幾百行代碼寫個Mybatis，原理搞的透透的！-有解無憂

作者：小傅哥
博客：https://bugstack.cn

沉淀、分享、成長，讓自己和他人都能有所識訓！😄

一、前言

Mybatis 最核心的原理也是它最便于使用的體現，為什么這說？

因為我們在使用 Mybatis 的時候，只需要定義一個不需要寫實作類的介面，就能通過注解或者配置SQL陳述句的方式，對資料庫進行 CRUD 操作，

那么這是怎么做到的呢，其中有一點非常重要，就是在 Spring 中可以把你的代理物件交給 Spring 容器，這個代理物件就是可以當做是 DAO 介面的具體實作類，而這個被代理的實作類就可以完成對資料庫的一個操作，也就是這個封裝程序被稱為 ORM 框架，

說了基本的流程，我們來做點測驗，讓大家可以動手操作起來！學知識，一定是上手，才能得到！你可以通過以下原始碼倉庫進行練習

原始碼：https://github.com/fuzhengwei/CodeGuide/wiki

二、把Bean塞到Spring容器，分幾步

Bean注冊

關于Bean注冊的技術場景，在我們日常用到的技術框架中，MyBatis 是最為常見的，通過在使用 MyBatis 時都只是定義一個介面不需要寫實作類，但是這個介面卻可以和配置的 SQL 陳述句關聯，執行相應的資料庫操作時可以回傳對應的結果，那么這個介面與資料庫的操作就用到的 Bean 的代理和注冊，
我們都知道類的呼叫是不能直接呼叫沒有實作的介面的，所以需要通過代理的方式給介面生成對應的實作類，接下來再通過把代理類放到 Spring 的 FactoryBean 的實作中，最后再把這個 FactoryBean 實作類注冊到 Spring 容器，那么現在你的代理類就已經被注冊到 Spring 容器了，接下來就可以通過注解的方式注入到屬性中，

按照這個實作方式，我們來操作一下，看看一個 Bean 的注冊程序在代碼中是如何實作的，

1. 定義介面

public interface IUserDao {

    String queryUserInfo();

}

先定義一個類似 DAO 的介面，基本這樣的介面在使用 MyBatis 時還是非常常見的，后面我們會對這個介面做代理和注冊，

2. 類代理實作

ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
Class<?>[] classes = {IUserDao.class};    

InvocationHandler handler = (proxy, method, args) -> "你被代理了 " + method.getName();
IUserDao userDao = (IUserDao) Proxy.newProxyInstance(classLoader, classes, handler); 

String res = userDao.queryUserInfo();
logger.info("測驗結果：{}", res);

Java 本身的代理方式使用起來還是比較簡單的，用法也很固定，
InvocationHandler 是個介面類，它對應的實作內容就是代理物件的具體實作，
最后就是把代理交給 Proxy 創建代理物件，Proxy.newProxyInstance，

3. 實作Bean工廠

public class ProxyBeanFactory implements FactoryBean {

    @Override
    public Object getObject() throws Exception {

        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        Class[] classes = {IUserDao.class};
        InvocationHandler handler = (proxy, method, args) -> "你被代理了 " + method.getName();

        return Proxy.newProxyInstance(classLoader, classes, handler);
    }

    @Override
    public Class<?> getObjectType() {
        return IUserDao.class;
    } 

}

FactoryBean 在 spring 起到著二當家的地位，它將近有70多個小弟(實作它的介面定義)，那么它有三個方法；
- T getObject() throws Exception; 回傳bean實體物件
- Class<?> getObjectType(); 回傳實體型別別
- boolean isSingleton(); 判斷是否單例，單例會放到Spring容器中單實體快取池中
在這里我們把上面使用Java代理的物件放到了 getObject() 方法中，那么現在再從 Spring 中獲取到的物件，就是我們的代理物件了，

4. Bean 注冊

public class RegisterBeanFactory implements BeanDefinitionRegistryPostProcessor {

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {

        GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
        beanDefinition.setBeanClass(ProxyBeanFactory.class);

        BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(beanDefinition, "userDao");
        BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, registry);
    }

}

在 Spring 的 Bean 管理中，所有的 Bean 最終都會被注冊到類 DefaultListableBeanFactory 中，以上這部分代碼主要的內容包括：

實作 BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry方法，獲取 Bean 注冊物件，
定義 Bean，GenericBeanDefinition，這里主要設定了我們的代理類工廠，
創建 Bean 定義處理類，BeanDefinitionHolder，這里需要的主要引數；定義 Bean 和名稱 setBeanClass(ProxyBeanFactory.class)，
最后將我們自己的bean注冊到spring容器中去，registry.registerBeanDefinition()

5. 測驗驗證

在上面我們已經把自定義代理的 Bean 注冊到了 Spring 容器中，接下來我們來測驗下這個代理的 Bean 被如何呼叫，

1. 定義 spring-config.xml

<bean id="userDao" class="org.itstack.interview.bean.RegisterBeanFactory"/>

這里我們把 RegisterBeanFactory 配置到 spring 的 xml 配置中，便于啟動時加載，

2. 單元測驗

@Test
public void test_IUserDao() {
    BeanFactory beanFactory = new ClassPathXmlApplicationContext("spring-config.xml");
    IUserDao userDao = beanFactory.getBean("userDao", IUserDao.class);
    String res = userDao.queryUserInfo();
    logger.info("測驗結果：{}", res);
}

測驗結果

22:53:14.759 [main] DEBUG o.s.c.e.PropertySourcesPropertyResolver - Could not find key 'spring.liveBeansView.mbeanDomain' in any property source
22:53:14.760 [main] DEBUG o.s.b.f.s.DefaultListableBeanFactory - Returning cached instance of singleton bean 'userDao'
22:53:14.796 [main] INFO  org.itstack.interview.test.ApiTest - 測驗結果：你被代理了 queryUserInfo

Process finished with exit code 0

從測驗結果可以看到，我們已經可以通過注入到Spring的代理Bean物件，實作我們的預期結果，
其實這個程序也是很多框架中用到的方式，尤其是在一些中間件開發，類似的 ORM 框架都需要使用到，

三、手寫個Mybatis

擴展上一篇原始碼分析工程；itstack-demo-mybatis，增加 like 包，模仿 Mybatis 工程，完整規程下載 https://github.com/fuzhengwei/CodeGuide/wiki

itstack-demo-mybatis
└── src
    ├── main
    │   ├── java
    │   │   └── org.itstack.demo
    │   │       ├── dao
    │   │       │	├── ISchool.java		
    │   │       │	└── IUserDao.java	
    │   │       ├── like
    │   │       │	├── Configuration.java
    │   │       │	├── DefaultSqlSession.java
    │   │       │	├── DefaultSqlSessionFactory.java
    │   │       │	├── Resources.java
    │   │       │	├── SqlSession.java
    │   │       │	├── SqlSessionFactory.java
    │   │       │	├── SqlSessionFactoryBuilder.java	
    │   │       │	└── SqlSessionFactoryBuilder.java	
    │   │       └── interfaces     
    │   │         	├── School.java	
    │   │        	└── User.java
    │   ├── resources	
    │   │   ├── mapper
    │   │   │   ├── School_Mapper.xml
    │   │   │   └── User_Mapper.xml
    │   │   ├── props	
    │   │   │   └── jdbc.properties
    │   │   ├── spring
    │   │   │   ├── mybatis-config-datasource.xml
    │   │   │   └── spring-config-datasource.xml
    │   │   ├── logback.xml
    │   │   ├── mybatis-config.xml
    │   │   └── spring-config.xml
    │   └── webapp
    │       └── WEB-INF
    └── test
         └── java
             └── org.itstack.demo.test
                 ├── ApiLikeTest.java
                 ├── MybatisApiTest.java
                 └── SpringApiTest.java

關于整個 Demo 版本，并不是把所有 Mybatis 全部實作一遍，而是撥絲抽繭將最核心的內容展示給你，從使用上你會感受一模一樣，但是實作類已經全部被替換，核心類包括；

Configuration
DefaultSqlSession
DefaultSqlSessionFactory
Resources
SqlSession
SqlSessionFactory
SqlSessionFactoryBuilder
XNode

1. 先測驗下整個DemoJdbc框架

ApiLikeTest.test_queryUserInfoById()

@Test
public void test_queryUserInfoById() {
    String resource = "spring/mybatis-config-datasource.xml";
    Reader reader;
    try {
        reader = Resources.getResourceAsReader(resource);
        SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);
        SqlSession session = sqlMapper.openSession();
		
        try {
            User user = session.selectOne("org.itstack.demo.dao.IUserDao.queryUserInfoById", 1L);
            System.out.println(JSON.toJSONString(user));
        } finally {
            session.close();
            reader.close();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

一切順利結果如下(新人往往會遇到各種問題)；

{"age":18,"createTime":1576944000000,"id":1,"name":"水水","updateTime":1576944000000}

Process finished with exit code 0

可能乍一看這測驗類完全和 MybatisApiTest.java 測驗的代碼一模一樣呀，也看不出區別，其實他們的引入的包是不一樣；

MybatisApiTest.java 里面引入的包

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

ApiLikeTest.java 里面引入的包

import org.itstack.demo.like.Resources;
import org.itstack.demo.like.SqlSession;
import org.itstack.demo.like.SqlSessionFactory;
import org.itstack.demo.like.SqlSessionFactoryBuilder;

好！接下來我們開始分析這部分核心代碼，

2. 加載XML組態檔

這里我們采用 mybatis 的組態檔結構進行決議，在不破壞原有結構的情況下，最大可能的貼近原始碼，mybatis 單獨使用的使用的時候使用了兩個組態檔；資料源配置、Mapper 映射配置，如下；

mybatis-config-datasource.xml & 資料源配置

<?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://127.0.0.1:3306/itstack?useUnicode=true"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="mapper/User_Mapper.xml"/>
        <mapper resource="mapper/School_Mapper.xml"/>
    </mappers>

</configuration>

User_Mapper.xml & Mapper 映射配置

<?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="org.itstack.demo.dao.IUserDao">

    <select id="queryUserInfoById" parameterType="java.lang.Long" resultType="org.itstack.demo.po.User">
        SELECT id, name, age, createTime, updateTime
        FROM user
        where id = #{id}
    </select>

    <select id="queryUserList" parameterType="org.itstack.demo.po.User" resultType="org.itstack.demo.po.User">
        SELECT id, name, age, createTime, updateTime
        FROM user
        where age = #{age}
    </select>

</mapper>

這里的加載程序與 mybaits 不同，我們采用 dom4j 方式，在案例中會看到最開始獲取資源，如下；

ApiLikeTest.test_queryUserInfoById() & 部分截取

String resource = "spring/mybatis-config-datasource.xml";
	Reader reader;
	try {
		reader = Resources.getResourceAsReader(resource);
	...

從上可以看到這是通過組態檔地址獲取到了讀取流的程序，從而為后面決議做基礎，首先我們先看 Resources 類，整個是我們的資源類，

Resources.java & 資源類

/**
 * 博 客 | https://bugstack.cn
 * Create by 小傅哥 @2020
 */
public class Resources {

    public static Reader getResourceAsReader(String resource) throws IOException {
        return new InputStreamReader(getResourceAsStream(resource));
    }

    private static InputStream getResourceAsStream(String resource) throws IOException {
        ClassLoader[] classLoaders = getClassLoaders();
        for (ClassLoader classLoader : classLoaders) {
            InputStream inputStream = classLoader.getResourceAsStream(resource);
            if (null != inputStream) {
                return inputStream;
            }
        }
        throw new IOException("Could not find resource " + resource);
    }

    private static ClassLoader[] getClassLoaders() {
        return new ClassLoader[]{
                ClassLoader.getSystemClassLoader(),
                Thread.currentThread().getContextClassLoader()};
    }

}

這段代碼方法的入口是getResourceAsReader，直到往下以此做了；

獲取 ClassLoader 集合，最大限度搜索組態檔
通過 classLoader.getResourceAsStream 讀取配置資源，找到后立即回傳，否則拋出例外

3. 決議XML組態檔

組態檔加載后開始進行決議操作，這里我們也仿照 mybatis 但進行簡化，如下；

SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);

SqlSessionFactoryBuilder.build() & 入口構建類

public DefaultSqlSessionFactory build(Reader reader) {
    SAXReader saxReader = new SAXReader();
    try {
        Document document = saxReader.read(new InputSource(reader));
        Configuration configuration = parseConfiguration(document.getRootElement());
        return new DefaultSqlSessionFactory(configuration);
    } catch (DocumentException e) {
        e.printStackTrace();
    }
    return null;
}

通過讀取流創建 xml 決議的 Document 類
parseConfiguration 進行決議 xml 檔案，并將結果設定到配置類中，包括；連接池、資料源、mapper關系

SqlSessionFactoryBuilder.parseConfiguration() & 決議程序

private Configuration parseConfiguration(Element root) {
    Configuration configuration = new Configuration();
    configuration.setDataSource(dataSource(root.selectNodes("//dataSource")));
    configuration.setConnection(connection(configuration.dataSource));
    configuration.setMapperElement(mapperElement(root.selectNodes("mappers")));
    return configuration;
}

在前面的 xml 內容中可以看到，我們需要決議出資料庫連接池資訊 datasource，還有資料庫陳述句映射關系 mappers

SqlSessionFactoryBuilder.dataSource() & 決議出資料源

private Map<String, String> dataSource(List<Element> list) {
    Map<String, String> dataSource = new HashMap<>(4);
    Element element = list.get(0);
    List content = element.content();
    for (Object o : content) {
        Element e = (Element) o;
        String name = e.attributeValue("name");
        String value = e.attributeValue("value");
        dataSource.put(name, value);
    }
    return dataSource;
}

這個程序比較簡單，只需要將資料源資訊獲取即可

SqlSessionFactoryBuilder.connection() & 獲取資料庫連接

private Connection connection(Map<String, String> dataSource) {
    try {
        Class.forName(dataSource.get("driver"));
        return DriverManager.getConnection(dataSource.get("url"), dataSource.get("username"), dataSource.get("password"));
    } catch (ClassNotFoundException | SQLException e) {
        e.printStackTrace();
    }
    return null;
}

這個就是jdbc最原始的代碼，獲取了資料庫連接池

SqlSessionFactoryBuilder.mapperElement() & 決議SQL陳述句

private Map<String, XNode> mapperElement(List<Element> list) {
    Map<String, XNode> map = new HashMap<>();
    Element element = list.get(0);
    List content = element.content();
    for (Object o : content) {
        Element e = (Element) o;
        String resource = e.attributeValue("resource");
        try {
            Reader reader = Resources.getResourceAsReader(resource);
            SAXReader saxReader = new SAXReader();
            Document document = saxReader.read(new InputSource(reader));
            Element root = document.getRootElement();
            //命名空間
            String namespace = root.attributeValue("namespace");
            // SELECT
            List<Element> selectNodes = root.selectNodes("select");
            for (Element node : selectNodes) {
                String id = node.attributeValue("id");
                String parameterType = node.attributeValue("parameterType");
                String resultType = node.attributeValue("resultType");
                String sql = node.getText();
                // ? 匹配
                Map<Integer, String> parameter = new HashMap<>();
                Pattern pattern = Pattern.compile("(#\\{(.*?)})");
                Matcher matcher = pattern.matcher(sql);
                for (int i = 1; matcher.find(); i++) {
                    String g1 = matcher.group(1);
                    String g2 = matcher.group(2);
                    parameter.put(i, g2);
                    sql = sql.replace(g1, "?");
                }
                XNode xNode = new XNode();
                xNode.setNamespace(namespace);
                xNode.setId(id);
                xNode.setParameterType(parameterType);
                xNode.setResultType(resultType);
                xNode.setSql(sql);
                xNode.setParameter(parameter);
                
                map.put(namespace + "." + id, xNode);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    return map;
}

這個程序首先包括是決議所有的sql陳述句，目前為了測驗只決議 select 相關
所有的 sql 陳述句為了確認唯一，都是使用；namespace + select中的id進行拼接，作為 key，之后與sql一起存放到 map 中，
在 mybaits 的 sql 陳述句配置中，都有占位符，用于傳參，where id = #{id} 所以我們需要將占位符設定為問號，另外需要將占位符的順序資訊與名稱存放到 map 結構，方便后續設定查詢時候的入參，

4. 創建DefaultSqlSessionFactory

最后將初始化后的配置類 Configuration，作為引數進行創建 DefaultSqlSessionFactory，如下；

public DefaultSqlSessionFactory build(Reader reader) {
    SAXReader saxReader = new SAXReader();
    try {
        Document document = saxReader.read(new InputSource(reader));
        Configuration configuration = parseConfiguration(document.getRootElement());
        return new DefaultSqlSessionFactory(configuration);
    } catch (DocumentException e) {
        e.printStackTrace();
    }
    return null;
}

DefaultSqlSessionFactory.java & SqlSessionFactory的實作類

public class DefaultSqlSessionFactory implements SqlSessionFactory {
    
	private final Configuration configuration;
    
	public DefaultSqlSessionFactory(Configuration configuration) {
        this.configuration = configuration;
    }
	
    @Override
    public SqlSession openSession() {
        return new DefaultSqlSession(configuration.connection, configuration.mapperElement);
    }
	
}

這個程序比較簡單，建構式只提供了配置類入參
實作 SqlSessionFactory 的 openSession()，用于創建 DefaultSqlSession，也就可以執行 sql 操作

5. 開啟SqlSession

SqlSession session = sqlMapper.openSession();

上面這一步就是創建了DefaultSqlSession，比較簡單，如下；

@Override
public SqlSession openSession() {
    return new DefaultSqlSession(configuration.connection, configuration.mapperElement);
}

6. 執行SQL陳述句

User user = session.selectOne("org.itstack.demo.dao.IUserDao.queryUserInfoById", 1L);

在 DefaultSqlSession 中通過實作 SqlSession，提供資料庫陳述句查詢和關閉連接池，如下；

SqlSession.java & 定義

public interface SqlSession {

    <T> T selectOne(String statement);

    <T> T selectOne(String statement, Object parameter);

    <T> List<T> selectList(String statement);

    <T> List<T> selectList(String statement, Object parameter);

    void close();
}

接下來看具體的執行程序，session.selectOne

DefaultSqlSession.selectOne() & 執行查詢

public <T> T selectOne(String statement, Object parameter) {
    XNode xNode = mapperElement.get(statement);
    Map<Integer, String> parameterMap = xNode.getParameter();
    try {
        PreparedStatement preparedStatement = connection.prepareStatement(xNode.getSql());
        buildParameter(preparedStatement, parameter, parameterMap);
        ResultSet resultSet = preparedStatement.executeQuery();
        List<T> objects = resultSet2Obj(resultSet, Class.forName(xNode.getResultType()));
        return objects.get(0);
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

selectOne 就objects.get(0);，selectList 就全部回傳

通過 statement 獲取最初決議 xml 時候的存盤的 select 標簽資訊；

<select id="queryUserInfoById" parameterType="java.lang.Long" resultType="org.itstack.demo.po.User">
	SELECT id, name, age, createTime, updateTime
	FROM user
	where id = #{id}
</select>

獲取 sql 陳述句后交給 jdbc 的 PreparedStatement 類進行執行

這里還需要設定入參，我們將入參設定進行抽取，如下；

private void buildParameter(PreparedStatement preparedStatement, Object parameter, Map<Integer, String> parameterMap) throws SQLException, IllegalAccessException {

      int size = parameterMap.size();
      // 單個引數
      if (parameter instanceof Long) {
          for (int i = 1; i <= size; i++) {
              preparedStatement.setLong(i, Long.parseLong(parameter.toString()));
          }
          return;
      }

      if (parameter instanceof Integer) {
          for (int i = 1; i <= size; i++) {
              preparedStatement.setInt(i, Integer.parseInt(parameter.toString()));
          }
          return;
      }

      if (parameter instanceof String) {
          for (int i = 1; i <= size; i++) {
              preparedStatement.setString(i, parameter.toString());
          }
          return;
      }

      Map<String, Object> fieldMap = new HashMap<>();
      // 物件引數
      Field[] declaredFields = parameter.getClass().getDeclaredFields();
      for (Field field : declaredFields) {
          String name = field.getName();
          field.setAccessible(true);
          Object obj = field.get(parameter);
          field.setAccessible(false);
          fieldMap.put(name, obj);
      }

      for (int i = 1; i <= size; i++) {
          String parameterDefine = parameterMap.get(i);
          Object obj = fieldMap.get(parameterDefine);

          if (obj instanceof Short) {
              preparedStatement.setShort(i, Short.parseShort(obj.toString()));
              continue;
          }

          if (obj instanceof Integer) {
              preparedStatement.setInt(i, Integer.parseInt(obj.toString()));
              continue;
          }

          if (obj instanceof Long) {
              preparedStatement.setLong(i, Long.parseLong(obj.toString()));
              continue;
          }

          if (obj instanceof String) {
              preparedStatement.setString(i, obj.toString());
              continue;
          }

          if (obj instanceof Date) {
              preparedStatement.setDate(i, (java.sql.Date) obj);
          }

      }

  }

單個引數比較簡單直接設定值即可，Long、Integer、String …
如果是一個類物件，需要通過獲取 Field 屬性，與引數 Map 進行匹配設定

設定引數后執行查詢 preparedStatement.executeQuery()

接下來需要將查詢結果轉換為我們的類（主要是反射類的操作），resultSet2Obj(resultSet, Class.forName(xNode.getResultType()));

private <T> List<T> resultSet2Obj(ResultSet resultSet, Class<?> clazz) {
	List<T> list = new ArrayList<>();
	try {
		ResultSetMetaData metaData = resultSet.getMetaData();
		int columnCount = metaData.getColumnCount();
		// 每次遍歷行值
		while (resultSet.next()) {
			T obj = (T) clazz.newInstance();
			for (int i = 1; i <= columnCount; i++) {
				Object value = resultSet.getObject(i);
				String columnName = metaData.getColumnName(i);
				String setMethod = "set" + columnName.substring(0, 1).toUpperCase() + columnName.substring(1);
				Method method;
				if (value instanceof Timestamp) {
					method = clazz.getMethod(setMethod, Date.class);
				} else {
					method = clazz.getMethod(setMethod, value.getClass());
				}
				method.invoke(obj, value);
			}
			list.add(obj);
		}
	} catch (Exception e) {
		e.printStackTrace();
	}
	return list;
}

主要通過反射生成我們的類物件，這個類的型別定義在 sql 標簽上
時間型別需要判斷后處理，Timestamp，與 java 不是一個型別

7. Sql查詢補充說明

sql 查詢有入參、有不需要入參、有查詢一個、有查詢集合，只需要合理包裝即可，例如下面的查詢集合，入參是物件型別；

ApiLikeTest.test_queryUserList()

@Test
public void test_queryUserList() {
    String resource = "spring/mybatis-config-datasource.xml";
    Reader reader;
    try {
        reader = Resources.getResourceAsReader(resource);
        SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);
        SqlSession session = sqlMapper.openSession();
        
		try {
            User req = new User();
            req.setAge(18);
            List<User> userList = session.selectList("org.itstack.demo.dao.IUserDao.queryUserList", req);
            System.out.println(JSON.toJSONString(userList));
        } finally {
            session.close();
            reader.close();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
	
}

**測驗結果：

[{"age":18,"createTime":1576944000000,"id":1,"name":"水水","updateTime":1576944000000},{"age":18,"createTime":1576944000000,"id":2,"name":"豆豆","updateTime":1576944000000}]

Process finished with exit code 0

四、原始碼分析(mybatis)

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.4.6</version>
</dependency>

Mybatis的整個原始碼還是很大的，以下主要將部分核心內容進行整理分析，以便于后續分析Mybatis與Spring整合的原始碼部分，簡要包括；容器初始化、組態檔決議、Mapper加載與動態代理，

1. 從一個簡單的案例開始

要學習Mybatis原始碼，最好的方式一定是從一個簡單的點進入，而不是從Spring整合開始分析，SqlSessionFactory是整個Mybatis的核心實體物件，SqlSessionFactory物件的實體又通過SqlSessionFactoryBuilder物件來獲得，SqlSessionFactoryBuilder物件可以從XML組態檔加載配置資訊，然后創建SqlSessionFactory，如下例子：

MybatisApiTest.java

public class MybatisApiTest {

    @Test
    public void test_queryUserInfoById() {
        String resource = "spring/mybatis-config-datasource.xml";
        Reader reader;
        try {
            reader = Resources.getResourceAsReader(resource);
            SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);

            SqlSession session = sqlMapper.openSession();
            try {
                User user = session.selectOne("org.itstack.demo.dao.IUserDao.queryUserInfoById", 1L);
                System.out.println(JSON.toJSONString(user));
            } finally {
                session.close();
                reader.close();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

dao/IUserDao.java

public interface IUserDao {

     User queryUserInfoById(Long id);

}

spring/mybatis-config-datasource.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://127.0.0.1:3306/itstack?useUnicode=true"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="mapper/User_Mapper.xml"/>
    </mappers>

</configuration>

如果一切順利，那么會有如下結果：

{"age":18,"createTime":1571376957000,"id":1,"name":"花花","updateTime":1571376957000}

從上面的代碼塊可以看到，核心代碼；SqlSessionFactoryBuilder().build(reader)，負責Mybatis組態檔的加載、決議、構建等職責，直到最終可以通過SqlSession來執行并回傳結果，

2. 容器初始化

從上面代碼可以看到，SqlSessionFactory是通過SqlSessionFactoryBuilder工廠類創建的，而不是直接使用構造器，容器的組態檔加載和初始化流程如下：

微信公眾號：bugstack蟲洞堆疊 & 初始化流程

流程核心類
- SqlSessionFactoryBuilder
- XMLConfigBuilder
- XPathParser
- Configuration

SqlSessionFactoryBuilder.java

public class SqlSessionFactoryBuilder {

  public SqlSessionFactory build(Reader reader) {
    return build(reader, null, null);
  }

  public SqlSessionFactory build(Reader reader, String environment) {
    return build(reader, environment, null);
  }

  public SqlSessionFactory build(Reader reader, Properties properties) {
    return build(reader, null, properties);
  }

  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

  public SqlSessionFactory build(InputStream inputStream) {
    return build(inputStream, null, null);
  }

  public SqlSessionFactory build(InputStream inputStream, String environment) {
    return build(inputStream, environment, null);
  }

  public SqlSessionFactory build(InputStream inputStream, Properties properties) {
    return build(inputStream, null, properties);
  }

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }
    
  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

}

從上面的原始碼可以看到，SqlSessionFactory提供三種方式build構建物件；

位元組流：java.io.InputStream
字符流：java.io.Reader
配置類：org.apache.ibatis.session.Configuration

那么，位元組流、字符流都會創建組態檔決議類：XMLConfigBuilder，并通過parser.parse()生成Configuration，最后呼叫配置類構建方法生成SqlSessionFactory，

XMLConfigBuilder.java

public class XMLConfigBuilder extends BaseBuilder {

  private boolean parsed;
  private final XPathParser parser;
  private String environment;
  private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

  ...
  public XMLConfigBuilder(Reader reader, String environment, Properties props) {
    this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
  }
  ...
}

XMLConfigBuilder對于XML檔案的加載和決議都委托于XPathParser，最終使用JDK自帶的javax.xml進行XML決議(XPath)
XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver)
1. reader：使用字符流創建新的輸入源，用于對XML檔案的讀取
2. validation：是否進行DTD校驗
3. variables：屬性配置資訊
4. entityResolver：Mybatis硬編碼了new XMLMapperEntityResolver()提供XML默認決議器

XMLMapperEntityResolver.java

public class XMLMapperEntityResolver implements EntityResolver {

  private static final String IBATIS_CONFIG_SYSTEM = "ibatis-3-config.dtd";
  private static final String IBATIS_MAPPER_SYSTEM = "ibatis-3-mapper.dtd";
  private static final String MYBATIS_CONFIG_SYSTEM = "mybatis-3-config.dtd";
  private static final String MYBATIS_MAPPER_SYSTEM = "mybatis-3-mapper.dtd";

  private static final String MYBATIS_CONFIG_DTD = "org/apache/ibatis/builder/xml/mybatis-3-config.dtd";
  private static final String MYBATIS_MAPPER_DTD = "org/apache/ibatis/builder/xml/mybatis-3-mapper.dtd";

  /*
   * Converts a public DTD into a local one
   * 
   * @param publicId The public id that is what comes after "PUBLIC"
   * @param systemId The system id that is what comes after the public id.
   * @return The InputSource for the DTD
   * 
   * @throws org.xml.sax.SAXException If anything goes wrong
   */
  @Override
  public InputSource resolveEntity(String publicId, String systemId) throws SAXException {
    try {
      if (systemId != null) {
        String lowerCaseSystemId = systemId.toLowerCase(Locale.ENGLISH);
        if (lowerCaseSystemId.contains(MYBATIS_CONFIG_SYSTEM) || lowerCaseSystemId.contains(IBATIS_CONFIG_SYSTEM)) {
          return getInputSource(MYBATIS_CONFIG_DTD, publicId, systemId);
        } else if (lowerCaseSystemId.contains(MYBATIS_MAPPER_SYSTEM) || lowerCaseSystemId.contains(IBATIS_MAPPER_SYSTEM)) {
          return getInputSource(MYBATIS_MAPPER_DTD, publicId, systemId);
        }
      }
      return null;
    } catch (Exception e) {
      throw new SAXException(e.toString());
    }
  }

  private InputSource getInputSource(String path, String publicId, String systemId) {
    InputSource source = null;
    if (path != null) {
      try {
        InputStream in = Resources.getResourceAsStream(path);
        source = new InputSource(in);
        source.setPublicId(publicId);
        source.setSystemId(systemId);        
      } catch (IOException e) {
        // ignore, null is ok
      }
    }
    return source;
  }

}

Mybatis依賴于dtd檔案進行進行決議，其中的ibatis-3-config.dtd主要是用于兼容用途
getInputSource(String path, String publicId, String systemId)的呼叫里面有兩個引數publicId（公共識別符號）和systemId（系統標示符）

XPathParser.java

public XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver) {
  commonConstructor(validation, variables, entityResolver);
  this.document = createDocument(new InputSource(reader));
}

private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) {
  this.validation = validation;
  this.entityResolver = entityResolver;
  this.variables = variables;
  XPathFactory factory = XPathFactory.newInstance();
  this.xpath = factory.newXPath();
}

private Document createDocument(InputSource inputSource) {
  // important: this must only be called AFTER common constructor
  try {
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    factory.setValidating(validation);
    factory.setNamespaceAware(false);
    factory.setIgnoringComments(true);
    factory.setIgnoringElementContentWhitespace(false);
    factory.setCoalescing(false);
    factory.setExpandEntityReferences(true);
    DocumentBuilder builder = factory.newDocumentBuilder();
    builder.setEntityResolver(entityResolver);
    builder.setErrorHandler(new ErrorHandler() {
      @Override
      public void error(SAXParseException exception) throws SAXException {
        throw exception;
      }
      @Override
      public void fatalError(SAXParseException exception) throws SAXException {
        throw exception;
      }
      @Override
      public void warning(SAXParseException exception) throws SAXException {
      }
    });
    return builder.parse(inputSource);
  } catch (Exception e) {
    throw new BuilderException("Error creating document instance.  Cause: " + e, e);
  }
  
}

從上到下可以看到主要是為了創建一個Mybatis的檔案決議器，最后根據builder.parse(inputSource)回傳Document

得到XPathParser實體后，接下來在呼叫方法：this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);

 XMLConfigBuilder.this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);

 private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
   super(new Configuration());
   ErrorContext.instance().resource("SQL Mapper Configuration");
   this.configuration.setVariables(props);
   this.parsed = false;
   this.environment = environment;
   this.parser = parser;
 }

其中呼叫了父類的建構式

public abstract class BaseBuilder {
  protected final Configuration configuration;
  protected final TypeAliasRegistry typeAliasRegistry;
  protected final TypeHandlerRegistry typeHandlerRegistry;

  public BaseBuilder(Configuration configuration) {
    this.configuration = configuration;
    this.typeAliasRegistry = this.configuration.getTypeAliasRegistry();
    this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();
  }
}

XMLConfigBuilder創建完成后，sqlSessionFactoryBuild呼叫parser.parse()創建Configuration

public class XMLConfigBuilder extends BaseBuilder {    
     public Configuration parse() {
       if (parsed) {
         throw new BuilderException("Each XMLConfigBuilder can only be used once.");
       }
       parsed = true;
       parseConfiguration(parser.evalNode("/configuration"));
       return configuration;
     }
}

3. 組態檔決議

這一部分是整個XML檔案決議和裝載的核心內容，其中包括；

屬性決議propertiesElement
加載settings節點settingsAsProperties
載自定義VFS loadCustomVfs
決議型別別名typeAliasesElement
加載插件pluginElement
加載物件工廠objectFactoryElement
創建物件包裝器工廠objectWrapperFactoryElement
加載反射工廠reflectorFactoryElement
元素設定settingsElement
加載環境配置environmentsElement
資料庫廠商標識加載databaseIdProviderElement
加載型別處理器typeHandlerElement
(核心)加載mapper檔案mapperElement

parseConfiguration(parser.evalNode("/configuration"));

private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      //屬性決議propertiesElement
      propertiesElement(root.evalNode("properties"));
      //加載settings節點settingsAsProperties
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      //加載自定義VFS loadCustomVfs
      loadCustomVfs(settings);
      //決議型別別名typeAliasesElement
      typeAliasesElement(root.evalNode("typeAliases"));
      //加載插件pluginElement
      pluginElement(root.evalNode("plugins"));
      //加載物件工廠objectFactoryElement
      objectFactoryElement(root.evalNode("objectFactory"));
      //創建物件包裝器工廠objectWrapperFactoryElement
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //加載反射工廠reflectorFactoryElement
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      //元素設定
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      //加載環境配置environmentsElement
      environmentsElement(root.evalNode("environments"));
      //資料庫廠商標識加載databaseIdProviderElement
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //加載型別處理器typeHandlerElement
      typeHandlerElement(root.evalNode("typeHandlers"));
      //加載mapper檔案mapperElement
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

所有的root.evalNode()底層都是呼叫XML DOM方法：Object evaluate(String expression, Object item, QName returnType)，運算式引數expression，通過XObject resultObject = eval( expression, item )回傳最終節點內容，可以參考http://mybatis.org/dtd/mybatis-3-config.dtd，如下；

<!ELEMENT configuration (properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?)>
 
<!ELEMENT databaseIdProvider (property*)>
<!ATTLIST databaseIdProvider
type CDATA #REQUIRED
>
 
<!ELEMENT properties (property*)>
<!ATTLIST properties
resource CDATA #IMPLIED
url CDATA #IMPLIED
>
 
<!ELEMENT property EMPTY>
<!ATTLIST property
name CDATA #REQUIRED
value CDATA #REQUIRED
>
 
<!ELEMENT settings (setting+)>
 
<!ELEMENT setting EMPTY>
<!ATTLIST setting
name CDATA #REQUIRED
value CDATA #REQUIRED
>
 
<!ELEMENT typeAliases (typeAlias*,package*)>
 
<!ELEMENT typeAlias EMPTY>
<!ATTLIST typeAlias
type CDATA #REQUIRED
alias CDATA #IMPLIED
>
 
<!ELEMENT typeHandlers (typeHandler*,package*)>
 
<!ELEMENT typeHandler EMPTY>
<!ATTLIST typeHandler
javaType CDATA #IMPLIED
jdbcType CDATA #IMPLIED
handler CDATA #REQUIRED
>
 
<!ELEMENT objectFactory (property*)>
<!ATTLIST objectFactory
type CDATA #REQUIRED
>
 
<!ELEMENT objectWrapperFactory EMPTY>
<!ATTLIST objectWrapperFactory
type CDATA #REQUIRED
>
 
<!ELEMENT reflectorFactory EMPTY>
<!ATTLIST reflectorFactory
type CDATA #REQUIRED
>
 
<!ELEMENT plugins (plugin+)>
 
<!ELEMENT plugin (property*)>
<!ATTLIST plugin
interceptor CDATA #REQUIRED
>
 
<!ELEMENT environments (environment+)>
<!ATTLIST environments
default CDATA #REQUIRED
>
 
<!ELEMENT environment (transactionManager,dataSource)>
<!ATTLIST environment
id CDATA #REQUIRED
>
 
<!ELEMENT transactionManager (property*)>
<!ATTLIST transactionManager
type CDATA #REQUIRED
>
 
<!ELEMENT dataSource (property*)>
<!ATTLIST dataSource
type CDATA #REQUIRED
>
 
<!ELEMENT mappers (mapper*,package*)>
 
<!ELEMENT mapper EMPTY>
<!ATTLIST mapper
resource CDATA #IMPLIED
url CDATA #IMPLIED
class CDATA #IMPLIED
>
 
<!ELEMENT package EMPTY>
<!ATTLIST package
name CDATA #REQUIRED
>

mybatis-3-config.dtd 定義檔案中有11個組態檔，如下；

properties?,
settings?,
typeAliases?,
typeHandlers?,
objectFactory?,
objectWrapperFactory?,
reflectorFactory?,
plugins?,
environments?,
databaseIdProvider?,
mappers?

以上每個配置都是可選，最終配置內容會保存到org.apache.ibatis.session.Configuration，如下；

public class Configuration {

  protected Environment environment;
  // 允許在嵌套陳述句中使用分頁（RowBounds），如果允許使用則設定為false，默認為false
  protected boolean safeRowBoundsEnabled;
  // 允許在嵌套陳述句中使用分頁（ResultHandler），如果允許使用則設定為false，
  protected boolean safeResultHandlerEnabled = true;
  // 是否開啟自動駝峰命名規則（camel case）映射，即從經典資料庫列名 A_COLUMN 到經典 Java 屬性名 aColumn 的類似映射，默認false
  protected boolean mapUnderscoreToCamelCase;
  // 當開啟時，任何方法的呼叫都會加載該物件的所有屬性，否則，每個屬性會按需加載，默認值false (true in ≤3.4.1)
  protected boolean aggressiveLazyLoading;
  // 是否允許單一陳述句回傳多結果集（需要兼容驅動），
  protected boolean multipleResultSetsEnabled = true;
  // 允許 JDBC 支持自動生成主鍵，需要驅動兼容，這就是insert時獲取mysql自增主鍵/oracle sequence的開關，注：一般來說,這是希望的結果,應該默認值為true比較合適，
  protected boolean useGeneratedKeys;
  // 使用列標簽代替列名,一般來說,這是希望的結果
  protected boolean useColumnLabel = true;
  // 是否啟用快取 {默認是開啟的，可能這也是你的面試題}
  protected boolean cacheEnabled = true;
  // 指定當結果集中值為 null 的時候是否呼叫映射物件的 setter（map 物件時為 put）方法，這對于有 Map.keySet() 依賴或 null 值初始化的時候是有用的，
  protected boolean callSettersOnNulls;
  // 允許使用方法簽名中的名稱作為陳述句引數名稱， 為了使用該特性，你的工程必須采用Java 8編譯，并且加上-parameters選項，（從3.4.1開始）
  protected boolean useActualParamName = true;
  //當回傳行的所有列都是空時，MyBatis默認回傳null， 當開啟這個設定時，MyBatis會回傳一個空實體， 請注意，它也適用于嵌套的結果集 (i.e. collectioin and association)，（從3.4.2開始） 注：這里應該拆分為兩個引數比較合適, 一個用于結果集，一個用于單記錄，通常來說，我們會希望結果集不是null，單記錄仍然是null
  protected boolean returnInstanceForEmptyRow;
  // 指定 MyBatis 增加到日志名稱的前綴，
  protected String logPrefix;
  // 指定 MyBatis 所用日志的具體實作，未指定時將自動查找，一般建議指定為slf4j或log4j
  protected Class <? extends Log> logImpl;
   // 指定VFS的實作, VFS是mybatis提供的用于訪問AS內資源的一個簡便介面
  protected Class <? extends VFS> vfsImpl;
  // MyBatis 利用本地快取機制（Local Cache）防止回圈參考（circular references）和加速重復嵌套查詢， 默認值為 SESSION，這種情況下會快取一個會話中執行的所有查詢， 若設定值為 STATEMENT，本地會話僅用在陳述句執行上，對相同 SqlSession 的不同呼叫將不會共享資料，
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  // 當沒有為引數提供特定的 JDBC 型別時，為空值指定 JDBC 型別， 某些驅動需要指定列的 JDBC 型別，多數情況直接用一般型別即可，比如 NULL、VARCHAR 或 OTHER，
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  // 指定物件的哪個方法觸發一次延遲加載，
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
  // 設定超時時間，它決定驅動等待資料庫回應的秒數，默認不超時
  protected Integer defaultStatementTimeout;
  // 為驅動的結果集設定默認獲取數量，
  protected Integer defaultFetchSize;
  // SIMPLE 就是普通的執行器；REUSE 執行器會重用預處理陳述句（prepared statements）； BATCH 執行器將重用陳述句并執行批量更新，
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  // 指定 MyBatis 應如何自動映射列到欄位或屬性， NONE 表示取消自動映射；PARTIAL 只會自動映射沒有定義嵌套結果集映射的結果集， FULL 會自動映射任意復雜的結果集（無論是否嵌套），
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
  // 指定發現自動映射目標未知列（或者未知屬性型別）的行為，這個值應該設定為WARNING比較合適
  protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior = AutoMappingUnknownColumnBehavior.NONE;
  // settings下的properties屬性
  protected Properties variables = new Properties();
  // 默認的反射器工廠,用于操作屬性、構造器方便
  protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
  // 物件工廠, 所有的類resultMap類都需要依賴于物件工廠來實體化
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  // 物件包裝器工廠,主要用來在創建非原生物件,比如增加了某些監控或者特殊屬性的代理類
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
  // 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，
  protected boolean lazyLoadingEnabled = false;
  // 指定 Mybatis 創建具有延遲加載能力的物件所用到的代理工具，MyBatis 3.3+使用JAVASSIST
  protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL
  // MyBatis 可以根據不同的資料庫廠商執行不同的陳述句，這種多廠商的支持是基于映射陳述句中的 databaseId 屬性，
  protected String databaseId;
  ...
}

以上可以看到，Mybatis把所有的配置；resultMap、Sql陳述句、插件、快取等都維護在Configuration中，這里還有一個小技巧，在Configuration還有一個StrictMap內部類，它繼承于HashMap完善了put時防重、get時取不到值的例外處理，如下；

protected static class StrictMap<V> extends HashMap<String, V> {

    private static final long serialVersionUID = -4950446264854982944L;
    private final String name;

    public StrictMap(String name, int initialCapacity, float loadFactor) {
      super(initialCapacity, loadFactor);
      this.name = name;
    }

    public StrictMap(String name, int initialCapacity) {
      super(initialCapacity);
      this.name = name;
    }

    public StrictMap(String name) {
      super();
      this.name = name;
    }

    public StrictMap(String name, Map<String, ? extends V> m) {
      super(m);
      this.name = name;
    }
}

(核心)加載mapper檔案mapperElement

Mapper檔案處理是Mybatis框架的核心服務，所有的SQL陳述句都撰寫在Mapper中，這塊也是我們分析的重點，其他模塊可以后續講解，

XMLConfigBuilder.parseConfiguration()->mapperElement(root.evalNode(“mappers”));

private void mapperElement(XNode parent) throws Exception {
   if (parent != null) {
     for (XNode child : parent.getChildren()) {
       // 如果要同時使用package自動掃描和通過mapper明確指定要加載的mapper，一定要確保package自動掃描的范圍不包含明確指定的mapper，否則在通過package掃描的interface的時候，嘗試加載對應xml檔案的loadXmlResource()的邏輯中出現判重出錯，報org.apache.ibatis.binding.BindingException例外，即使xml檔案中包含的內容和mapper介面中包含的陳述句不重復也會出錯，包括加載mapper介面時自動加載的xml mapper也一樣會出錯，
       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.");
         }
       }
     }
   }
}

Mybatis提供了兩類配置Mapper的方法，第一類是使用package自動搜索的模式，這樣指定package下所有介面都會被注冊為mapper，也是在Spring中比較常用的方式，例如：
```
<mappers>
  <package name="org.itstack.demo"/>
</mappers>
```

另外一類是明確指定Mapper，這又可以通過resource、url或者class進行細分，例如；

<mappers>
    <mapper resource="mapper/User_Mapper.xml"/>
    <mapper class=""/>
    <mapper url=""/>
</mappers>

4. Mapper加載與動態代理

通過package方式自動搜索加載，生成對應的mapper代理類，代碼塊和流程，如下；

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 {
        ...
      }
    }
  }
}

微信公眾號：bugstack蟲洞堆疊 & 動態代理程序

Mapper加載到生成代理物件的流程中，主要的核心類包括；

XMLConfigBuilder
Configuration
MapperRegistry
MapperAnnotationBuilder
MapperProxyFactory

MapperRegistry.java

決議加載Mapper

public void addMappers(String packageName, Class<?> superType) {
  // mybatis框架提供的搜索classpath下指定package以及子package中符合條件(注解或者繼承于某個類/介面)的類，默認使用Thread.currentThread().getContextClassLoader()回傳的加載器,和spring的工具類殊途同歸，
  ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();   
  // 無條件的加載所有的類,因為呼叫方傳遞了Object.class作為父類,這也給以后的指定mapper介面預留了余地
  resolverUtil.find(new ResolverUtil.IsA(superType), packageName); 
 // 所有匹配的calss都被存盤在ResolverUtil.matches欄位中
  Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
  for (Class<?> mapperClass : mapperSet) {   
    //呼叫addMapper方法進行具體的mapper類/介面決議
    addMapper(mapperClass);
  }
}

生成代理類：MapperProxyFactory

public <T> void addMapper(Class<T> type) {    
  // 對于mybatis mapper介面檔案，必須是interface，不能是class
  if (type.isInterface()) {
    if (hasMapper(type)) {
      throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
    }
    boolean loadCompleted = false;
    try {      
      // 為mapper介面創建一個MapperProxyFactory代理
      knownMappers.put(type, new MapperProxyFactory<T>(type));
      // It's important that the type is added before the parser is run
      // otherwise the binding may automatically be attempted by the
      // mapper parser. If the type is already known, it won't try.
      MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
      parser.parse();
      loadCompleted = true;
    } finally {
      if (!loadCompleted) {
        knownMappers.remove(type);
      }
    }
  }
}

在MapperRegistry中維護了介面類與代理工程的映射關系，knownMappers；

private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

MapperProxyFactory.java

public class MapperProxyFactory<T> {
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }
  public Class<T> getMapperInterface() {
    return mapperInterface;
  }
  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }
  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }
}

如上是Mapper的代理類工程，建構式中的mapperInterface就是對應的介面類，當實體化時候會獲得具體的MapperProxy代理，里面主要包含了SqlSession，

五、原始碼分析(mybatis-spring)

<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis-spring</artifactId>
    <version>1.3.2</version>
</dependency>

作為一款好用的ORM框架，一定是蘿莉臉(單純)、御姐心(強大)，鋪的了床(屏蔽與JDBC直接打交道)、暖的了房(速度性能好)！鑒于這些優點幾乎在國內互聯網大部分開發框架都會使用到Mybatis，尤其在一些需要高性能的場景下需要優化sql那么一定需要手寫sql在xml中，那么，準備好了嗎！開始分析分析它的原始碼；

1. 從一個簡單的案例開始

與分析mybatis原始碼一樣，先做一個簡單的案例；定義dao、撰寫組態檔、junit單元測驗；

SpringApiTest.java

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:spring-config.xml")
public class SpringApiTest {

    private Logger logger = LoggerFactory.getLogger(SpringApiTest.class);

    @Resource
    private ISchoolDao schoolDao;
    @Resource
    private IUserDao userDao;

    @Test
    public void test_queryRuleTreeByTreeId(){
        School ruleTree = schoolDao.querySchoolInfoById(1L);
        logger.info(JSON.toJSONString(ruleTree));

        User user = userDao.queryUserInfoById(1L);
        logger.info(JSON.toJSONString(user));
    }

}

spring-config-datasource.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">

    <!-- 1.資料庫連接池： DriverManagerDataSource 也可以使用DBCP2-->
    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="driverClassName" value="${db.jdbc.driverClassName}"/>
        <property name="url" value="${db.jdbc.url}"/>
        <property name="username" value="${db.jdbc.username}"/>
        <property name="password" value="${db.jdbc.password}"/>
    </bean>

    <!-- 2.配置SqlSessionFactory物件 -->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <!-- 注入資料庫連接池 -->
        <property name="dataSource" ref="dataSource"/>
        <!-- 配置MyBaties全域組態檔:mybatis-config.xml -->
        <property name="configLocation" value="classpath:mybatis-config.xml"/>
        <!-- 掃描entity包 使用別名 -->
        <property name="typeAliasesPackage" value="org.itstack.demo.po"/>
        <!-- 掃描sql組態檔:mapper需要的xml檔案 -->
        <property name="mapperLocations" value="classpath:mapper/*.xml"/>
    </bean>

    <!-- 3.配置掃描Dao介面包，動態實作Dao介面，注入到spring容器中 -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <!-- 注入sqlSessionFactory -->
        <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
        <!-- 給出需要掃描Dao介面包，多個逗號隔開 -->
        <property name="basePackage" value="org.itstack.demo.dao"/>
    </bean>
              
</beans>

如果一切順利，那么會有如下結果：

{"address":"北京市海淀區頤和園路5號","createTime":1571376957000,"id":1,"name":"北京大學","updateTime":1571376957000}
{"age":18,"createTime":1571376957000,"id":1,"name":"花花","updateTime":1571376957000}

從上面單元測驗的代碼可以看到，兩個沒有方法體的注解就這么神奇的執行了我們的xml中的配置陳述句并輸出了結果，其實主要得益于以下兩個類；

org.mybatis.spring.SqlSessionFactoryBean
org.mybatis.spring.mapper.MapperScannerConfigurer

2. 掃描裝配注冊(MapperScannerConfigurer)

MapperScannerConfigurer為整個Dao介面層生成動態代理類注冊，啟動到了核心作用，這個類實作了如下介面，用來對掃描的Mapper進行處理：

BeanDefinitionRegistryPostProcessor
InitializingBean
ApplicationContextAware
BeanNameAware

整體類圖如下；

微信公眾號：bugstack蟲洞堆疊 & MapperScannerConfigurer類圖

執行流程如下；

微信公眾號：bugstack蟲洞堆疊 & 執行流程圖

上面的類圖+流程圖，其實已經很清楚的描述了MapperScannerConfigurer初始化程序，但對于頭一次看的新人來說依舊是我太難了，好繼續！

MapperScannerConfigurer.java & 部分截取

@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
  if (this.processPropertyPlaceHolders) {
    processPropertyPlaceHolders();
  }
  ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
  scanner.setAddToConfig(this.addToConfig);
  scanner.setAnnotationClass(this.annotationClass);
  scanner.setMarkerInterface(this.markerInterface);
  scanner.setSqlSessionFactory(this.sqlSessionFactory);
  scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
  scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
  scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
  scanner.setResourceLoader(this.applicationContext);
  scanner.setBeanNameGenerator(this.nameGenerator);
  scanner.registerFilters();
  scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}

實作了BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry用于注冊Bean到Spring容器中
306行：new ClassPathMapperScanner(registry); 硬編碼類路徑掃描器，用于決議Mybatis的Mapper檔案
317行：scanner.scan 對Mapper進行掃描，這里包含了一個繼承類實作關系的呼叫，也就是本文開頭的測驗題，

ClassPathMapperScanner.java & 部分截取

@Override
public Set<BeanDefinitionHolder> doScan(String... basePackages) {
  Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
  if (beanDefinitions.isEmpty()) {
    logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
  } else {
    processBeanDefinitions(beanDefinitions);
  }
  return beanDefinitions;
}

優先呼叫父類的super.doScan(basePackages);進行注冊Bean資訊

ClassPathBeanDefinitionScanner.java & 部分截取

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
	Assert.notEmpty(basePackages, "At least one base package must be specified");
	Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
	for (String basePackage : basePackages) {
		Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
		for (BeanDefinition candidate : candidates) {
			ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
			candidate.setScope(scopeMetadata.getScopeName());
			String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
			if (candidate instanceof AbstractBeanDefinition) {
				postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
			}
			if (candidate instanceof AnnotatedBeanDefinition) {
				AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate)
			}
			if (checkCandidate(beanName, candidate)) {
				BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
				definitionHolder =
						AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.regi
				beanDefinitions.add(definitionHolder);
				registerBeanDefinition(definitionHolder, this.registry);
			}
		}
	}
	return beanDefinitions;
}

優先呼叫了父類的doScan方法，用于Mapper掃描和Bean的定義以及注冊到DefaultListableBeanFactory，｛DefaultListableBeanFactory是Spring中IOC容器的始祖，所有需要實體化的類都需要注冊進來，之后在初始化｝
272行：findCandidateComponents(basePackage)，掃描package包路徑，對于注解類的有另外的方式，大同小異
288行：registerBeanDefinition(definitionHolder, this.registry);注冊Bean資訊的程序，最侄訓呼叫到：org.springframework.beans.factory.support.DefaultListableBeanFactory

ClassPathMapperScanner.java & 部分截取

**processBeanDefinitions(beanDefinitions);**

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
  GenericBeanDefinition definition;
  for (BeanDefinitionHolder holder : beanDefinitions) {
    definition = (GenericBeanDefinition) holder.getBeanDefinition();
    if (logger.isDebugEnabled()) {
      logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() 
        + "' and '" + definition.getBeanClassName() + "' mapperInterface");
    }
    // the mapper interface is the original class of the bean
    // but, the actual class of the bean is MapperFactoryBean
    definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
    definition.setBeanClass(this.mapperFactoryBean.getClass());
    definition.getPropertyValues().add("addToConfig", this.addToConfig);
    boolean explicitFactoryUsed = false;
    if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
      definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
      explicitFactoryUsed = true;
    } else if (this.sqlSessionFactory != null) {
      definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
      explicitFactoryUsed = true;
    }
    if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
      if (explicitFactoryUsed) {
        logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
      }
      definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
      explicitFactoryUsed = true;
    } else if (this.sqlSessionTemplate != null) {
      if (explicitFactoryUsed) {
        logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
      }
      definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
      explicitFactoryUsed = true;
    }
    if (!explicitFactoryUsed) {
      if (logger.isDebugEnabled()) {
        logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
      }
      definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
    }
  }
}

163行：super.doScan(basePackages);，呼叫完父類方法后開始執行內部方法：processBeanDefinitions(beanDefinitions)
186行：definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); 設定BeanName引數，也就是我們的：ISchoolDao、IUserDao
187行：definition.setBeanClass(this.mapperFactoryBean.getClass());，設定BeanClass，介面本身是沒有類的，那么這里將MapperFactoryBean類設定進來，最終所有的dao層介面類都是這個MapperFactoryBean

MapperFactoryBean.java & 部分截取

這個類有繼承也有介面實作，最好先了解下整體類圖，如下；

微信公眾號：bugstack蟲洞堆疊 & MapperFactoryBean類圖

這個類就非常重要了，最終所有的sql資訊執行都會通過這個類獲取getObject()，也就是SqlSession獲取mapper的代理類：MapperProxyFactory->MapperProxy

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  private Class<T> mapperInterface;

  private boolean addToConfig = true;

  public MapperFactoryBean() {
    //intentionally empty 
  }
  
  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**  
   * 當SpringBean容器初始化時候會呼叫到checkDaoConfig()，他是繼承類中的抽象方法
   * {@inheritDoc}
   */
  @Override
  protected void checkDaoConfig() {
    super.checkDaoConfig();

    notNull(this.mapperInterface, "Property 'mapperInterface' is required");

    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        configuration.addMapper(this.mapperInterface);
      } catch (Exception e) {
        logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
        throw new IllegalArgumentException(e);
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

  ...
}

72行：checkDaoConfig()，當SpringBean容器初始化時候會呼叫到checkDaoConfig()，他是繼承類中的抽象方法

95行：getSqlSession().getMapper(this.mapperInterface);，通過介面獲取Mapper(代理類)，呼叫程序如下；

DefaultSqlSession.getMapper(Class type)，獲取Mapper
Configuration.getMapper(Class type, SqlSession sqlSession)，從配置中獲取

MapperRegistry.getMapper(Class type, SqlSession sqlSession)，從注冊中心獲取到實體化生成

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
  final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
  if (mapperProxyFactory == null) {
    throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
  }
  try {
    return mapperProxyFactory.newInstance(sqlSession);
  } catch (Exception e) {
    throw new BindingException("Error getting mapper instance. Cause: " + e, e);
  }
}

mapperProxyFactory.newInstance(sqlSession);，通過反射工程生成MapperProxy

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
  final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
  return newInstance(mapperProxy);
}

MapperProxy.java & 部分截取

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

  @UsesJava7
  private Object invokeDefaultMethod(Object proxy, Method method, Object[] args)
      throws Throwable {
    final Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
        .getDeclaredConstructor(Class.class, int.class);
    if (!constructor.isAccessible()) {
      constructor.setAccessible(true);
    }
    final Class<?> declaringClass = method.getDeclaringClass();
    return constructor
        .newInstance(declaringClass,
            MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
                | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC)
        .unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
  }

  ...
}

58行：final MapperMethod mapperMethod = cachedMapperMethod(method);，從快取中獲取MapperMethod

59行：mapperMethod.execute(sqlSession, args);，執行SQL陳述句，并回傳結果(到這關于查詢獲取結果就到骨頭(干)層了)；INSERT、UPDATE、DELETE、SELECT

public Object execute(SqlSession sqlSession, Object[] args) {
  Object result;
  switch (command.getType()) {
    case INSERT: {
    Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
      break;
    }
    case UPDATE: {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
      break;
    }
    case DELETE: {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
      break;
    }
    case SELECT:
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else if (method.returnsCursor()) {
        result = executeForCursor(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
      break;
    case FLUSH:
      result = sqlSession.flushStatements();
      break;
    default:
      throw new BindingException("Unknown execution method for: " + command.getName());
  }
  if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
    throw new BindingException("Mapper method '" + command.getName() 
        + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
  }
  return result;
}

以上對于MapperScannerConfigurer這一層就分析完了，從掃描定義注入到為Spring容器準備Bean的資訊，代理、反射、SQL執行，基本就包括全部核心內容了，接下來在分析下SqlSessionFactoryBean

3. SqlSession容器工廠初始化(SqlSessionFactoryBean)

SqlSessionFactoryBean初始化程序中需要對一些自身內容進行處理，因此也需要實作如下介面；

FactoryBean
InitializingBean -> void afterPropertiesSet() throws Exception
ApplicationListener

微信公眾號：bugstack蟲洞堆疊 & SqlSessionFactoryBean初始化流程

以上的流程其實已經很清晰的描述整個核心流程，但同樣對于新手上路會有障礙，那么！好，繼續！

SqlSessionFactoryBean.java & 部分截取

public void afterPropertiesSet() throws Exception {
  notNull(dataSource, "Property 'dataSource' is required");
  notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
  state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
            "Property 'configuration' and 'configLocation' can not specified with together");
  this.sqlSessionFactory = buildSqlSessionFactory();
}

afterPropertiesSet()，InitializingBean介面為bean提供了初始化方法的方式，它只包括afterPropertiesSet方法，凡是繼承該介面的類，在初始化bean的時候都會執行該方法，
380行：buildSqlSessionFactory();內部方法構建，核心功能繼續往下看，

SqlSessionFactoryBean.java & 部分截取

protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
  Configuration configuration;
  XMLConfigBuilder xmlConfigBuilder = null;
  
  ...

  if (!isEmpty(this.mapperLocations)) {
    for (Resource mapperLocation : this.mapperLocations) {
      if (mapperLocation == null) {
        continue;
      }
      try {
        XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
            configuration, mapperLocation.toString(), configuration.getSqlFragments());
        xmlMapperBuilder.parse();
      } catch (Exception e) {
        throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
      } finally {
        ErrorContext.instance().reset();
      }
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
      }
    }
  } else {
    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
    }
  }
  return this.sqlSessionFactoryBuilder.build(configuration);
}

513行：for (Resource mapperLocation : this.mapperLocations) 回圈決議Mapper內容
519行：XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(…) 決議XMLMapperBuilder
521行：xmlMapperBuilder.parse() 執行決議，具體如下；

XMLMapperBuilder.java & 部分截取

public class XMLMapperBuilder extends BaseBuilder {
   private final XPathParser parser;
   private final MapperBuilderAssistant builderAssistant;
   private final Map<String, XNode> sqlFragments;
   private final String resource;

   private void bindMapperForNamespace() {
     String namespace = builderAssistant.getCurrentNamespace();
     if (namespace != null) {
       Class<?> boundType = null;
       try {
         boundType = Resources.classForName(namespace);
       } catch (ClassNotFoundException e) {
         //ignore, bound type is not required
       }
       if (boundType != null) {
         if (!configuration.hasMapper(boundType)) {
           // Spring may not know the real resource name so we set a flag
           // to prevent loading again this resource from the mapper interface
           // look at MapperAnnotationBuilder#loadXmlResource
           configuration.addLoadedResource("namespace:" + namespace);
           configuration.addMapper(boundType);
         }
       }
     }
   }
}

這里413行非常重要，configuration.addMapper(boundType);，真正到了添加Mapper到配置中心

MapperRegistry.java & 部分截取

public class MapperRegistry {

  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }
  
}

67行：創建代理工程knownMappers.put(type, new MapperProxyFactory(type));

截至到這，MapperScannerConfigurer、SqlSessionFactoryBean，兩個類干的事情就相融合了；

第一個用于掃描Dao介面設定代理類注冊到IOC中，用于后續生成Bean物體類，MapperFactoryBean，并可以通過mapperInterface從Configuration獲取Mapper
另一個用于生成SqlSession工廠初始化，決議Mapper里的XML配置進行動態代理MapperProxyFactory->MapperProxy注入到Configuration的Mapper
最終在注解類的幫助下進行方法注入，等執行操作時候即可獲得動態代理物件，從而執行相應的CRUD操作
```
@Resource
private ISchoolDao schoolDao;

schoolDao.querySchoolInfoById(1L);
```

六、綜上總結

分析程序較長篇幅也很大，不一定一天就能看懂整個流程，但當耐下心來一點點研究，還是可以獲得很多的識訓的，以后在遇到這類的例外就可以迎刃而解了，同時也有助于面試、招聘！
之所以分析Mybatis最開始是想在Dao上加自定義注解，發現切面攔截不到，想到這是被動態代理的類，之后層層往往下扒直到MapperProxy.invoke！當然，Mybatis提供了自定義插件開發，
以上的原始碼分析只是對部分核心內容進行分析，如果希望了解全部可以參考資料；MyBatis 3原始碼深度決議，并除錯代碼，IDEA中還是很方便看原始碼的，包括可以查看類圖、呼叫順序等，
mybatis、mybatis-spring中其實最重要的是將Mapper組態檔決議與介面類組裝成代理類進行映射，以此來方便對資料庫的CRUD操作，從原始碼分析后，可以獲得更多的編程經驗(套路)，
Mybatis相關鏈接；
- https://github.com/mybatis/mybatis-3
- https://mybatis.org/mybatis-3/zh/index.html

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