对于MyBatis而言，初始化指从开始到构建SqlSessionFactory的过程，通常SqlSessionFactoryBuilder使用XML的构建SqlSessionFactory实例。我们可以从以下四个方面对初始化过程一探究竟：

1、做了什么

2、如何做的

3、抽象对象

4、设计模式

一、做了什么

官方文档提供两种初始化方法，使用XML或者JavaAPI

使用XML方式：

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession();
try {  
    Blog blog = session.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);
} finally {  
    session.close();
}

使用JavaAPI方式：

DataSource dataSource = BlogDataSourceFactory.getBlogDataSource();
TransactionFactory transactionFactory = new JdbcTransactionFactory();
Environment environment = new Environment("development", transactionFactory, dataSource);
Configuration configuration = new Configuration(environment);
configuration.addMapper(BlogMapper.class);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);
//生成session并执行查询代码略
...

可知，最终都是调用了SqlSessionFactoryBuilder.build()不同参数的重载方法：

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      //1、解析作为inputStream的xml配置文件
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      //2、构建Configration对象
      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) {
    //3、构建DefaultSqlSessionFactory对象
    return new DefaultSqlSessionFactory(config);
  }

说白了，build方法其实是构建Configuration对象，并依此生成SqlSessionFactory，具体可分为三步，如上图注解所示，下面对这三步进行具体说明。

二、如何做的

1.1解析主配置文件

第一步XMLConfigBuilder对象创建过程中，可追溯到下面的代码，通过dom方式对主配置文件进行解析：

private Document createDocument(InputSource inputSource) {
    // important: this must only be called AFTER common constructor
    try {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      //设置参数
      ...
      DocumentBuilder builder = factory.newDocumentBuilder();
      //设置参数
      ...
      //解析输入源
      return builder.parse(inputSource);
    } catch (Exception e) {
      throw new BuilderException("Error creating document instance.  Cause: " + e, e);
    }
  }

下面再看DocumentBuilder的实现类DocumentBuilderImpl中的方法：

public Document parse(InputSource is) throws SAXException, IOException {
        if (is == null) {
            throw new IllegalArgumentException(
                DOMMessageFormatter.formatMessage(DOMMessageFormatter.DOM_DOMAIN,
                "jaxp-null-input-source", null));
        }
        if (fSchemaValidator != null) {
            if (fSchemaValidationManager != null) {
                fSchemaValidationManager.reset();
                fUnparsedEntityHandler.reset();
            }
            resetSchemaValidator();
        }
        //通过dom解析类解析输入源
        domParser.parse(is);
        Document doc = domParser.getDocument();
        domParser.dropDocumentReferences();
        return doc;
    }

代码执行时序图如下：

1.2构建Configuration对象

 XmlConfigBuilder中的parse方法：

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

  private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      //解析mapper.xml文件的方法
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

可见，parseConfiguration中对于主配置文件的节点都进行了解析，其实每个方法都非常类似，都是将xml中的内容进行解析并放到configuration中。

解析properties的方法如下：

private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      //1、在 properties 元素体内指定的属性首先被读取。
      Properties defaults = context.getChildrenAsProperties();
      String resource = context.getStringAttribute("resource");
      String url = context.getStringAttribute("url");
      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.");
      }
      //2、然后根据 properties 元素中的 resource 属性读取类路径下属性文件或根据 url 属性指定的路径读取属性文件，并覆盖已读取的同名属性。
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }
      //3、最后读取作为方法参数传递的属性，并覆盖已读取的同名属性。
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      parser.setVariables(defaults);
      configuration.setVariables(defaults);
    }
  }

将settings解析为Properties ：

private Properties settingsAsProperties(XNode context) {
    if (context == null) {
      return new Properties();
    }
    Properties props = context.getChildrenAsProperties();
    // Check that all settings are known to the configuration class
    MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
    for (Object key : props.keySet()) {
      if (!metaConfig.hasSetter(String.valueOf(key))) {
        throw new BuilderException("The setting " + key + " is not known.  Make sure you spelled it correctly (case sensitive).");
      }
    }
    return props;
  }

对VFS和logImpl进行设置略，接下来是别名的设置：

private void typeAliasesElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        //根据包名设置
        if ("package".equals(child.getName())) {
          String typeAliasPackage = child.getStringAttribute("name");
          configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
        } else {
          //根据类型设置
          String alias = child.getStringAttribute("alias");
          String type = child.getStringAttribute("type");
          try {
            Class<?> clazz = Resources.classForName(type);
            //处理别名为空的情况
            if (alias == null) {
              typeAliasRegistry.registerAlias(clazz);
            } else {
              typeAliasRegistry.registerAlias(alias, clazz);
            }
          } catch (ClassNotFoundException e) {
            throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
          }
        }
      }
    }
  }

 值得一提的是，如果缺少alias属性，如下所示：

<typeAliases>
    <typeAlias type="org.apache.ibatis.builder.XmlConfigBuilderTest"/>
</typeAliases>

则程序会设置一个全小写的类名作为别名，即xmlconfigbuildertest。

MyBatis 允许你在已映射语句执行过程中的某一点使用插件来拦截，解析并注册插件的代码如下：

private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
        interceptorInstance.setProperties(properties);
        //往interceptorchain里面添加interceptor
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

每次创建结果对象的新实例时，都会使用一个对象工厂（ObjectFactory）实例来完成，解析对象工厂和对象包装工厂方法如下：

private void objectFactoryElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties properties = context.getChildrenAsProperties();
      ObjectFactory factory = (ObjectFactory) resolveClass(type).newInstance();
      factory.setProperties(properties);
      configuration.setObjectFactory(factory);
    }
  }

  private void objectWrapperFactoryElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      ObjectWrapperFactory factory = (ObjectWrapperFactory) resolveClass(type).newInstance();
      configuration.setObjectWrapperFactory(factory);
    }
  }

 反射工厂的代码：

private void reflectorFactoryElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      ReflectorFactory factory = (ReflectorFactory) resolveClass(type).newInstance();
      configuration.setReflectorFactory(factory);
    }
  }

 此时，才将settings进行赋值：

private void settingsElement(Properties props) {
    configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));
    configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
    configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
    configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));
    configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));
    configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));
    configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));
    configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));
    configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));
    configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));
    configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));
    configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));
    configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));
    configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));
    configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));
    configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));
    configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));
    configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));
    configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));
    configuration.setDefaultEnumTypeHandler(resolveClass(props.getProperty("defaultEnumTypeHandler")));
    configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));
    configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));
    configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));
    configuration.setLogPrefix(props.getProperty("logPrefix"));
    configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
  }

 environment用于配置成适应多种环境，这种机制有助于将 SQL 映射应用于多种数据库之中，解析代码如下：

private void environmentsElement(XNode context) throws Exception {
    if (context != null) {
      if (environment == null) {
        environment = context.getStringAttribute("default");
      }
      for (XNode child : context.getChildren()) {
        String id = child.getStringAttribute("id");
        if (isSpecifiedEnvironment(id)) {
          TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
          DataSource dataSource = dsFactory.getDataSource();
          Environment.Builder environmentBuilder = new Environment.Builder(id)
              .transactionFactory(txFactory)
              .dataSource(dataSource);
          configuration.setEnvironment(environmentBuilder.build());
        }
      }
    }
  }

 databaseId支持根据数据库厂商执行不同的语句，解析代码如下：

private void databaseIdProviderElement(XNode context) throws Exception {
    DatabaseIdProvider databaseIdProvider = null;
    if (context != null) {
      String type = context.getStringAttribute("type");
      // awful patch to keep backward compatibility
      if ("VENDOR".equals(type)) {
        type = "DB_VENDOR";
      }
      Properties properties = context.getChildrenAsProperties();
      databaseIdProvider = (DatabaseIdProvider) resolveClass(type).newInstance();
      databaseIdProvider.setProperties(properties);
    }
    Environment environment = configuration.getEnvironment();
    if (environment != null && databaseIdProvider != null) {
      String databaseId = databaseIdProvider.getDatabaseId(environment.getDataSource());
      configuration.setDatabaseId(databaseId);
    }
  }

typeHandler类型处理器将获取的值以合适的方式转换成 Java 类型，解析代码如下：

private void typeHandlerElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String typeHandlerPackage = child.getStringAttribute("name");
          typeHandlerRegistry.register(typeHandlerPackage);
        } else {
          String javaTypeName = child.getStringAttribute("javaType");
          String jdbcTypeName = child.getStringAttribute("jdbcType");
          String handlerTypeName = child.getStringAttribute("handler");
          Class<?> javaTypeClass = resolveClass(javaTypeName);
          JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
          Class<?> typeHandlerClass = resolveClass(handlerTypeName);
          if (javaTypeClass != null) {
            if (jdbcType == null) {
              typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
            } else {
              typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
            }
          } else {
            typeHandlerRegistry.register(typeHandlerClass);
          }
        }
      }
    }
  }

mapper的解析如下：

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        //方式一：包名
        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");
          //方式二：resource
          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();
          //方式三：url
          } 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();
          //方式四：mapperClass
          } 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.");
          }
        }
      }
    }
  }

这里分为四种配置方式，一是设置mapper接口所在的包名，二是设置resource，三是设置url，四是设置mapper接口（即mapperClass），方式一和方式四是将mapperClass直接添加到MapperRegistry.knownMappers，而方式二和方式三则是对xml文件进行解析，然后根据namespace进行反射，将反射类（即mapperClass）添加到MapperRegistry.knownMappers，所以mapper配置文件的namespace是必填项，对此官方文档有明确解释：http://www.mybatis.org/mybatis-3/zh/getting-started.html。

下面是方式一用到的addMappers和方式四用到的addMapper方法：

public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
}

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<>(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);
        }
      }
    }
  }

下面是方式二和方式三用到的方法：

 

public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
      configurationElement(parser.evalNode("/mapper"));
      configuration.addLoadedResource(resource);
      bindMapperForNamespace();
    }

    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
}

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);
        }
      }
    }
  }

1.3构建DefaultSqlSessionFactory对象

因为Configuration对象包含了所有的配置项以及程序运行的各类参数，所以DefaultSqlSessionFactory对象的创建仅仅是对其configuration属性进行赋值：

public DefaultSqlSessionFactory(Configuration configuration) {
    this.configuration = configuration;
  }

至此，可以将整个初始化过程补充完整：

三、抽象对象

初始化阶段涉及的主要对象是相关配置的抽象Configuration，包含了会深深影响 MyBatis 行为的设置（settings）和属性（properties）信息。文档的顶层结构如下：

• configuration 配置
  • properties 属性
  • settings 设置
  • typeAliases 类型别名
  • typeHandlers 类型处理器
  • objectFactory 对象工厂
  • plugins 插件
  • environments 环境
    • environment 环境变量
      • transactionManager 事务管理器
      • dataSource 数据源
  • databaseIdProvider 数据库厂商标识
  • mappers 映射器

在代码中的反映：

看起来非常多的属性其实就是前面顶层结构提到的内容，其中settings设置包含了较多的属性变量。

具体到每个变量的作用可以参考官方文档：settings 设置。

四、设计模式

其中涉及的模式主要有：工厂模式、创建者模式及拦截链等。