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Mybatis 基础介绍与逆向工程的构建

Mybatis系列：
Mybatis 基础介绍与逆向工程的构建 ：https://www.jianshu.com/p/1c18db4d7a38
Mybatis 源码分析（一）之 Mybatis 的Executor的初始化：https://www.jianshu.com/p/c7425c841337
Mybatis 源码分析（二）之 Mybatis 操作数据库的流程 ：https://www.jianshu.com/p/11d354ec3612
Mybatis 源码分析（三）之 Mybatis 的一级缓存和二级缓存 ：https://www.jianshu.com/p/5515640d14fe

优秀博客：
http://www.mybatis.org/mybatis-3/zh/index.html
http://www.mybatis.org/generator/index.html

为什么要用Mybatis？

先看下我们传统JDBC连接数据库的弊端：

• jdbc 底层没有用连接池、操作数据库需要频繁的创建和关联链接。消耗很大的资源

• 写原生的 jdbc 代码在java中，一旦我们要修改sql的话，java需要整体编译，不利于系统维护

• 使用 PreparedStatement 预编译的话对变量进行设置 123 数字，这样的序号不利于维护

• 返回 result 结果集也需要硬编码。

什么是 MyBatis ？

MyBatis 是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息，将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。

这么看一下，是不是感觉mybatis全给解决了。

开始构建Mybatis工程

pom.xml

    <?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

<groupId>com.demo</groupId>
<artifactId>mybatis-test</artifactId>
<version>1.0-SNAPSHOT</version>

<dependencies>
    <!-- mybatis核心包 -->
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.3.0</version>
    </dependency>
    <!-- mysql驱动包 -->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.29</version>
    </dependency>
    <!-- junit测试包 -->
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.11</version>
        <scope>test</scope>
    </dependency>
    <!-- 日志文件管理包 -->
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.17</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-api</artifactId>
        <version>1.7.12</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
        <version>1.7.12</version>
    </dependency>

</dependencies>

<!-- 逆向工程插件 -->
<build>
    <pluginManagement>
        <plugins>
            <plugin>
                <groupId>org.mybatis.generator</groupId>
                <artifactId>mybatis-generator-maven-plugin</artifactId>
                <version>1.3.7</version>
                <dependencies>
                    <dependency>
                        <groupId>mysql</groupId>
                        <artifactId>mysql-connector-java</artifactId>
                        <version>5.1.29</version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </pluginManagement>
</build>


</project>

项目目录结构

项目目录结构.png

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="mysql.properties"/>
    <settings>
        <!--全局性设置懒加载。如果设为‘false’，则所有相关联的都会被初始化加载,默认值为false-->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!--当设置为‘true’的时候，懒加载的对象可能被任何懒属性全部加载。否则，每个属性都按需加载。默认值为true-->
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>
    <typeAliases>
        <!-- 其实就是将bean的替换成一个短的名字-->
        <typeAlias type="com.demo.mybatis.pojo.User" alias="User"/>
    </typeAliases>
    <!--对事务的管理和连接池的配置-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED"><!--POOLED：使用Mybatis自带的数据库连接池来管理数据库连接-->
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <!--mapping文件路径配置-->
    <mappers>
        <!--<mapper resource="mapper/UserMapper.xml"/>-->
        <mapper class="com.demo.mybatis.mapper.UserMapper"/>
        <!--<package name="com.demo.mybatis.mapper"/>-->
        <!--<mapper url=""/>-->
    </mappers>

</configuration>

其中的SQL映射文件mappers如下：

<!--mapping文件路径配置-->
<mappers>
    <!--<mapper resource="mapper/UserMapper.xml"/>-->
    <mapper class="com.demo.mybatis.mapper.UserMapper"/>
    <!--<package name="com.demo.mybatis.mapper"/>-->
    <!--<mapper url=""/>-->
</mappers>

mapper文件可以配置4种，源码种最先的是package，如果package没有，则是其余三种，4种里面只能选择一种配置。
源码地址：

org.apache.ibatis.builder.xml.XMLConfigBuilder#mapperElement

 //mapper元素解析，获得各种crud操作或者配置文件  
 private void mapperElement(XNode parent) throws     Exception {  
  if (parent != null) {  
  for (XNode child : parent.getChildren()) {  
    if ("package".equals(child.getName())) {  
      String mapperPackage = child.getStringAttribute("name");  
      configuration.addMappers(mapperPackage);  
    } else {  
      String resource = child.getStringAttribute("resource");  
      String url = child.getStringAttribute("url");  
      String mapperClass = child.getStringAttribute("class");  
      if (resource != null && url == null && mapperClass == null) {  
        ErrorContext.instance().resource(resource);  
        InputStream inputStream = Resources.getResourceAsStream(resource);  
        XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());  
        mapperParser.parse();  
      } else if (resource == null && url != null && mapperClass == null) {  
        ErrorContext.instance().resource(url);  
        InputStream inputStream = Resources.getUrlAsStream(url);  
        XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());  
        mapperParser.parse();  
      } else if (resource == null && url == null && mapperClass != null) {  
        Class<?> mapperInterface = Resources.classForName(mapperClass);  
        configuration.addMapper(mapperInterface);  
      } else {  
        throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");  
      }  
    }  
  }  
}  

}

mysql.properties （安装mysql数据库）

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://192.168.5.104:3306/mybatis
jdbc.username=root
jdbc.password=root

建表语句

    CREATE TABLE `user` (
  `id` int(16) NOT NULL AUTO_INCREMENT COMMENT '用户ID',
  `name` varchar(32) DEFAULT NULL COMMENT '用户名',
  `age` int(4) DEFAULT NULL COMMENT '年龄',
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_id` (`id`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

log4j.properties

### 设置###
log4j.rootLogger = debug,stdout,D,E

### 输出信息到控制抬 ###
log4j.appender.stdout = org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target = System.out
log4j.appender.stdout.layout = org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%n

### 输出DEBUG 级别以上的日志到=E://logs/error.log ###
log4j.appender.D = org.apache.log4j.DailyRollingFileAppender
log4j.appender.D.File = E://logs/log.log
log4j.appender.D.Append = true
log4j.appender.D.Threshold = DEBUG 
log4j.appender.D.layout = org.apache.log4j.PatternLayout
log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

### 输出ERROR 级别以上的日志到=E://logs/error.log ###
log4j.appender.E = org.apache.log4j.DailyRollingFileAppender
log4j.appender.E.File =E://logs/error.log 
log4j.appender.E.Append = true
log4j.appender.E.Threshold = ERROR 
log4j.appender.E.layout = org.apache.log4j.PatternLayout
log4j.appender.E.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

什么是逆向工程(http://www.mybatis.org/generator/index.html):

MyBatis 的一个主要的特点就是需要程序员自己编写 sql，那么如果表太多的话，难免会很麻烦，所以 mybatis 官方提供了一个逆向工程，可以针对单表自动生成 mybatis 执行所需要的代码（包括 mapper.xml、mapper.java、po..）。一般在开发中，常用的逆向工程方式是通过数据库的表生成

generatorConfig.xml

    <?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE generatorConfiguration
        PUBLIC "-//mybatis.org//DTD MyBatis Generator Configuration 1.0//EN"
        "http://mybatis.org/dtd/mybatis-generator-config_1_0.dtd">

<generatorConfiguration>
    <context id="sqlserverTables" targetRuntime="MyBatis3">
        <!-- 生成的pojo，将implements Serializable -->
        <plugin type="org.mybatis.generator.plugins.SerializablePlugin"></plugin>
        <commentGenerator>
            <!-- 是否去除自动生成的注释 true：是 ： false:否 -->
            <property name="suppressAllComments" value="true" />
        </commentGenerator>

    <!-- 数据库链接URL、用户名、密码 -->
    <jdbcConnection driverClass="com.mysql.jdbc.Driver"
                    connectionURL="jdbc:mysql://192.168.5.104:3306/mybatis"
                    userId="root" password="root">
    </jdbcConnection>

    <!-- 默认false，把JDBC DECIMAL 和 NUMERIC 类型解析为 Integer true，把JDBC DECIMAL
        和 NUMERIC 类型解析为java.math.BigDecimal -->
    <javaTypeResolver>
        <property name="forceBigDecimals" value="false" />
    </javaTypeResolver>

    <!-- 生成model模型，对应的包路径，以及文件存放路径(targetProject)，targetProject可以指定具体的路径,如./src/main/java，
        也可以使用“MAVEN”来自动生成，这样生成的代码会在target/generatord-source目录下 -->
    <!--<javaModelGenerator targetPackage="com.joey.mybaties.test.pojo" targetProject="MAVEN"> -->
    <javaModelGenerator targetPackage="com.demo.mybatis.pojo"
                        targetProject="./src/main/java">
        <property name="enableSubPackages" value="true" />
        <!-- 从数据库返回的值被清理前后的空格 -->
        <property name="trimStrings" value="true" />
    </javaModelGenerator>

    <!--对应的mapper.xml文件 -->
    <sqlMapGenerator targetPackage="mappers"
                     targetProject="./src/main/resources">
        <property name="enableSubPackages" value="true" />
    </sqlMapGenerator>

    <!-- 对应的Mapper接口类文件 XMLMAPPER/ANNOTATEDMAPPER-->
    <javaClientGenerator type="ANNOTATEDMAPPER"
                         targetPackage="com.demo.mybatis.mapper" targetProject="./src/main/java">
        <property name="enableSubPackages" value="true" />
    </javaClientGenerator>


    <!-- 列出要生成代码的所有表，这里配置的是不生成Example文件 -->
    <table tableName="food" domainObjectName="Food"
           enableCountByExample="false" enableUpdateByExample="false"
           enableDeleteByExample="false" enableSelectByExample="false"
           selectByExampleQueryId="false">
        <property name="useActualColumnNames" value="false" />
    </table>

    <table tableName="user" domainObjectName="User"
           enableCountByExample="false" enableUpdateByExample="false"
           enableDeleteByExample="false" enableSelectByExample="false"
           selectByExampleQueryId="false">
        <property name="useActualColumnNames" value="false" />
    </table>
</context>
</generatorConfiguration>

这些文件创建好之后，进入你的pom文件地址，使用maven命令，逆向生成我们需要的pojo、mapper等文件。

mvn mybatis-generator:generate

生成情况如下

D:\dev\code\demo\mybatis-test>mvn mybatis-generator:generate
[INFO] Scanning for projects...
[INFO]
[INFO] -----------------------< com.demo:mybatis-test >------------------------
[INFO] Building mybatis-test 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- mybatis-generator-maven-plugin:1.3.7:generate (default-cli) @ mybatis-test ---
[WARNING] Existing file not overwritten, the generated file is saved as D:\dev\code\demo\mybatis-test\.\src\main\java\com\demo\mybatis\pojo\Food.java.2
[WARNING] Existing file not overwritten, the generated file is saved as D:\dev\code\demo\mybatis-test\.\src\main\java\com\demo\mybatis\mapper\FoodMapper.java.1
[WARNING] Existing file not overwritten, the generated file is saved as D:\dev\code\demo\mybatis-test\.\src\main\java\com\demo\mybatis\mapper\FoodSqlProvider.java.1
[WARNING] Existing file not overwritten, the generated file is saved as D:\dev\code\demo\mybatis-test\.\src\main\java\com\demo\mybatis\pojo\User.java.2
[WARNING] Existing file not overwritten, the generated file is saved as D:\dev\code\demo\mybatis-test\.\src\main\java\com\demo\mybatis\mapper\UserMapper.java.1
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.802 s
[INFO] Finished at: 2019-01-26T19:48:50+08:00
[INFO] ------------------------------------------------------------------------

D:\dev\code\demo\mybatis-test>

创建测试类：MybatisTest.java

package com.demo.mybatis;

import com.demo.mybatis.mapper.UserMapper;
import com.demo.mybatis.pojo.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.io.Reader;

/**
 * com.demo.mybatis
 *
 * @author Zyy
 * @date 2019/1/26 18:13
 */
public class MybatisTest {
    private static final Logger logger = LoggerFactory.getLogger(MybatisTest.class);
    @Test
    public void test01() throws IOException {
        String resource = "mybatis-config.xml";
        Reader reader = Resources.getResourceAsReader(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        SqlSession session = sqlSessionFactory.openSession();
        User user = session.selectOne("selectByPrimaryKey", 1);
        session.commit();
        logger.info(user.toString());
    }


    @Test
    public void test02() throws IOException {
        String resource = "mybatis-config.xml";
        Reader reader = Resources.getResourceAsReader(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.selectByPrimaryKey(2);
        logger.info(user.toString());
    }

    //user.dir可以用来打印你当然目录地址，如果有需要可以替换配置文件中的地址
    @Test
    public void test03() throws IOException {
        System.out.println(System.getProperty("user.dir"));
    }
}

执行结果信息：

[DEBUG] 2019-01-26 23:34:08,755 method:org.apache.ibatis.logging.LogFactory.setImplementation(LogFactory.java:135)
Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
[DEBUG] 2019-01-26 23:34:08,891 method:org.apache.ibatis.datasource.pooled.PooledDataSource.forceCloseAll(PooledDataSource.java:316)
PooledDataSource forcefully closed/removed all connections.
[DEBUG] 2019-01-26 23:34:08,892 method:org.apache.ibatis.datasource.pooled.PooledDataSource.forceCloseAll(PooledDataSource.java:316)
PooledDataSource forcefully closed/removed all connections.
[DEBUG] 2019-01-26 23:34:08,892 method:org.apache.ibatis.datasource.pooled.PooledDataSource.forceCloseAll(PooledDataSource.java:316)
PooledDataSource forcefully closed/removed all connections.
[DEBUG] 2019-01-26 23:34:08,893 method:org.apache.ibatis.datasource.pooled.PooledDataSource.forceCloseAll(PooledDataSource.java:316)
PooledDataSource forcefully closed/removed all connections.
[DEBUG] 2019-01-26 23:34:08,958 method:org.apache.ibatis.transaction.jdbc.JdbcTransaction.openConnection(JdbcTransaction.java:138)
Opening JDBC Connection
[DEBUG] 2019-01-26 23:34:09,231 method:org.apache.ibatis.datasource.pooled.PooledDataSource.popConnection(PooledDataSource.java:387)
Created connection 1511785794.
[DEBUG] 2019-01-26 23:34:09,232 method:org.apache.ibatis.transaction.jdbc.JdbcTransaction.setDesiredAutoCommit(JdbcTransaction.java:102)
Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@5a1c0542]
[DEBUG] 2019-01-26 23:34:09,252 method:org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:142)
==>  Preparing: select id, name, age from user where id = ? 
[DEBUG] 2019-01-26 23:34:09,301 method:org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:142)
==> Parameters: 1(Integer)
[DEBUG] 2019-01-26 23:34:09,331 method:org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:142)
<==      Total: 1
[INFO ] 2019-01-26 23:34:09,332 method:com.demo.mybatis.MybatisTest.test01(MybatisTest.java:32)
User{id=1, name='ayang', age=18}

其中的注意事项：

逆向工程配置文件
generatorConfig.xml中，javaClientGenerator可以选择XMLMAPPER（xml文件形式）、ANNOTATEDMAPPER（注解形式）两种类型，还有一种example，感觉这种不好使，是在另个参数里面配置的。

<!-- 对应的Mapper接口类文件 XMLMAPPER/ANNOTATEDMAPPER-->
    <javaClientGenerator type="ANNOTATEDMAPPER"
                         targetPackage="com.demo.mybatis.mapper" targetProject="./src/main/java">
        <property name="enableSubPackages" value="true" />
    </javaClientGenerator>

选择XMLMAPPER（xml文件形式）生成的如下：

<select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
select 
<include refid="Base_Column_List" />
from user
where id = #{id,jdbcType=INTEGER}
 </select>

ANNOTATEDMAPPER（注解形式）生成的如下：

@Select({
        "select",
        "id, name, age",
        "from user",
        "where id = #{id,jdbcType=INTEGER}"
})
@Results({
        @Result(column="id", property="id", jdbcType=JdbcType.INTEGER, id=true),
        @Result(column="name", property="name", jdbcType=JdbcType.VARCHAR),
        @Result(column="age", property="age", jdbcType=JdbcType.INTEGER)
})
User selectByPrimaryKey(Integer id);

对比：

Xml：增加 xml 文件、麻烦、条件不确定、容易出错，特殊字符转义

注释：不适合复杂 sql，收集 sql 不方便，重新编译

parameterType 与 parameterMap 区别：

通过 parameterType 指定输入参数的类型，类型可以是简单类型、hashmap、pojo 的包装类型

resultType 与 resultMap 区别

使用 resultType 进行输出映射，只有查询出来的列名和 pojo 中的属性名一致，该列才可以
映射成功。

mybatis 中使用 resultMap 完成高级输出结果映射。

参数标记符号之#与$的区别

# 预编译，防止 sql 注入(推荐)

$可以 sql 注入，代替作用

搞定：）
接下来进行重要特性了解与源码分析：）