneratedKeys</code></td> 
   <td align="left">（仅适用于 insert 和 update）这会令 MyBatis 使用 JDBC 的 getGeneratedKeys 方法来取出由数据库内部生成的主键（比如：像 MySQL 和 SQL Server 这样的关系型数据库管理系统的自动递增字段），默认值：false。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>keyProperty</code></td> 
   <td align="left">（仅适用于 insert 和 update）指定能够唯一识别对象的属性，MyBatis 会使用 getGeneratedKeys 的返回值或 insert 语句的 selectKey 子元素设置它的值，默认值：未设置（<code>unset</code>）。如果生成列不止一个，可以用逗号分隔多个属性名称。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>keyColumn</code></td> 
   <td align="left">（仅适用于 insert 和 update）设置生成键值在表中的列名，在某些数据库（像 PostgreSQL）中，当主键列不是表中的第一列的时候，是必须设置的。如果生成列不止一个，可以用逗号分隔多个属性名称。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>databaseId</code></td> 
   <td align="left">如果配置了数据库厂商标识（databaseIdProvider），MyBatis 会加载所有不带 databaseId 或匹配当前 databaseId 的语句；如果带和不带的语句都有，则不带的会被忽略。</td> 
  </tr> 
 </tbody> 
</table>

**测试**

*  测试方法中，增加了六个用户
 *  增删改操作之后必须提交事务，否则无效

/**
    * 增加用户
    */
    @Test
    public void testAddUser(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        mapper.addUser(new User(IdUtils.getId(),"ZWZ1",18));
        mapper.addUser(new User(IdUtils.getId(),"ZWZ2",18));
        mapper.addUser(new User(IdUtils.getId(),"ZWZ3",18));
        mapper.addUser(new User(IdUtils.getId(),"ZWZ4",18));
        mapper.addUser(new User(IdUtils.getId(),"ZWZ5",18));
        mapper.addUser(new User(IdUtils.getId(),"ZWZ6",18));
        session.commit();
        session.close();
    }

**日志输出**

可以看到，系统执行了六次insert SQL语句，成功插入

Created connection 1552326679.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@5c86a017]
    ==>  Preparing: insert into user(id, name, age) VALUES (?,?,?); 
    ==> Parameters: 9e3eed6af6da4214b520535cac13f13a(String), ZWZ1(String), 18(Integer)
    <==    Updates: 1
    ==>  Preparing: insert into user(id, name, age) VALUES (?,?,?); 
    ==> Parameters: 7322170480104981a58b28f51fdb2288(String), ZWZ2(String), 18(Integer)
    <==    Updates: 1
    ==>  Preparing: insert into user(id, name, age) VALUES (?,?,?); 
    ==> Parameters: f0d102cd857645c9aaca4ded53abb0e8(String), ZWZ3(String), 18(Integer)
    <==    Updates: 1
    ==>  Preparing: insert into user(id, name, age) VALUES (?,?,?); 
    ==> Parameters: e4993aecfd4f4322905eb6dc0cf039b9(String), ZWZ4(String), 18(Integer)
    <==    Updates: 1
    ==>  Preparing: insert into user(id, name, age) VALUES (?,?,?); 
    ==> Parameters: 4e387cafc50840468c4dfeec54940e1c(String), ZWZ5(String), 18(Integer)
    <==    Updates: 1
    ==>  Preparing: insert into user(id, name, age) VALUES (?,?,?); 
    ==> Parameters: 35ee11b46afa47a68be7713ecdeccd7e(String), ZWZ6(String), 18(Integer)
    <==    Updates: 1
    Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@5c86a017]
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@5c86a017]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@5c86a017]
    Returned connection 1552326679 to pool.
    
    Process finished with exit code 0

### 2.3 删除用户 ###

**mapper层接口**

删除、修改、查询原理同增加用户

// 删除用户
    int deleteUser(@Param("id") String id);

**XML具体实现**

<delete id="deleteUser" parameterType="string">
        delete from user where id = #{id};
    </delete>

**测试**

其中删除方法deleteUser中的参数ID，根据实际情况修改

/**
    * 删除用户
    */
    @Test
    public void testDeleteUser(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        // 根据数据库用户ID自行填写
        mapper.deleteUser("35ee11b46afa47a68be7713ecdeccd7e");
        session.commit();
        session.close();
    }

**日志输出**

可以看出，系统执行了一条delete语句，用户成功被删除

Created connection 1473981203.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@57db2b13]
    ==>  Preparing: delete from user where id = ?; 
    ==> Parameters: 35ee11b46afa47a68be7713ecdeccd7e(String)
    <==    Updates: 1
    Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@57db2b13]
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@57db2b13]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@57db2b13]
    Returned connection 1473981203 to pool.
    
    Process finished with exit code 0

### 2.4 修改用户 ###

**mapper层接口**

// 指定ID查询用户
    User findUserById(@Param("id") String id);

**XML具体实现**

<update id="updateUser" parameterType="user">
        update user set name = #{name}, age = #{age} where id = #{id};
    </update>

**测试**

/**
    * 修改用户
    */
    @Test
    public void testUpdateUser(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        // 根据数据库用户ID自行填写
        mapper.updateUser(new User("9e3eed6af6da4214b520535cac13f13a","ZWZ1改",18));
        session.commit();
        session.close();
    }

**日志输出**

Created connection 454884231.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1b1cfb87]
    ==>  Preparing: update user set name = ?, age = ? where id = ?; 
    ==> Parameters: ZWZ1改(String), 18(Integer), 9e3eed6af6da4214b520535cac13f13a(String)
    <==    Updates: 1
    Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1b1cfb87]
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1b1cfb87]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1b1cfb87]
    Returned connection 454884231 to pool.
    
    Process finished with exit code 0

### 2.5 单个查询 ###

**mapper层接口**

其中传递非引用类型参数，建议加上@Param注解，注解中的字符串和XML文件\#\{\}中对应

// 指定ID查询用户
    User findUserById(@Param("id") String id);

**XML具体实现**

<select id="findUserById" resultType="user" parameterType="string">
        select * from user where id = #{id};
    </select>

Select 元素的属性如下

<table> 
 <thead> 
  <tr> 
   <th align="left">属性</th> 
   <th align="left">描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><code>id</code></td> 
   <td align="left">在命名空间中唯一的标识符，可以被用来引用这条语句。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>parameterType</code></td> 
   <td align="left">将会传入这条语句的参数的类全限定名或别名。这个属性是可选的，因为 MyBatis 可以通过类型处理器（TypeHandler）推断出具体传入语句的参数，默认值为未设置（unset）。</td> 
  </tr> 
  <tr> 
   <td align="left">parameterMap</td> 
   <td align="left">用于引用外部 parameterMap 的属性，目前已被废弃。请使用行内参数映射和 parameterType 属性。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>resultType</code></td> 
   <td align="left">期望从这条语句中返回结果的类全限定名或别名。 注意，如果返回的是集合，那应该设置为集合包含的类型，而不是集合本身的类型。 resultType 和 resultMap 之间只能同时使用一个。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>resultMap</code></td> 
   <td align="left">对外部 resultMap 的命名引用。结果映射是 MyBatis 最强大的特性，如果你对其理解透彻，许多复杂的映射问题都能迎刃而解。 resultType 和 resultMap 之间只能同时使用一个。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>flushCache</code></td> 
   <td align="left">将其设置为 true 后，只要语句被调用，都会导致本地缓存和二级缓存被清空，默认值：false。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>useCache</code></td> 
   <td align="left">将其设置为 true 后，将会导致本条语句的结果被二级缓存缓存起来，默认值：对 select 元素为 true。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>timeout</code></td> 
   <td align="left">这个设置是在抛出异常之前，驱动程序等待数据库返回请求结果的秒数。默认值为未设置（unset）（依赖数据库驱动）。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>fetchSize</code></td> 
   <td align="left">这是一个给驱动的建议值，尝试让驱动程序每次批量返回的结果行数等于这个设置值。 默认值为未设置（unset）（依赖驱动）。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>statementType</code></td> 
   <td align="left">可选 STATEMENT，PREPARED 或 CALLABLE。这会让 MyBatis 分别使用 Statement，PreparedStatement 或 CallableStatement，默认值：PREPARED。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>resultSetType</code></td> 
   <td align="left">FORWARD_ONLY，SCROLL_SENSITIVE, SCROLL_INSENSITIVE 或 DEFAULT（等价于 unset） 中的一个，默认值为 unset （依赖数据库驱动）。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>databaseId</code></td> 
   <td align="left">如果配置了数据库厂商标识（databaseIdProvider），MyBatis 会加载所有不带 databaseId 或匹配当前 databaseId 的语句；如果带和不带的语句都有，则不带的会被忽略。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>resultOrdered</code></td> 
   <td align="left">这个设置仅针对嵌套结果 select 语句：如果为 true，将会假设包含了嵌套结果集或是分组，当返回一个主结果行时，就不会产生对前面结果集的引用。 这就使得在获取嵌套结果集的时候不至于内存不够用。默认值：<code>false</code>。</td> 
  </tr> 
  <tr> 
   <td align="left"><code>resultSets</code></td> 
   <td align="left">这个设置仅适用于多结果集的情况。它将列出语句执行后返回的结果集并赋予每个结果集一个名称，多个名称之间以逗号分隔。</td> 
  </tr> 
 </tbody> 
</table>

**测试**

/**
     * 查询单个用户
     */
    @Test
    public void testFindUserById(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        User user = mapper.findUserById("9e3eed6af6da4214b520535cac13f13a");
        System.out.println(user);
    }

**日志输出**

可以看出系统执行了一条查询语句

Created connection 2113748097.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7dfd3c81]
    ==>  Preparing: select * from user where id = ?; 
    ==> Parameters: 9e3eed6af6da4214b520535cac13f13a(String)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==      Total: 1
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    
    Process finished with exit code 0

### 2.6 查询全部 ###

**mapper层接口**

<select id="findAllUsers" resultType="user">
        select * from user;
    </select>

**XML具体实现**

// 查询全部用户
    List<User> findAllUsers();

**测试**

@Test
    public void testFindAllUsers(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        List<User> users = mapper.findAllUsers();
        for (User user : users) {
        
            System.out.println(user);
        }
    }

**日志输出**

Created connection 352367347.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1500b2f3]
    ==>  Preparing: select * from user; 
    ==> Parameters: 
    <==    Columns: id, name, age
    <==        Row: 4e387cafc50840468c4dfeec54940e1c, ZWZ5, 18
    <==        Row: 7322170480104981a58b28f51fdb2288, ZWZ2, 18
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==        Row: e4993aecfd4f4322905eb6dc0cf039b9, ZWZ4, 18
    <==        Row: f0d102cd857645c9aaca4ded53abb0e8, ZWZ3, 18
    <==      Total: 5
    User(id=4e387cafc50840468c4dfeec54940e1c, name=ZWZ5, age=18)
    User(id=7322170480104981a58b28f51fdb2288, name=ZWZ2, age=18)
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    User(id=e4993aecfd4f4322905eb6dc0cf039b9, name=ZWZ4, age=18)
    User(id=f0d102cd857645c9aaca4ded53abb0e8, name=ZWZ3, age=18)
    
    Process finished with exit code 0

## 三、resultMap结果集映射 ##

*  resultMap 是 MyBatis 中最重要最强大的元素。
 *  它可以进行一些 JDBC 不支持的操作。
 *  设计思想，对简单的语句做到零配置，对于复杂语句，只需要描述语句之间的关系

### 需要实现的功能 ###

使用resultMap ，查询所有的User

### mapper接口 ###

mapper接口和之前一样，约束需要实现的功能

public interface UserMapper {
        
        List<User> findAllUsers();
    }

### XML具体实现 ###

*  findAllUsers 方法配置了 resultMap
 *  userMap配置了数据库列和Java实体类的关系

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.UserMapper">
        <select id="findAllUsers" resultMap="userMap">
            select * from user;
        </select>
    
        <resultMap id="userMap" type="user">
            <result column="id" property="id"/>
            <result column="name" property="name"/>
            <result column="age" property="age"/>
        </resultMap>
    </mapper>

### 测试 ###

@Test
    public void testResultMap(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        List<User> users = mapper.findAllUsers();
        for (User user : users) {
        
            System.out.println(user);
        }
    }

### 日志输出 ###

可以看到和之前一样，也实现了查询操作

Created connection 126189538.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7857fe2]
    ==>  Preparing: select * from user; 
    ==> Parameters: 
    <==    Columns: id, name, age
    <==        Row: 4e387cafc50840468c4dfeec54940e1c, ZWZ5, 18
    <==        Row: 7322170480104981a58b28f51fdb2288, ZWZ2, 18
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==        Row: e4993aecfd4f4322905eb6dc0cf039b9, ZWZ4, 18
    <==        Row: f0d102cd857645c9aaca4ded53abb0e8, ZWZ3, 18
    <==      Total: 5
    User(id=4e387cafc50840468c4dfeec54940e1c, name=ZWZ5, age=18)
    User(id=7322170480104981a58b28f51fdb2288, name=ZWZ2, age=18)
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    User(id=e4993aecfd4f4322905eb6dc0cf039b9, name=ZWZ4, age=18)
    User(id=f0d102cd857645c9aaca4ded53abb0e8, name=ZWZ3, age=18)
    
    Process finished with exit code 0

## 四、分页 ##

### 4.1 XML版 limit ###

**mapper层接口**

public interface UserMapper {
        
        // 使用Limit语句分页
        List<User> findAllByPageLimit(@Param("startPage") int startPage,@Param("pageSize") int pageSize);
    }

**XML具体实现**

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.UserMapper">
    
        <select id="findAllByPageLimit" resultType="user">
            select * from user limit #{startPage},#{pageSize};
        </select>
    </mapper>

**测试**

下标从0开始，所以是输出第三个元素开始，输出三个用户

@Test
    public void testFindAllByPageLimit(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        List<User> users = mapper.findAllByPageLimit(2, 3);
        for (User user : users) {
        
            System.out.println(user);
        }
    }

**日志输出**

Created connection 1041109062.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3e0e1046]
    ==>  Preparing: select * from user limit ?,?; 
    ==> Parameters: 2(Integer), 3(Integer)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==        Row: e4993aecfd4f4322905eb6dc0cf039b9, ZWZ4, 18
    <==        Row: f0d102cd857645c9aaca4ded53abb0e8, ZWZ3, 18
    <==      Total: 3
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    User(id=e4993aecfd4f4322905eb6dc0cf039b9, name=ZWZ4, age=18)
    User(id=f0d102cd857645c9aaca4ded53abb0e8, name=ZWZ3, age=18)
    
    Process finished with exit code 0

### 4.2 Java注解版 limit ###

XML可以实现分页，Java直接也可以实现分页

**mapper接口**

public interface UserMapper {
        
    
        // 使用Java注解分页
        @Select("select * from user limit #{startPage},#{pageSize};")
        List<User> findAllByJava(@Param("startPage") int startPage,@Param("pageSize") int pageSize);
    }

**测试**

@Test
    public void testFindAllByJava(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        List<User> users = mapper.findAllByJava(2, 3);
        for (User user : users) {
        
            System.out.println(user);
        }
    }

**日志输出**

Created connection 1825719826.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@6cd24612]
    ==>  Preparing: select * from user limit ?,?; 
    ==> Parameters: 2(Integer), 3(Integer)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==        Row: e4993aecfd4f4322905eb6dc0cf039b9, ZWZ4, 18
    <==        Row: f0d102cd857645c9aaca4ded53abb0e8, ZWZ3, 18
    <==      Total: 3
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    User(id=e4993aecfd4f4322905eb6dc0cf039b9, name=ZWZ4, age=18)
    User(id=f0d102cd857645c9aaca4ded53abb0e8, name=ZWZ3, age=18)
    
    Process finished with exit code 0

--------------------

## 五、多对一 ##

### 需求 ###

*  对于学生而言，都有一位老师
 *  对于老师而言，有多名学生
 *  在学生角度看，学生和老师就是多对一的关系
 *  要求输出学生的所有信息，包括他的老师

### 新实体类 ###

**学生**

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

**老师**

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

--------------------

### 方法一 ###

**先查询学生信息，再根据老师ID，查询对应老师信息**

**mapper接口**

public interface StudentMapper {
        
        /**
         * 查询学生所有信息，包括其对应的老师 方法1
         * @return
         */
        List<Student> findAllStudentMessages1();
    }

**XML具体实现**

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.StudentMapper">
    
        
    
        
        <select id="findAllStudentMessages1" resultMap="teacherAndStudent1">
            select * from student;
        </select>
    
        
        <resultMap id="teacherAndStudent1" type="student">
            <result column="id" property="id"/>
            <result column="name" property="name"/>
            
            <association property="teacher" column="teacher" javaType="zwz.pojo.Teacher" select="findTeacherById"/>
        </resultMap>
    
        
        <select id="findTeacherById" resultType="teacher">
            select * from teacher
            where id = #{teacher};
        </select>
    </mapper>

**测试**

@Test
    public void testFindAllStudentMessages1(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        List<Student> students = mapper.findAllStudentMessages1();
        for (Student student : students) {
        
            System.out.println(student);
        }
    }

**日志输出**

可以看出，系统先执行 **select \* from student**，然后根据查询到的老师ID，再执行 **select \* from teacher where id = ?**

Created connection 1651162064.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@626abbd0]
    ==>  Preparing: select * from student; 
    ==> Parameters: 
    <==    Columns: id, name, teacher
    <==        Row: 5992b89d9cfd4f0ba79a2991e108d017, ZWZ5, 5992b89d9cfd4f0ba79a2991e108d0b7
    ====>  Preparing: select * from teacher where id = ?; 
    ====> Parameters: 5992b89d9cfd4f0ba79a2991e108d0b7(String)
    <====    Columns: id, name
    <====        Row: 5992b89d9cfd4f0ba79a2991e108d0b7, 郑老师
    <====      Total: 1
    <==        Row: 5992b89d9cfd4f0ba79a2991e108d1b7, ZWZ1, 5992b89d9cfd4f0ba79a2991e108d0b7
    <==      Total: 2
    Student(id=5992b89d9cfd4f0ba79a2991e108d017, name=ZWZ5, teacher=Teacher(id=5992b89d9cfd4f0ba79a2991e108d0b7, name=郑老师))
    Student(id=5992b89d9cfd4f0ba79a2991e108d1b7, name=ZWZ1, teacher=Teacher(id=5992b89d9cfd4f0ba79a2991e108d0b7, name=郑老师))
    
    Process finished with exit code 0

### 方法二 ###

**使用SQL语句做关联查询，再匹配到对应字段**

**mapper接口**

public interface StudentMapper {
        
        /**
         * 查询学生所有信息，包括其对应的老师 方法2
         * @return
         */
        List<Student> findAllStudentMessages2();
    }

**XML具体实现**

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.StudentMapper">
    
        
        
        
        <select id="findAllStudentMessages2" resultMap="teacherAndStudent2">
            select t.id tid,t.name tname,s.id,s.name,s.teacher
            from teacher t,student s
            where s.teacher = t.id;
        </select>
    
        
        <resultMap id="teacherAndStudent2" type="student">
            <result column="id" property="id"/>
            <result column="name" property="name"/>
            <association property="teacher" javaType="teacher">
                <result column="tid" property="id"/>
                <result column="tname" property="name"/>
            </association>
        </resultMap>
    </mapper>

**测试**

/**
     * 方法二
     */
    @Test
    public void testFindAllStudentMessages2(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        List<Student> students = mapper.findAllStudentMessages2();
        for (Student student : students) {
        
            System.out.println(student);
        }
    }

**日志输出**

可以看出，系统直接执行 **select t.id tid,t.name tname,s.id,s.name,s.teacher from teacher t,student s where s.teacher = t.id;**

Created connection 1304589447.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@4dc27487]
    ==>  Preparing: select t.id tid,t.name tname,s.id,s.name,s.teacher from teacher t,student s where s.teacher = t.id; 
    ==> Parameters: 
    <==    Columns: tid, tname, id, name, teacher
    <==        Row: 5992b89d9cfd4f0ba79a2991e108d0b7, 郑老师, 5992b89d9cfd4f0ba79a2991e108d017, ZWZ5, 5992b89d9cfd4f0ba79a2991e108d0b7
    <==        Row: 5992b89d9cfd4f0ba79a2991e108d0b7, 郑老师, 5992b89d9cfd4f0ba79a2991e108d1b7, ZWZ1, 5992b89d9cfd4f0ba79a2991e108d0b7
    <==      Total: 2
    Student(id=5992b89d9cfd4f0ba79a2991e108d017, name=ZWZ5, teacher=Teacher(id=5992b89d9cfd4f0ba79a2991e108d0b7, name=郑老师))
    Student(id=5992b89d9cfd4f0ba79a2991e108d1b7, name=ZWZ1, teacher=Teacher(id=5992b89d9cfd4f0ba79a2991e108d0b7, name=郑老师))
    
    Process finished with exit code 0

## 六、一对多 ##

### 需求 ###

*  对于学生而言，都有一位老师
 *  对于老师而言，有多名学生
 *  在老师角度看，老师和学生就是一对多的关系
 *  要求输出老师的所有信息，包括他的所有学生

### 实体类 ###

**学生**

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Student {
        
        private String id;
        private String name;
        // 对应老师的ID即可
        private String teacher;
    }

**老师**

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Teacher {
        
        private String id;
        private String name;
        // 老师拥有的学生列表
        private List<Student> students;
    }

### 具体实现 ###

**mapper接口**

public interface TeacherMapper {
        
        List<Teacher> findAllTeacherMessage();
    }

**XML具体实现**

collection 是一个一个复杂类型的集合

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.TeacherMapper">
    
        <select id="findAllTeacherMessage" resultMap="teacherAndStudent">
            select t.id,t.name,s.id sid,s.name sname,s.teacher sTeacher
            from teacher t,student s
            where t.id = s.teacher;
        </select>
    
        <resultMap id="teacherAndStudent" type="teacher">
            <result property="id" column="id"/>
            <result property="name" column="name"/>
            <collection property="students" ofType="student">
                <result property="id" column="sid"/>
                <result property="name" column="sname"/>
                <result property="teacher" column="sTeacher"/>
            </collection>
        </resultMap>
    </mapper>

**测试**

@Test
    public void testFindAllTeacherMessage(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        TeacherMapper mapper = session.getMapper(TeacherMapper.class);
        List<Teacher> teachers = mapper.findAllTeacherMessage();
        for (Teacher teacher : teachers) {
        
            System.out.println(teacher);
        }
    }

**日志输出**

可以看出系统直接执行 **select t.id,t.name,s.id sid,s.name sname,s.teacher sTeacher from teacher t,student s where t.id = s.teacher;** 查询到了结果

Created connection 2142565033.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7fb4f2a9]
    ==>  Preparing: select t.id,t.name,s.id sid,s.name sname,s.teacher sTeacher from teacher t,student s where t.id = s.teacher; 
    ==> Parameters: 
    <==    Columns: id, name, sid, sname, sTeacher
    <==        Row: 5992b89d9cfd4f0ba79a2991e108d0b7, 郑老师, 5992b89d9cfd4f0ba79a2991e108d017, ZWZ5, 5992b89d9cfd4f0ba79a2991e108d0b7
    <==        Row: 5992b89d9cfd4f0ba79a2991e108d0b7, 郑老师, 5992b89d9cfd4f0ba79a2991e108d1b7, ZWZ1, 5992b89d9cfd4f0ba79a2991e108d0b7
    <==      Total: 2
    Teacher(id=5992b89d9cfd4f0ba79a2991e108d0b7, name=郑老师, students=[Student(id=5992b89d9cfd4f0ba79a2991e108d017, name=ZWZ5, teacher=5992b89d9cfd4f0ba79a2991e108d0b7), Student(id=5992b89d9cfd4f0ba79a2991e108d1b7, name=ZWZ1, teacher=5992b89d9cfd4f0ba79a2991e108d0b7)])
    
    Process finished with exit code 0

## 七、动态SQL ##

### 简介 ###

*  动态 SQL 是 MyBatis 的强大特性之一
 *  动态 SQL 用来拼接 SQL 语句

### 新实体类 ###

**Book**

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Book {
        
        private String id;
        private String name;
        private double money;
        private Date buyDate;
    }

--------------------

### IF ###

**mapper接口**

public interface BookMapper {
        
        /**
         * if 标签测试
         *
         * 模拟查询书名和比某价格便宜的书
         * 两个条件都看
         */
        List<Book> findBookByNameAndMoney(@Param("name") String name, @Param("money") Double money);
    }

**XML具体实现**

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.BookMapper">
    
        <select id="findBookByNameAndMoney" resultType="book">
            select * from book
            where 1 = 1
            <if test="name != ''">
                and name like #{name}
            </if>
            <if test="money != 0.0">
                and money < #{money}
            </if>
        </select>
    </mapper>

**测试**

/**
     * 模拟查询书名和比某价格便宜的书
     * 如果name为空   忽略
     * 如果价格为0.0  忽略
     */
    @Test
    public void testDynamicChoose(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        BookMapper mapper = session.getMapper(BookMapper.class);
        List<Book> books = mapper.findBookByNameOrMoney("%设计%", 80.0);
        for (Book book : books) {
        
            System.out.println(book);
        }
    }

--------------------

### Choose ###

public interface BookMapper {
        
        /**
         * choose 标签测试
         *
         * 模拟查询书名和比某价格便宜的书
         * 如果姓名不为空，只看姓名
         * 如果姓名为空，就看价格
         *
         * @param name
         * @param money
         * @return
         */
        List<Book> findBookByNameOrMoney(@Param("name") String name, @Param("money") Double money);
    }

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.BookMapper">
    
        <select id="findBookByNameOrMoney" resultType="book">
            select * from book
            where 1 = 1
            <choose>
                <when test="name != ''">
                    and name like #{name}
                </when>
                <when test="money != 0.0">
                    and money < #{money}
                </when>
                <otherwise>
                    and 1 = 1
                </otherwise>
            </choose>
        </select>
    </mapper>

### Foreach ###

public interface BookMapper {
        
        /**
         * foreach 标签测试
         *
         * 查询指定书名的书列表
         *
         * @param books
         * @return
         */
        List<Book> findInName(@Param("books") List<String> books);
    }

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="zwz.mapper.BookMapper">
        <select id="findInName" resultType="book" parameterType="list">
            select * from book
            where 1 = 1 and name in
            <foreach collection="books" index="index" item="item" open="(" close=")" separator=",">
                #{item}
            </foreach>
        </select>
    </mapper>

/**
     * 查询指定书名的书列表
     */
    @Test
    public void testDynamicForeach(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        BookMapper mapper = session.getMapper(BookMapper.class);
        List<String> bookNames = new ArrayList<>();
        bookNames.add("JavaScript程序设计");
        bookNames.add("JAVA程序设计");
        bookNames.add("C++程序设计");
        List<Book> books = mapper.findInName(bookNames);
        for (Book book : books) {
        
            System.out.println(book);
        }
    }

## 八、缓存 ##

### 8.1 一级缓存 ###

MyBatis 内置了一个强大的事务性查询缓存机制，它可以非常方便地配置和定制

默认情况下，**只启用了本地的会话缓存（一级缓存）**，它仅仅对一个会话中的数据进行缓存

*  映射语句文件中的所有 select 语句的结果将会被缓存。
 *  映射语句文件中的所有 insert、update 和 delete 语句会刷新缓存。
 *  缓存会使用最近最少使用算法（LRU, Least Recently Used）算法来清除不需要的缓存。
 *  缓存不会定时进行刷新（也就是说，没有刷新间隔）。
 *  缓存会保存列表或对象（无论查询方法返回哪种）的 1024 个引用。
 *  缓存会被视为读/写缓存，这意味着获取到的对象并不是共享的，可以安全地被调用者修改，而不干扰其他调用者或线程所做的潜在修改。

**比如两次查询同样的用户，可以发现系统只执行了一次SQL指令**

@Test
    public void testCache1(){
        
        SqlSession session = MybatisUtils.getSqlSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        // 第一次查询
        User user = mapper.findUserById("7a0f791f1e7b416ea01b860470fa9aeb");
        System.out.println(user);
        
        System.out.println("+++++++++++++++++++++++++++++++++++++++");
        // 第二次查询
        User user2 = mapper.findUserById("7a0f791f1e7b416ea01b860470fa9aeb");
        System.out.println(user2);
    
        System.out.println(user == user2);
        session.close();
    }

**输出日志如下**

Created connection 508512860.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1e4f4a5c]
    ==>  Preparing: select * from user where id = ?; 
    ==> Parameters: 9e3eed6af6da4214b520535cac13f13a(String)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==      Total: 1
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    +++++++++++++++++++++++++++++++++++++++
    User(id=9e3eed6af6da4214b520535cac13f13a, name=ZWZ1改, age=18)
    true
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@1e4f4a5c]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@1e4f4a5c]
    Returned connection 508512860 to pool.
    
    Process finished with exit code 0

--------------------

### 8.2 二级缓存 ###

#### 关闭二级缓存 ####

如果没有开启二级缓存，在两个不同的SqlSession中发起查询，**系统会执行两次SQL指令**

@Test
    public void testCache2(){
        
        SqlSession session1 = MybatisUtils.getSqlSession();
        UserMapper2 mapper1 = session1.getMapper(UserMapper2.class);
        User2 user1 = mapper1.findUserById("9e3eed6af6da4214b520535cac13f13a");
        session1.close();
        SqlSession session2 = MybatisUtils.getSqlSession();
        UserMapper2 mapper2 = session2.getMapper(UserMapper2.class);
        User2 user2 = mapper2.findUserById("9e3eed6af6da4214b520535cac13f13a");
        session2.close();
    }

**输出日志如下**

Created connection 2075952726.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7bbc8656]
    ==>  Preparing: select * from user where id = ?; 
    ==> Parameters: 9e3eed6af6da4214b520535cac13f13a(String)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==      Total: 1
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7bbc8656]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7bbc8656]
    Returned connection 2075952726 to pool.
    Opening JDBC Connection
    Checked out connection 2075952726 from pool.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7bbc8656]
    ==>  Preparing: select * from user where id = ?; 
    ==> Parameters: 9e3eed6af6da4214b520535cac13f13a(String)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==      Total: 1
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@7bbc8656]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@7bbc8656]
    Returned connection 2075952726 to pool.
    
    Process finished with exit code 0

--------------------

#### 开启二级缓存 ####

**在mybatis-config.xml中配置全局缓存**

**在mapper层xml中，配置cache**

**测试**

从日志输出中可以发现，**系统只执行了一次SQL指令**

Created connection 307829448.
    Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@12591ac8]
    ==>  Preparing: select * from user where id = ?; 
    ==> Parameters: 9e3eed6af6da4214b520535cac13f13a(String)
    <==    Columns: id, name, age
    <==        Row: 9e3eed6af6da4214b520535cac13f13a, ZWZ1改, 18
    <==      Total: 1
    Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@12591ac8]
    Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@12591ac8]
    Returned connection 307829448 to pool.
    Cache Hit Ratio [cache2.mapper.UserMapper2]: 0.5
    
    Process finished with exit code 0

[https_blog.csdn.net_qq_41464123_article_details_108172276]: https://blog.csdn.net/qq_41464123/article/details/108172276
[https_blog.csdn.net_qq_41464123]: https://blog.csdn.net/qq_41464123