Spring Boot: Mybatis 手写实现

简介

MyBatis框架的核心功能其实不难，无非就是动态代理和jdbc的操作，难的是写出来可扩展，高内聚，低耦合的规范的代码。

上图分析总结： 1、mybatis的配置文件有2类

mybatisconfig.xml，配置文件的名称不是固定的，配置了全局的参数的配置，全局只能有一个配置文件。
Mapper.xml 配置多个statemement，也就是多个sql，整个mybatis框架中可以有多个Mappe.xml配置文件。

2、通过mybatis配置文件得到SqlSessionFactory

3、通过SqlSessionFactory得到SqlSession，用SqlSession就可以操作数据了。

4、SqlSession通过底层的Executor（执行器），执行器有2类实现：

基本实现
带有缓存功能的实现 5、MappedStatement是通过Mapper.xml中定义statement生成的对象。

6、参数输入执行并输出结果集，无需手动判断参数类型和参数下标位置，且自动将结果集映射为Java对象

HashMap，KV格式的数据类型
Java的基本数据类型
POJO，java的对象

手动实现Mybatis

根据上文Mybatis流程，我简化了下，分为以下步骤：

1.读取xml文件，建立连接从图中可以看出，MyConfiguration负责与人交互。待读取xml后，将属性和连接数据库的操作封装在MyConfiguration对象中供后面的组件调用。本文将使用dom4j来读取xml文件，它具有性能优异和非常方便使用的特点。

2.创建SqlSession，搭建Configuration和Executor之间的桥梁我们经常在使用框架时看到Session，Session到底是什么呢？一个Session仅拥有一个对应的数据库连接。类似于一个前段请求Request，它可以直接调用exec(SQL)来执行SQL语句。

从流程图中的箭头可以看出，MySqlSession的成员变量中必须得有MyExecutor和MyConfiguration去集中做调配，箭头就像是一种关联关系。我们自己的MySqlSession将有一个getMapper方法，然后使用动态代理生成对象后，就可以做数据库的操作了。

3.创建Executor，封装JDBC操作数据库 Executor是一个执行器，负责SQL语句的生成和查询缓存（缓存还没完成）的维护，也就是jdbc的代码将在这里完成，不过本文只实现了单表，有兴趣的同学可以尝试完成多表。

4.创建MapperProxy，使用动态代理生成Mapper对象我们只是希望对指定的接口生成一个对象，使得执行它的时候能运行一句sql罢了，而接口无法直接调用方法，所以这里使用动态代理生成对象，在执行时还是回到MySqlSession中调用查询，最终由MyExecutor做JDBC查询。这样设计是为了单一职责，可扩展性更强。

代码实现

Pom配置

 
<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.liugh</groupId>
  <artifactId>liugh-mybatis</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>
  
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    <java.version>1.8</java.version>
  </properties>
  
  <dependencies>
       <!-- 读取xml文件 -->
    <dependency>
      <groupId>dom4j</groupId>
      <artifactId>dom4j</artifactId>
      <version>1.6.1</version>
    </dependency>
    
    <!-- MySQL -->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>5.1.29</version>
    </dependency>
    </dependencies>
</project>

数据库配置

 
<?xml version="1.0" encoding="UTF-8"?>
<database>
  <property name="driverClassName">com.mysql.jdbc.Driver</property>
  <property name="url">jdbc:mysql://localhost:3306/test?useUnicode=true&amp;characterEncoding=utf8</property>
  <property name="username">root</property>
  <property name="password">123456</property>
</database>

数据库表配置

 
CREATE TABLE `user` (
  `id` varchar(64) NOT NULL,
  `password` varchar(255) DEFAULT NULL,
  `username` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
INSERT INTO `test`.`user` (`id`, `password`, `username`) VALUES ('1', '123456', 'liugh');

User类及UserMapper定义

 
package com.liugh.bean;
 
public class User {
    private String id;
    private String username;
    private String password;
    //省略get set toString方法...
}
 
//接口配置
package com.liugh.mapper;
import com.liugh.bean.User;
public interface UserMapper {
  public User getUserById(String id);  
}
 
//xml配置
<?xml version="1.0" encoding="UTF-8"?>
<mapper nameSpace="com.liugh.mapper.UserMapper">
    <select id="getUserById" resultType ="com.liugh.bean.User">
        select * from user where id = ?
    </select>
</mapper>

MyConfiguration定义

 
package com.liugh.sqlSession;
 
import java.io.InputStream;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;
import com.liugh.config.Function;
import com.liugh.config.MapperBean;
 
/**
 * 读取与解析配置信息，并返回处理后的Environment
 */
public class MyConfiguration {
  private static ClassLoader loader = ClassLoader.getSystemClassLoader();
 
  /**
   * 读取xml信息并处理
   */
  public  Connection build(String resource){
      try {
          InputStream stream = loader.getResourceAsStream(resource);
      SAXReader reader = new SAXReader();
      Document document = reader.read(stream);
      Element root = document.getRootElement();
      return evalDataSource(root);
    } catch (Exception e) {
      throw new RuntimeException("error occured while evaling xml " + resource);
    }
  }
  
  private  Connection evalDataSource(Element node) throws ClassNotFoundException {
        if (!node.getName().equals("database")) {
          throw new RuntimeException("root should be <database>");
        }
    String driverClassName = null;
    String url = null;
    String username = null;
    String password = null;
    //获取属性节点
    for (Object item : node.elements("property")) {
      Element i = (Element) item;      
      String value = getValue(i);
      String name = i.attributeValue("name");
      if (name == null || value == null) {
        throw new RuntimeException("[database]: <property> should contain name and value");
      }
      //赋值
      switch (name) {
        case "url" : url = value; break;
        case "username" : username = value; break;
        case "password" : password = value; break;
        case "driverClassName" : driverClassName = value; break; 
        default : throw new RuntimeException("[database]: <property> unknown name"); 
      }
    }
    
     Class.forName(driverClassName); 
     Connection connection = null;
    try {
      //建立数据库链接
      connection = DriverManager.getConnection(url, username, password);
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
    return connection;
  }
  
  //获取property属性的值,如果有value值,则读取 没有设置value,则读取内容
  private  String getValue(Element node) {
    return node.hasContent() ? node.getText() : node.attributeValue("value");
  }
  
    @SuppressWarnings("rawtypes")
    public  MapperBean readMapper(String path){
        MapperBean mapper = new MapperBean();
        try{
          InputStream stream = loader.getResourceAsStream(path);
       SAXReader reader = new SAXReader();
       Document document = reader.read(stream);
       Element root = document.getRootElement();
            mapper.setInterfaceName(root.attributeValue("nameSpace").trim()); //把mapper节点的nameSpace值存为接口名
            List<Function> list = new ArrayList<Function>(); //用来存储方法的List
            for(Iterator rootIter = root.elementIterator();rootIter.hasNext();) {//遍历根节点下所有子节点
                Function fun = new Function();    //用来存储一条方法的信息
                Element e = (Element) rootIter.next(); 
                String sqltype = e.getName().trim();
                String funcName = e.attributeValue("id").trim();
                String sql = e.getText().trim();
                String resultType = e.attributeValue("resultType").trim();
                fun.setSqltype(sqltype);
                fun.setFuncName(funcName);
                Object newInstance=null;
        try {
          newInstance = Class.forName(resultType).newInstance();
        } catch (InstantiationException e1) {
          e1.printStackTrace();
        } catch (IllegalAccessException e1) {
          e1.printStackTrace();
        } catch (ClassNotFoundException e1) {
          e1.printStackTrace();
        }
                fun.setResultType(newInstance);
                fun.setSql(sql);
                list.add(fun);
            }
            mapper.setList(list);
            
        } catch (DocumentException e) {
            e.printStackTrace();
        }
        return mapper;
    }
}

用面向对象的思想设计读取xml配置后：

 
package com.liugh.config;
 
import java.util.List;
public class MapperBean {
 
    private String interfaceName; //接口名
    private List<Function> list; //接口下所有方法
    //省略 get  set方法...
 
}

Function对象包括sql的类型、方法名、sql语句、返回类型和参数类型。

 
package com.liugh.config;
 
public class Function {
   private String sqltype;  
    private String funcName;  
    private String sql;       
    private Object resultType;  
    private String parameterType; 
  //省略 get set方法
}

接下来实现我们的MySqlSession,首先的成员变量里得有Excutor和MyConfiguration，代码的精髓就在getMapper的方法里。

 
package com.liugh.sqlSession;
 
import java.lang.reflect.Proxy;
 
public class MySqlsession {
  
  private Excutor excutor= new MyExcutor();  
  
  private MyConfiguration myConfiguration = new MyConfiguration();
  
   public <T> T selectOne(String statement,Object parameter){  
       return excutor.query(statement, parameter);  
   }  
        
   @SuppressWarnings("unchecked")
   public <T> T getMapper(Class<T> clas){ 
      //动态代理调用
        return (T)Proxy.newProxyInstance(clas.getClassLoader(),new Class[]{clas},
            new MyMapperProxy(myConfiguration,this));  
    }  
}

紧接着创建Excutor和实现类：

 
package com.liugh.sqlSession;
 
public interface Excutor {
  public <T> T query(String statement,Object parameter);  
}

MyExcutor中封装了JDBC的操作：

 
package com.liugh.sqlSession;
 
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import com.liugh.bean.User;
 
public class MyExcutor implements Excutor{
   private MyConfiguration xmlConfiguration = new MyConfiguration();
  
   @Override  
   public <T> T query(String sql, Object parameter) {  
         Connection connection=getConnection();  
         ResultSet set =null;
         PreparedStatement pre =null;
         try {  
             pre = connection.prepareStatement(sql); 
             //设置参数
             pre.setString(1, parameter.toString());
             set = pre.executeQuery();  
             User u=new User();  
             //遍历结果集
             while(set.next()){  
                 u.setId(set.getString(1));
                 u.setUsername(set.getString(2)); 
                 u.setPassword(set.getString(3));
             }  
             return (T) u;  
         } catch (SQLException e) {  
             e.printStackTrace();  
         } finally{
                try{  
                    if(set!=null){  
                      set.close();  
                    }if(pre!=null){  
                      pre.close();  
                    }if(connection!=null){  
                      connection.close();  
                    }  
                }catch(Exception e2){  
                    e2.printStackTrace();  
                }  
            }   
         return null;  
     }  
   
     private Connection getConnection() {  
         try {  
             Connection connection =xmlConfiguration.build("config.xml");
             return connection;  
         } catch (Exception e) {  
             e.printStackTrace();  
         }  
         return null;  
     }  
}

MyMapperProxy代理类完成xml方法和真实方法对应，执行查询：

 
package com.liugh.sqlSession;
 
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.List;
import com.liugh.config.Function;
import com.liugh.config.MapperBean;
 
public class MyMapperProxy implements InvocationHandler{
  
  private  MySqlsession mySqlsession;  
  
  private MyConfiguration myConfiguration;
     
    public MyMapperProxy(MyConfiguration myConfiguration,MySqlsession mySqlsession) {  
        this.myConfiguration=myConfiguration;  
        this.mySqlsession=mySqlsession;  
    }  
 
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    MapperBean readMapper = myConfiguration.readMapper("UserMapper.xml");
    //是否是xml文件对应的接口
    if(!method.getDeclaringClass().getName().equals(readMapper.getInterfaceName())){
      return null;  
    }
    List<Function> list = readMapper.getList();
    if(null != list || 0 != list.size()){
      for (Function function : list) {
      //id是否和接口方法名一样
       if(method.getName().equals(function.getFuncName())){  
                return mySqlsession.selectOne(function.getSql(), String.valueOf(args[0]));  
            }  
      }
    }
       return null;  
  }
}

到这里，就完成了自己的Mybatis框架，我们测试一下：

 
package com.liugh;
 
import com.liugh.bean.User;
import com.liugh.mapper.UserMapper;
import com.liugh.sqlSession.MySqlsession;
 
public class TestMybatis {
  
    public static void main(String[] args) {  
        MySqlsession sqlsession=new MySqlsession();  
        UserMapper mapper = sqlsession.getMapper(UserMapper.class);  
        User user = mapper.getUserById("1");  
        System.out.println(user);
    } 
}

执行结果：

查询一个不存在的用户试试：

到这里我们就大功告成了！

其他

参考: https://cloud.tencent.com/developer/article/1486206

	<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.liugh</groupId>
	<artifactId>liugh-mybatis</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<properties>
	<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
	<maven.compiler.source>1.8</maven.compiler.source>
	<maven.compiler.target>1.8</maven.compiler.target>
	<java.version>1.8</java.version>
	</properties>

	<dependencies>
	<!-- 读取xml文件 -->
	<dependency>
	<groupId>dom4j</groupId>
	<artifactId>dom4j</artifactId>
	<version>1.6.1</version>
	</dependency>

	<!-- MySQL -->
	<dependency>
	<groupId>mysql</groupId>
	<artifactId>mysql-connector-java</artifactId>
	<version>5.1.29</version>
	</dependency>
	</dependencies>
	</project>

	<?xml version="1.0" encoding="UTF-8"?>
	<database>
	<property name="driverClassName">com.mysql.jdbc.Driver</property>
	<property name="url">jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=utf8</property>
	<property name="username">root</property>
	<property name="password">123456</property>
	</database>

	CREATE TABLE `user` (
	`id` varchar(64) NOT NULL,
	`password` varchar(255) DEFAULT NULL,
	`username` varchar(255) DEFAULT NULL,
	PRIMARY KEY (`id`)
	) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
	INSERT INTO `test`.`user` (`id`, `password`, `username`) VALUES ('1', '123456', 'liugh');

	package com.liugh.bean;

	public class User {
	private String id;
	private String username;
	private String password;
	//省略get set toString方法...
	}

	//接口配置
	package com.liugh.mapper;
	import com.liugh.bean.User;
	public interface UserMapper {
	public User getUserById(String id);
	}

	//xml配置
	<?xml version="1.0" encoding="UTF-8"?>
	<mapper nameSpace="com.liugh.mapper.UserMapper">
	<select id="getUserById" resultType ="com.liugh.bean.User">
	select * from user where id = ?
	</select>
	</mapper>

	package com.liugh.sqlSession;

	import java.io.InputStream;
	import java.sql.Connection;
	import java.sql.DriverManager;
	import java.sql.SQLException;
	import java.util.ArrayList;
	import java.util.Iterator;
	import java.util.List;
	import org.dom4j.Document;
	import org.dom4j.DocumentException;
	import org.dom4j.Element;
	import org.dom4j.io.SAXReader;
	import com.liugh.config.Function;
	import com.liugh.config.MapperBean;

	/**
	* 读取与解析配置信息，并返回处理后的Environment
	*/
	public class MyConfiguration {
	private static ClassLoader loader = ClassLoader.getSystemClassLoader();

	/**
	* 读取xml信息并处理
	*/
	public Connection build(String resource){
	try {
	InputStream stream = loader.getResourceAsStream(resource);
	SAXReader reader = new SAXReader();
	Document document = reader.read(stream);
	Element root = document.getRootElement();
	return evalDataSource(root);
	} catch (Exception e) {
	throw new RuntimeException("error occured while evaling xml " + resource);
	}
	}

	private Connection evalDataSource(Element node) throws ClassNotFoundException {
	if (!node.getName().equals("database")) {
	throw new RuntimeException("root should be <database>");
	}
	String driverClassName = null;
	String url = null;
	String username = null;
	String password = null;
	//获取属性节点
	for (Object item : node.elements("property")) {
	Element i = (Element) item;
	String value = getValue(i);
	String name = i.attributeValue("name");
	if (name == null \|\| value == null) {
	throw new RuntimeException("[database]: <property> should contain name and value");
	}
	//赋值
	switch (name) {
	case "url" : url = value; break;
	case "username" : username = value; break;
	case "password" : password = value; break;
	case "driverClassName" : driverClassName = value; break;
	default : throw new RuntimeException("[database]: <property> unknown name");
	}
	}

	Class.forName(driverClassName);
	Connection connection = null;
	try {
	//建立数据库链接
	connection = DriverManager.getConnection(url, username, password);
	} catch (SQLException e) {
	// TODO Auto-generated catch block
	e.printStackTrace();
	}
	return connection;
	}

	//获取property属性的值,如果有value值,则读取没有设置value,则读取内容
	private String getValue(Element node) {
	return node.hasContent() ? node.getText() : node.attributeValue("value");
	}

	@SuppressWarnings("rawtypes")
	public MapperBean readMapper(String path){
	MapperBean mapper = new MapperBean();
	try{
	InputStream stream = loader.getResourceAsStream(path);
	SAXReader reader = new SAXReader();
	Document document = reader.read(stream);
	Element root = document.getRootElement();
	mapper.setInterfaceName(root.attributeValue("nameSpace").trim()); //把mapper节点的nameSpace值存为接口名
	List<Function> list = new ArrayList<Function>(); //用来存储方法的List
	for(Iterator rootIter = root.elementIterator();rootIter.hasNext();) {//遍历根节点下所有子节点
	Function fun = new Function(); //用来存储一条方法的信息
	Element e = (Element) rootIter.next();
	String sqltype = e.getName().trim();
	String funcName = e.attributeValue("id").trim();
	String sql = e.getText().trim();
	String resultType = e.attributeValue("resultType").trim();
	fun.setSqltype(sqltype);
	fun.setFuncName(funcName);
	Object newInstance=null;
	try {
	newInstance = Class.forName(resultType).newInstance();
	} catch (InstantiationException e1) {
	e1.printStackTrace();
	} catch (IllegalAccessException e1) {
	e1.printStackTrace();
	} catch (ClassNotFoundException e1) {
	e1.printStackTrace();
	}
	fun.setResultType(newInstance);
	fun.setSql(sql);
	list.add(fun);
	}
	mapper.setList(list);

	} catch (DocumentException e) {
	e.printStackTrace();
	}
	return mapper;
	}
	}

	package com.liugh.config;

	import java.util.List;
	public class MapperBean {

	private String interfaceName; //接口名
	private List<Function> list; //接口下所有方法
	//省略 get set方法...

	}

	package com.liugh.config;

	public class Function {
	private String sqltype;
	private String funcName;
	private String sql;
	private Object resultType;
	private String parameterType;
	//省略 get set方法
	}

	package com.liugh.sqlSession;

	import java.lang.reflect.Proxy;

	public class MySqlsession {

	private Excutor excutor= new MyExcutor();

	private MyConfiguration myConfiguration = new MyConfiguration();

	public <T> T selectOne(String statement,Object parameter){
	return excutor.query(statement, parameter);
	}

	@SuppressWarnings("unchecked")
	public <T> T getMapper(Class<T> clas){
	//动态代理调用
	return (T)Proxy.newProxyInstance(clas.getClassLoader(),new Class[]{clas},
	new MyMapperProxy(myConfiguration,this));
	}
	}

	package com.liugh.sqlSession;

	public interface Excutor {
	public <T> T query(String statement,Object parameter);
	}

	package com.liugh.sqlSession;

	import java.sql.Connection;
	import java.sql.PreparedStatement;
	import java.sql.ResultSet;
	import java.sql.SQLException;
	import com.liugh.bean.User;

	public class MyExcutor implements Excutor{
	private MyConfiguration xmlConfiguration = new MyConfiguration();

	@Override
	public <T> T query(String sql, Object parameter) {
	Connection connection=getConnection();
	ResultSet set =null;
	PreparedStatement pre =null;
	try {
	pre = connection.prepareStatement(sql);
	//设置参数
	pre.setString(1, parameter.toString());
	set = pre.executeQuery();
	User u=new User();
	//遍历结果集
	while(set.next()){
	u.setId(set.getString(1));
	u.setUsername(set.getString(2));
	u.setPassword(set.getString(3));
	}
	return (T) u;
	} catch (SQLException e) {
	e.printStackTrace();
	} finally{
	try{
	if(set!=null){
	set.close();
	}if(pre!=null){
	pre.close();
	}if(connection!=null){
	connection.close();
	}
	}catch(Exception e2){
	e2.printStackTrace();
	}
	}
	return null;
	}

	private Connection getConnection() {
	try {
	Connection connection =xmlConfiguration.build("config.xml");
	return connection;
	} catch (Exception e) {
	e.printStackTrace();
	}
	return null;
	}
	}

	package com.liugh.sqlSession;

	import java.lang.reflect.InvocationHandler;
	import java.lang.reflect.Method;
	import java.util.List;
	import com.liugh.config.Function;
	import com.liugh.config.MapperBean;

	public class MyMapperProxy implements InvocationHandler{

	private MySqlsession mySqlsession;

	private MyConfiguration myConfiguration;

	public MyMapperProxy(MyConfiguration myConfiguration,MySqlsession mySqlsession) {
	this.myConfiguration=myConfiguration;
	this.mySqlsession=mySqlsession;
	}

	@Override
	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	MapperBean readMapper = myConfiguration.readMapper("UserMapper.xml");
	//是否是xml文件对应的接口
	if(!method.getDeclaringClass().getName().equals(readMapper.getInterfaceName())){
	return null;
	}
	List<Function> list = readMapper.getList();
	if(null != list \|\| 0 != list.size()){
	for (Function function : list) {
	//id是否和接口方法名一样
	if(method.getName().equals(function.getFuncName())){
	return mySqlsession.selectOne(function.getSql(), String.valueOf(args[0]));
	}
	}
	}
	return null;
	}
	}

	package com.liugh;

	import com.liugh.bean.User;
	import com.liugh.mapper.UserMapper;
	import com.liugh.sqlSession.MySqlsession;

	public class TestMybatis {

	public static void main(String[] args) {
	MySqlsession sqlsession=new MySqlsession();
	UserMapper mapper = sqlsession.getMapper(UserMapper.class);
	User user = mapper.getUserById("1");
	System.out.println(user);
	}
	}