Mybatis源码解析之Mybatis初始化过程

一、搭建一个简单的Mybatis工程

为了了解Mybatis的初始化过程，这里需要搭建一个简单的Mybatis工程操作数据库，工程结构如下：

一个UserBean.java

private int id;
    private String username;
    private String password;
    private int age;
    public UserBean(String username, String password, int age) {
        super();
        this.username = username;
        this.password = password;
        this.age = age;
    }

操作数据库接口以及对应的Mapper

public interface UserMapper {

    void insertUser(UserBean user);

}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zjp.mapper.UserMapper">

    <insert id="insertUser" parameterType="userBean">
        insert into user_t (id,user_name,password,age) values(#{id},#{username},#{password},#{age})
    </insert>

</mapper>

mybatis配置文件mybatis-config以及jdbc配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <!-- 引入外部配置文件 -->
    <properties resource="jdbc.properties">
        <property name="dialect" value="mysql" />
    </properties>
    <settings>
        <setting name="logImpl" value="LOG4J" />
    </settings>

    <typeAliases>
        <typeAlias type="com.zjp.bean.UserBean" alias="userBean" />
    </typeAliases>

    <!-- 配置mybatis运行环境 -->
    <environments default="cybatis">
        <environment id="cybatis">
            <!-- type="JDBC" 代表使用JDBC的提交和回滚来管理事务 -->
            <transactionManager type="JDBC" />

            <!-- mybatis提供了3种数据源类型，分别是：POOLED,UNPOOLED,JNDI -->
            <!-- POOLED 表示支持JDBC数据源连接池 -->
            <!-- UNPOOLED 表示不支持数据源连接池 -->
            <!-- JNDI 表示支持外部数据源连接池 -->
            <dataSource type="POOLED">
                <property name="driver" value="${driver}" />
                <property name="url" value="${url}" />
                <property name="username" value="${username}" />
                <property name="password" value="${password}" />
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <!-- 映射文件方式1，一个一个的配置-->
        <mapper resource="com/zjp/mapper/UserBeanMapper.xml" /> 
    </mappers>
</configuration>

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useUnicode=true&characterEncoding=utf8
username=root
password=root
#\u5B9A\u4E49\u521D\u59CB\u8FDE\u63A5\u6570  
initialSize=0  
#\u5B9A\u4E49\u6700\u5927\u8FDE\u63A5\u6570  
maxActive=20  
#\u5B9A\u4E49\u6700\u5927\u7A7A\u95F2  
maxIdle=20  
#\u5B9A\u4E49\u6700\u5C0F\u7A7A\u95F2  
minIdle=1  
#\u5B9A\u4E49\u6700\u957F\u7B49\u5F85\u65F6\u95F4  
maxWait=60000  

这样就完成了一个简单的mybatis工程搭建，然后新建一个main方法对数据库进行操作

package com.zjp;

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

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 com.zjp.bean.UserBean;
import com.zjp.mapper.UserMapper;

public class Test {

    public static void main(String[] args) throws IOException {
        // 使用MyBatis提供的Resources类加载mybatis的配置文件
        Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
        // 构建sqlSession的工厂
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        SqlSession session = sqlSessionFactory.openSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        UserBean user = new UserBean("张三", "123456", 7);
        try {
            mapper.insertUser(user);
            System.out.println(user.toString());
            session.commit();
        } catch (Exception e) {
            e.printStackTrace();
            session.rollback();
        }
    }
}

二、Mybatis配置文件初始化过程

在Main方法中前面两行代码就是配置文件的初始化过程

1、创建SqlSessionFactoryBuilder对象

// 使用MyBatis提供的Resources类加载mybatis的配置文件
Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
// 构建sqlSession的工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);

第一步是获取配置文件的reader，然后获取SqlSessionFactory ，SqlSessionFactory是MyBatis的关键对象,它是个单个数据库映射关系经过编译后的内存镜像通过源码可知SqlSessionFactory 是接口，

public interface SqlSessionFactory {

  //8个方法可以用来创建SqlSession实例
  SqlSession openSession();

  //自动提交
  SqlSession openSession(boolean autoCommit);
  //连接
  SqlSession openSession(Connection connection);
  //事务隔离级别
  SqlSession openSession(TransactionIsolationLevel level);

  //执行器的类型
  SqlSession openSession(ExecutorType execType);
  SqlSession openSession(ExecutorType execType, boolean autoCommit);
  SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
  SqlSession openSession(ExecutorType execType, Connection connection);

  Configuration getConfiguration();

}

SqlSessionFactoryBuilder就是创建，通过build方法对配置文件进行解析并初始化，通过源码可知build方法有很多重载，
通过源码可知，所有的build方法最后都是通过

public SqlSessionFactory build(Reader reader, String environment, Properties properties)

这方法来执行，所以SqlSessionFactoryBuilder的功能就是对输入的配置文件流进行解析最后生成SqlSessionFactory 。

2 创建SqlSessionFactory

进入最终都需要进入的build方法

/**
     * 第4种方法是最常用的，它使用了一个参照了XML文档或更特定的SqlMapConfig.xml文件的Reader实例。
     * 可选的参数是environment和properties。Environment决定加载哪种环境(开发环境/生产环境)，包括数据源和事务管理器。
     * 如果使用properties，那么就会加载那些properties（属性配置文件），那些属性可以用${propName}语法形式多次用在配置文件中
     */
    public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
        try {
            // 委托XMLConfigBuilder来解析xml文件，并构建
            XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
            return build(parser.parse());
        } catch (Exception e) {
            // 这里是捕获异常，包装成自己的异常并抛出的idiom？，最后还要reset ErrorContext
            throw ExceptionFactory.wrapException("Error building SqlSession.", e);
        } finally {
            ErrorContext.instance().reset();
            try {
                reader.close();
            } catch (IOException e) {
                // Intentionally ignore. Prefer previous error.
            }
        }
    }

build函数首先会构造一个XMLConfigBuilder对象，从名字上可以看出来，该对象是用来解析XML配置文件的。通过XMLConfigBuilder将配置信息转换成paser

//构造函数，转换成XPathParser再去调用构造函数
  public XMLConfigBuilder(Reader reader, String environment, Properties props) {
    //构造一个需要验证，XMLMapperEntityResolver的XPathParser
    this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
  }

 //上面6个构造函数最后都合流到这个函数，传入XPathParser
  private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
    //首先调用父类初始化Configuration
    super(new Configuration());
    //错误上下文设置成SQL Mapper Configuration(XML文件配置),以便后面出错了报错用吧
    ErrorContext.instance().resource("SQL Mapper Configuration");
    //将Properties全部设置到Configuration里面去
    this.configuration.setVariables(props);
    this.parsed = false;
    this.environment = environment;
    this.parser = parser;
  }

在这里有一个很重要的类Configuration，最后所有的配置信息都会封装到这个类里面，这个里面拥有很多参数

//环境
  protected Environment environment;

  //---------以下都是<settings>节点-------
  protected boolean safeRowBoundsEnabled = false;
  protected boolean safeResultHandlerEnabled = true;
  protected boolean mapUnderscoreToCamelCase = false;
  protected boolean aggressiveLazyLoading = true;
  protected boolean multipleResultSetsEnabled = true;
  protected boolean useGeneratedKeys = false;
  protected boolean useColumnLabel = true;
  //默认启用缓存
  protected boolean cacheEnabled = true;
  protected boolean callSettersOnNulls = false;

  protected String logPrefix;
  protected Class <? extends Log> logImpl;
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
  protected Integer defaultStatementTimeout;
  //默认为简单执行器
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
  //---------以上都是<settings>节点-------

  protected Properties variables = new Properties();
  //对象工厂和对象包装器工厂
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
  //映射注册机
  protected MapperRegistry mapperRegistry = new MapperRegistry(this);

  //默认禁用延迟加载
  protected boolean lazyLoadingEnabled = false;
  protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL

  protected String databaseId;
  /**
   * Configuration factory class.
   * Used to create Configuration for loading deserialized unread properties.
   *
   * @see <a href='https://code.google.com/p/mybatis/issues/detail?id=300'>Issue 300</a> (google code)
   */
  protected Class<?> configurationFactory;

  protected final InterceptorChain interceptorChain = new InterceptorChain();
  //类型处理器注册机
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry();
  //类型别名注册机
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();

  //映射的语句,存在Map里
  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");
  //缓存,存在Map里
  protected final Map<String, Cache> caches = new StrictMap<Cache>("Caches collection");
  //结果映射,存在Map里
  protected final Map<String, ResultMap> resultMaps = new StrictMap<ResultMap>("Result Maps collection");
  protected final Map<String, ParameterMap> parameterMaps = new StrictMap<ParameterMap>("Parameter Maps collection");
  protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<KeyGenerator>("Key Generators collection");

  protected final Set<String> loadedResources = new HashSet<String>();
  protected final Map<String, XNode> sqlFragments = new StrictMap<XNode>("XML fragments parsed from previous mappers");

  //不完整的SQL语句
  protected final Collection<XMLStatementBuilder> incompleteStatements = new LinkedList<XMLStatementBuilder>();
  protected final Collection<CacheRefResolver> incompleteCacheRefs = new LinkedList<CacheRefResolver>();
  protected final Collection<ResultMapResolver> incompleteResultMaps = new LinkedList<ResultMapResolver>();
  protected final Collection<MethodResolver> incompleteMethods = new LinkedList<MethodResolver>();

  /*
   * A map holds cache-ref relationship. The key is the namespace that
   * references a cache bound to another namespace and the value is the
   * namespace which the actual cache is bound to.
   */
  protected final Map<String, String> cacheRefMap = new HashMap<String, String>();

  public Configuration(Environment environment) {
    this();
    this.environment = environment;
  }

3、解析配置文件

获取到了XMLConfigBuilder之后就可以对配置进行解析了

//解析配置
  public Configuration parse() {
    //如果已经解析过了，报错
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //根节点是configuration
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

解析配置文件的重要逻辑

//解析配置
  private void parseConfiguration(XNode root) {
    try {
      //分步骤解析
      //issue #117 read properties first
      //1.properties
      propertiesElement(root.evalNode("properties"));
      //2.类型别名
      typeAliasesElement(root.evalNode("typeAliases"));
      //3.插件
      pluginElement(root.evalNode("plugins"));
      //4.对象工厂
      objectFactoryElement(root.evalNode("objectFactory"));
      //5.对象包装工厂
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //6.设置
      settingsElement(root.evalNode("settings"));
      // read it after objectFactory and objectWrapperFactory issue #631
      //7.环境
      environmentsElement(root.evalNode("environments"));
      //8.databaseIdProvider
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //9.类型处理器
      typeHandlerElement(root.evalNode("typeHandlers"));
      //10.映射器
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

通过上面的代码就可以明确的看出来，mybatis对配置文件的每个不同节点的解析过程，由于解析的节点太多，平时在开发接触较多的properties、typeAliases、environments、mappers

3.1properties的解析

假如有一个这样的节点需要解析

 <properties resource="jdbc.properties">
      <property name="username" value="root"/>
      <property name="password" value="root"/>
  </properties>

解析过程

 private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      //传入方式是调用构造函数时传入，public XMLConfigBuilder(Reader reader, String environment, Properties props)

      //1.XNode.getChildrenAsProperties函数方便得到孩子所有Properties
      Properties defaults = context.getChildrenAsProperties();
      //2.然后查找resource或者url,加入前面的Properties
      String resource = context.getStringAttribute("resource");
      String url = context.getStringAttribute("url");
      // resource和url不能同时存在
      if (resource != null && url != null) {
        throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");
      }
      if (resource != null) {
          // 将所有的配置信息加载到Properties中
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      //3.Variables也全部加入Properties
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      parser.setVariables(defaults);
      configuration.setVariables(defaults);
    }
  }

• 获取到下面的所有子节点信息并将这些信息放到Properties 中；
• 然后读取resource或者url中的配置加入到Properties 中；

• 最后将所有的配置信息保存到configuration中

  如果在这些地方,属性多于一个的话,MyBatis 按照如下的顺序加载它们:
  1.在 properties 元素体内指定的属性首先被读取。
  2.从类路径下资源或 properties 元素的 url 属性中加载的属性第二被读取,它会覆盖已经存在的完全一样的属性。
  3.作为方法参数传递的属性最后被读取, 它也会覆盖任一已经存在的完全一样的属性,这些属性可能是从 properties 元素体内和资源/url 属性中加载的。

3.2 typeAliases的解析

别名的解析

private void typeAliasesElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          //如果是package
          String typeAliasPackage = child.getStringAttribute("name");
          //（一）调用TypeAliasRegistry.registerAliases，去包下找所有类,然后注册别名(有@Alias注解则用，没有则取类的simpleName)
          configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
        } else {
          //如果是typeAlias
          String alias = child.getStringAttribute("alias");
          String type = child.getStringAttribute("type");
          try {
            Class<?> clazz = Resources.classForName(type);
            //根据Class名字来注册类型别名
            //（二）调用TypeAliasRegistry.registerAlias
            if (alias == null) {
              //alias可以省略
              typeAliasRegistry.registerAlias(clazz);
            } else {
              typeAliasRegistry.registerAlias(alias, clazz);
            }
          } catch (ClassNotFoundException e) {
            throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
          }
        }
      }
    }
  }

通过上面的代码可知，别名的配置方式有package和typeAlias两种。

<typeAliases>
  <package name="com.zjp.bean"/>
</typeAliases>  

<typeAliases>
  <typeAlias type="com.zjp.bean.UserBean" alias="userBean" />
</typeAliases>

通过上面的两种方式之后就会为所有的类起一个别名，为类首字母小写。
最终这些别名都会注册到configuration的typeAliasRegistry中。

3.3 setting的解析

<settings>
        <setting name="logImpl" value="LOG4J" />
    </settings>

通过源码可知，这个解析较为简单，就是将读取到的设置信息设置到configuration中

//显式定义用什么log框架，不定义则用默认的自动发现jar包机制
configuration.setLogImpl(resolveClass(props.getProperty("logImpl")));

3.4 mappers的解析

mappers节点解析是mybatis中比较重要，通过源码可以知道定义方式有如下四种：

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          //10.4自动扫描包下所有映射器
          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) {
            //10.1使用类路径
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            //映射器比较复杂，调用XMLMapperBuilder
            //注意在for循环里每个mapper都重新new一个XMLMapperBuilder，来解析
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) {
            //10.2使用绝对url路径
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            //映射器比较复杂，调用XMLMapperBuilder
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            //10.3使用java类名
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            //直接把这个映射加入配置
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

通过上面的源码可以得出这四种分别是package、resource、url和class。
进入Mapper的解析首先会遍历所有的子节点，然后判断不同的方式对Mapper进行解析，解析之后的信息也是保存到configuration中。
其中package的解析方式较为简单，其他的解析方式会使用到一个映射器进行解析

//映射器比较复杂，调用XMLMapperBuilder
//注意在for循环里每个mapper都重新new一个XMLMapperBuilder，来解析
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
mapperParser.parse();

首先在解析之前会创建一个XMLMapperBuilder ，获取到之后再进行解析

//解析
  public void parse() {
    //如果mapper文件没有加载过再加载，防止重复加载
    if (!configuration.isResourceLoaded(resource)) {
      //配置mapper
      configurationElement(parser.evalNode("/mapper"));
      //标记一下，已经加载过了
      configuration.addLoadedResource(resource);
      //绑定映射器到namespace
      bindMapperForNamespace();
    } 
    parsePendingResultMaps();
    parsePendingChacheRefs();
    parsePendingStatements();
  }

解析mapper

private void configurationElement(XNode context) {
    try {
      //1.配置namespace
      String namespace = context.getStringAttribute("namespace");
      if (namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      //2.配置cache-ref
      cacheRefElement(context.evalNode("cache-ref"));
      //3.配置cache
      cacheElement(context.evalNode("cache"));
      //4.配置parameterMap(已经废弃,老式风格的参数映射)
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      //5.配置resultMap(高级功能)
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      //6.配置sql(定义可重用的 SQL 代码段)
      sqlElement(context.evalNodes("/mapper/sql"));
      //7.配置select|insert|update|delete TODO
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
    }
  }

resultMapElements函数
该函数用于解析映射文件中所有的节点，这些节点会被解析成ResultMap对象，存储在Configuration对象的resultMaps容器中。

resultMap解析过程：

//5.1 配置resultMap
  private ResultMap resultMapElement(XNode resultMapNode, List<ResultMapping> additionalResultMappings) throws Exception {
    //错误上下文
//取得标示符   ("resultMap[userResultMap]")
//    <resultMap id="userResultMap" type="User">
//      <id property="id" column="user_id" />
//      <result property="username" column="username"/>
//      <result property="password" column="password"/>
//    </resultMap>
    ErrorContext.instance().activity("processing " + resultMapNode.getValueBasedIdentifier());
    String id = resultMapNode.getStringAttribute("id",
        resultMapNode.getValueBasedIdentifier());
    //一般拿type就可以了，后面3个难道是兼容老的代码？
    String type = resultMapNode.getStringAttribute("type",
        resultMapNode.getStringAttribute("ofType",
            resultMapNode.getStringAttribute("resultType",
                resultMapNode.getStringAttribute("javaType"))));
    //高级功能，还支持继承?
//  <resultMap id="carResult" type="Car" extends="vehicleResult">
//    <result property="doorCount" column="door_count" />
//  </resultMap>
    String extend = resultMapNode.getStringAttribute("extends");
    //autoMapping
    Boolean autoMapping = resultMapNode.getBooleanAttribute("autoMapping");
    Class<?> typeClass = resolveClass(type);
    Discriminator discriminator = null;
    List<ResultMapping> resultMappings = new ArrayList<ResultMapping>();
    resultMappings.addAll(additionalResultMappings);
    List<XNode> resultChildren = resultMapNode.getChildren();
    for (XNode resultChild : resultChildren) {
      if ("constructor".equals(resultChild.getName())) {
        //解析result map的constructor
        processConstructorElement(resultChild, typeClass, resultMappings);
      } else if ("discriminator".equals(resultChild.getName())) {
        //解析result map的discriminator
        discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
      } else {
        List<ResultFlag> flags = new ArrayList<ResultFlag>();
        if ("id".equals(resultChild.getName())) {
          flags.add(ResultFlag.ID);
        }
        //调5.1.1 buildResultMappingFromContext,得到ResultMapping
        resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags));
      }
    }
    //最后再调ResultMapResolver得到ResultMap
    ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
    try {
      return resultMapResolver.resolve();
    } catch (IncompleteElementException  e) {
      configuration.addIncompleteResultMap(resultMapResolver);
      throw e;
    }
  }

解析完成之后ResultMapResolver将resultMap封装到configuration中，完成解析。
最后buildStatementFromContext构建sql语句

//7.1构建语句
  private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
      //构建所有语句,一个mapper下可以有很多select
      //语句比较复杂，核心都在这里面，所以调用XMLStatementBuilder
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
          //核心XMLStatementBuilder.parseStatementNode
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
          //如果出现SQL语句不完整，把它记下来，塞到configuration去
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

最后将sql语句封装到configuration中。

4 SQLSessionFactory对象的创建

通过上面的一些列解析以及封装，把mybatis的配置信息全部都封装到configuration中了，所以mybatis中configuration是一个很重要的类

// 最后一个build方法使用了一个Configuration作为参数,并返回DefaultSqlSessionFactory
    public SqlSessionFactory build(Configuration config) {
        return new DefaultSqlSessionFactory(config);
    }

最终就获取到了SqlSessionFactory了，以上就是mybatis的初始化过程。