mybatis启动流程

SqlSessionFactoryBuilder

SqlSessionFactoryBuilder使用了Builder模式用来创建SqlSessionFactory；

创建 SqlSessionFactory 对象两种方式：

1. 使用 Configuration 对象（mybaits全局配置对象），创建一个默认的DefaultSqlSessionFactory对象；
2. 通过配置文件*.xml文件流、environment、properties 等解析为Configuration对象，然后通过Configuration对象，创建默认的DefaultSqlSessionFactory对象；

  public SqlSessionFactory build(InputStream inputStream, Properties properties) {
    return build(inputStream, 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(Configuration config) {
    return new DefaultSqlSessionFactory(config);
}

SqlSessionManager

• SqlSessionManager 实现了SqlSessionFactory、 SqlSession 接口，重写了这两个接口的所有方法；

public class SqlSessionManager implements SqlSessionFactory, SqlSession 

image.png

• 包含两个成员变量 sqlSessionFactory 、 sqlSessionProxy，通过newInstance()方法调用构造方法进行赋值；

public static SqlSessionManager newInstance(Reader reader) {
    return new SqlSessionManager(new SqlSessionFactoryBuilder().build(reader, null, null));
}
private SqlSessionManager(SqlSessionFactory sqlSessionFactory) {
    this.sqlSessionFactory = sqlSessionFactory;
    this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[]{SqlSession.class},
        new SqlSessionInterceptor());
}

• sqlSessionProxy 为一个代理对象，通过 Java 反射技术生成代理对象。

private class SqlSessionInterceptor implements InvocationHandler {
    public SqlSessionInterceptor() {
        // Prevent Synthetic Access
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      final SqlSession sqlSession = SqlSessionManager.this.localSqlSession.get();
      if (sqlSession != null) {
        try {
          return method.invoke(sqlSession, args);
        } catch (Throwable t) {
          throw ExceptionUtil.unwrapThrowable(t);
        }
      } else {
        try (SqlSession autoSqlSession = openSession()) {
          try {
            final Object result = method.invoke(autoSqlSession, args);
            autoSqlSession.commit();
            return result;
          } catch (Throwable t) {
            autoSqlSession.rollback();
            throw ExceptionUtil.unwrapThrowable(t);
          }
        }
      }
    }
}

• SqlSeesion 接口定义了 close() 、 commit() 、 rollback()、clearCache() 等方法，为了操作同一个SqlSeesion对象，引入了 ThreadLocal 变量，通过localSqlSession确保操作同一个SqlSession，保证了线程安全。

private final ThreadLocal<SqlSession> localSqlSession = new ThreadLocal<>();
//通过此方法，对localSqlSession赋值
public void startManagedSession() {
    this.localSqlSession.set(openSession());
}

BaseBuilder

BaseBuilder 是一个基础抽象类，定义了一些通用方法，有多个具体实现类；成员变量包括：解决数据类型别名的注册器 typeAliasRegistry，及处理 JDBC 类型和 Java 数据类型对应的、类型处理转换器注册中心 typeHandlerRegistry,；还有一些简单的工具方法；

包含3个成员变量：

• Configuration mybatis 全局配置对象；
• TypeAliasRegistry 类型别名注册中心，处理各种类型别名；
• TypeHandlerRegistry 类型处理器注册中心，处理各种TypeHandler ；

多个具体实现类图，介绍其中的XMLConfigBuilder

image.png

XMLConfigBuilder

XMLConfigBuilder 是一个读取并解析 mybatis-config.xml 配置的文件的类；
s
成员变量:

  private boolean parsed;  // 标识是否已经解析 mybats-config.xml 配置丈件
  private final XPathParser parser;  //用于解析 mybatis config xml 配置文件的 XPathParser 对象
  private String environment;   //标识＜environment＞配置的名称，默认读取＜environment> 标签的 default 属性
  //ReflectorFactory 负责创建和缓存 Reflector 对象
  private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

使用

在 SqlSessionFactoryBuilder 中创建 XMLConfigBuilder 对象，并解析 mybatis-config.xml 文件，返回全局的 Configuration 对象；

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      // 解析配置文件
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      // 构建 SqlSessionFactory 对象
      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.
      }
    }
  }

方法

• parse() 解析 mybatis-config 文件，并返回全局 Configuration 对象

  // 解析 mybatis-config 文件，并返回全局 Configuration 对象
  public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //在 mybatis-config xml 配置文件中查找＜configuration＞节点，并开始解析
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

• parseConfiguration(XNode root) 解析 mybatis-config.xml 文件中的各个节点

  private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      //<properties resource="org/apache/ibatis/databases/blog/blog-derby.properties"/>
      //解析＜properties＞节
      propertiesElement(root.evalNode("properties"));
      //  <settings>
      //    <setting name="cacheEnabled" value="true"/>
      //    <setting name="lazyLoadingEnabled" value="false"/>
      //  </settings>
      //解析＜settings＞节点
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      // TODO: 2020/8/4  vfs 虚拟文件系统
      loadCustomVfs(settings);    // 设置 vfsimpl 字段
      loadCustomLogImpl(settings);
      //解析＜typeAliases＞节点
      typeAliasesElement(root.evalNode("typeAliases"));
      //解析＜plugins＞节
      pluginElement(root.evalNode("plugins"));
      //解析＜objectFactory＞节点
      objectFactoryElement(root.evalNode("objectFactory"));
      //解析 <objectWrapperFactory> 节点
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //解析＜reflectorFactory>节点
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      // 解析各种设置，在 mybatis 环境中生效
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      //解析＜environments>节点
      environmentsElement(root.evalNode("environments"));
      //解析＜databaseidProvider＞节点
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //解析＜typeHandlers＞节点
      typeHandlerElement(root.evalNode("typeHandlers"));
      //解析<mappers＞节点
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

• 解析中具体使用到的方法和注解说明

  // 检测 setting 属性是否是 Configuration 中指定的，包含 setter 方法， 就是指定的 Configuration 属性
  // 使用 MetaClass 检测 key 定的属性在 configuration 类中是否有对应 setter 方法的步骤。
  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
    // 检查 setting 设置的参数，是否是指定的，并可被解析的
    // 创建 Configuration 类的元信息包装类，localReflectorFactory 为 DefaultReflectorFactory 新建的对象；
    // metaConfig 为短暂的方法生命周期对象，近在此方法中临时创建，并近存活于此方法中；
    // 检测 Configuration 是否定义了 key 指定属性相应的 setter 方法
    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;
  }

  private void loadCustomVfs(Properties props) throws ClassNotFoundException {
    String value = props.getProperty("vfsImpl");
    if (value != null) {
      String[] clazzes = value.split(",");
      for (String clazz : clazzes) {
        if (!clazz.isEmpty()) {
          @SuppressWarnings("unchecked")
          Class<? extends VFS> vfsImpl = (Class<? extends VFS>)Resources.classForName(clazz);
          configuration.setVfsImpl(vfsImpl);
        }
      }
    }
  }

  private void loadCustomLogImpl(Properties props) {
    Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
    configuration.setLogImpl(logImpl);
  }
 /**
  * 解析类的别名
   <typeAliases>
    <typeAlias alias="BlogAuthor" type="org.apache.ibatis.domain.blog.Author"/>
    <typeAlias type="org.apache.ibatis.domain.blog.Blog"/>
    <typeAlias type="org.apache.ibatis.domain.blog.Post"/>
    <package name="org.apache.ibatis.domain.jpetstore"/>
  </typeAliases>
  */
  private void typeAliasesElement(XNode parent) {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        // 如果包含 package,解析 package 下类,进行别名注册
        // 处理＜package ＞节点
        if ("package".equals(child.getName())) {
          String typeAliasPackage = child.getStringAttribute("name");
          // 通过 TypeAliasRegistry 扫描指定包中所有的类，并解析＠Alias 注解，完成别名注册
          configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
        } else {
          String alias = child.getStringAttribute("alias");  // 获取指定的别名
          String type = child.getStringAttribute("type");    // 获取别名对应的类型
          try {
            Class<?> clazz = Resources.classForName(type);
            if (alias == null) {
              typeAliasRegistry.registerAlias(clazz); // 扫描 @Alias 完成注册 ，完成注册
            } else {
              typeAliasRegistry.registerAlias(alias, clazz);
            }
          } catch (ClassNotFoundException e) {
            throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
          }
        }
      }
    }
  }

  // 以通过添加自定义插件在 SQL 语句执行过程中的某点进行拦截。 MyBatis 中的自定义插件只需实现 Interceptor 接口，并通过注解指定想
  // 要拦截的方法签名即可

  /**
   * <plugins>
   *   <plugin interceptor="org.apache.ibatis.builder.ExamplePlugin">
   *      <property name="pluginProperty" value="100"/>
   *   </plugin>
   *  </plugins>
   * @param parent
   * @throws Exception
   */
  private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        // 创建插件实例,插件是 Interceptor 的具体实现
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
        // 设置插件的属性
        interceptorInstance.setProperties(properties);
        // 设置插件到拦截器链中
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

  /**
   *  <objectFactory type="org.apache.ibatis.builder.ExampleObjectFactory">
   *    <property name="objectFactoryProperty" value="100"/>
   *  </objectFactory>
   * @param context
   * @throws Exception
   */
  // 解析 objectFactory 节点, objectFactory 创建对象的工厂
  // 可以通过添加自定义 ObjectFactory 实现类、ObjectWrapperFactory 实现类、ReflectorFactory 实现类对 Mybatis 进行扩展
  private void objectFactoryElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties properties = context.getChildrenAsProperties();
      ObjectFactory factory = (ObjectFactory) resolveClass(type).getDeclaredConstructor().newInstance(); // 创建 objectFactory 对象
      factory.setProperties(properties);  // 设置对象属性
      configuration.setObjectFactory(factory); // 反写 configuration 对象中的 objectFactory 属性
    }
  }
  /**
   * 解析 <objectWrapperFactory> 节点
   * @param context
   * @throws Exception
   */
  private void objectWrapperFactoryElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      ObjectWrapperFactory factory = (ObjectWrapperFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      configuration.setObjectWrapperFactory(factory);
    }
  }
  /**
   * 解析＜reflectorFactory>节点
   * @param context
   * @throws Exception
   */
  private void reflectorFactoryElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      ReflectorFactory factory = (ReflectorFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      configuration.setReflectorFactory(factory);
    }
  }
  //解析properties节点
  private void propertiesElement(XNode context) throws Exception {
    if (context != null) {
      //得到所有的孩子属性；
      Properties defaults = context.getChildrenAsProperties();
      //parse only one resource element ： resource or url
      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.");
      }
      if (resource != null) {
        defaults.putAll(Resources.getResourceAsProperties(resource));
      } else if (url != null) {
        defaults.putAll(Resources.getUrlAsProperties(url));
      }

      // ．．．与 Configurat on 对象中 variables 集合合并（略）
      Properties vars = configuration.getVariables();
      if (vars != null) {
        defaults.putAll(vars);
      }
      //设置 解析器 variables; 在后面的解析过程中，会使用该 Properties 对象中的信息替换占位符。
      parser.setVariables(defaults);
      //更新 configuration 的 variables 变量
      configuration.setVariables(defaults);
    }
  }

  /**
   * 解析全局配置，并应用到 mybatis 全局变量，使配置生效
   * @param props
   */
  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.setDefaultResultSetType(resolveResultSetType(props.getProperty("defaultResultSetType")));
    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")));
  }

  /**
   * <environments default="development">
   *     <environment id="development">
   *       <transactionManager type="JDBC">
   *         <property name="" value="" />
   *       </transactionManager>
   *       <dataSource type="UNPOOLED">
   *         <property name="driver" value="org.hsqldb.jdbcDriver" />
   *         <property name="url" value="jdbc:hsqldb:mem:localtime" />
   *         <property name="username" value="sa" />
   *       </dataSource>
   *     </environment>
   *   </environments>
   *
   * @param context
   * @throws Exception
   */
  private void environmentsElement(XNode context) throws Exception {
    if (context != null) {
      //未指定 XMLConfigBuilder environment 字段，则使用 default 性指定的 <environment>
      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")); // 创建 TransactionFactory
          DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); // 创建 DataSourceFactory
          DataSource dataSource = dsFactory.getDataSource();        // 创建 DataSource
          Environment.Builder environmentBuilder = new Environment.Builder(id)
              .transactionFactory(txFactory)
              .dataSource(dataSource);
          configuration.setEnvironment(environmentBuilder.build()); // 创建 Environment,并反写 configuration 对象
        }
      }
    }
  }

  /**
   *   <databaseIdProvider type="DB_VENDOR">
   *     <property name="Apache Derby" value="derby"/>
   *   </databaseIdProvider>
   * @param context
   * @throws Exception
   */
  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)) {  // 为了保证兼容性，修改 type 取值
        type = "DB_VENDOR";
      }
      Properties properties = context.getChildrenAsProperties();
      databaseIdProvider = (DatabaseIdProvider) resolveClass(type).getDeclaredConstructor().newInstance();
      databaseIdProvider.setProperties(properties);
    }
    Environment environment = configuration.getEnvironment();
    if (environment != null && databaseIdProvider != null) {
      // 根据前面确定的 DataSource 确定当前使用的数据库产品,得到不同厂商数据库Id
      String databaseId = databaseIdProvider.getDatabaseId(environment.getDataSource());
      configuration.setDatabaseId(databaseId);
    }
  }

  /**
   * 根据 配置文件中的节点，创建 TransactionFactory 对象
   * @param context
   * @return
   * @throws Exception
   */
  private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      TransactionFactory factory = (TransactionFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a TransactionFactory.");
  }

  /**
   * 根据 配置文件中的节点，创建 DataSourceFactory 对象
   * @param context
   * @return
   * @throws Exception
   */
  private DataSourceFactory dataSourceElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      DataSourceFactory factory = (DataSourceFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a DataSourceFactory.");
  }
  /**
   * 解析＜typeHandlers＞节点
   * @param parent
   */
  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);
          }
        }
      }
    }
  }

  /**
   * 解析实体 mapper 文件；
   *   <mappers>
   *     <mapper resource="org/apache/ibatis/builder/BlogMapper.xml"/>
   *     <mapper url="file:./src/test/java/org/apache/ibatis/builder/NestedBlogMapper.xml"/>
   *     <mapper class="org.apache.ibatis.builder.CachedAuthorMapper"/>
   *     <package name="org.apache.ibatis.builder.mapper"/>
   *   </mappers>
   * @param parent
   * @throws Exception
   */
  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) { // <package ＞子节点
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          // 获取＜mapper ＞节点的 resource url class 属性，这三个属性互斥
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          // 如果＜mapper ＞节点指定了 resource 或是 url 属性，则创建 XMLMapperBuilder 对象
          // 并通过该对象解析 resource 或是 url 属性指定的 Mapper 配置文件
          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());
            // 解析 mapper 节点
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            // 如果＜mapper＞节点指定了 class 属性，则 MapperRegistry 注册该 Mapper 接口
            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.");
          }
        }
      }
    }
  }

  /**
   * 检测是否是，指定的环境
   * @param id
   * @return
   */
  private boolean isSpecifiedEnvironment(String id) {
    if (environment == null) {
      throw new BuilderException("No environment specified.");x
    } else if (id == null) {
      throw new BuilderException("Environment requires an id attribute.");
    } else if (environment.equals(id)) {
      return true;
    }
    return false;
  }

创建SqlSessionFactory

经过上面的解析和加载过程，已经创建了 SqlSessionFactory 对象，并初始化了 Configuration 全局配置对象，最后返回一个 SqlSessionFactory 供 SqlSessionManager 使用。然后通过构造方法创建了 SqlSessionManager 对象。

  // 通过 newInstance() 方法创建 SqlSessionManager 对象
  public static SqlSessionManager newInstance(Reader reader) {
    // 创建 SqlSessionFactory 对象，作为成员变量
    return new SqlSessionManager(new SqlSessionFactoryBuilder().build(reader, null, null));
// SqlSessionManager 构造方法
private SqlSessionManager(SqlSessionFactory sqlSessionFactory) {
    this.sqlSessionFactory = sqlSessionFactory;
    // 使用动态代理的方式创建 SqlSession 的代理对象
    this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[]{SqlSession.class},
        new SqlSessionInterceptor());
  }
  }

启动流程

• 解析 mybatis-congfig.xml 配置文件
• 解析各个实体 mapper.xml 文件
• 解析配置文件创建全局配置对象 Configuration 对象
• 通过 Configuration 成员变量，构建 SqlSessionFactory 对象
• 通过 SqlSessionFactory.openSession() 创建 SqlSession 对象
• 创建 SqlSession 对象同时，也创建了 Transaction、Executor 等对象

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      // 获取 mybatis-config.xml 配置文件中配置的 Environment 对象
      final Environment environment = configuration.getEnvironment();
      // 获取 TransactionFactory
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      // 创建 Transaction 对象
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      // 创建 Executor 对象
      final Executor executor = configuration.newExecutor(tx, execType);
      // 创建 DefaultSqlSession 对象
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      // 关闭 Transaction
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

• 还会通过 Configuration 对象创建 StatementHandler、parameterHandler、resultSetHandler 对象

StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql);
ParameterHandler parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
ResultSetHandler resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);

至此为止 MyBatis 已经启动完毕，创建了 SqlSession、Executor 、StatementHandler、parameterHandler、resultSetHandler等关键对象，为后续数据查询奠定了基础。