MyBatis运行原理(一) : 核心组件、SqlSessionFactory的构建过程(源码分析)

MyBatis的工作原理 MyBatis运行原理图

• SqISessionFactoryBuilder (构建器): 根据配置信息或Java代码来构建 SqlSessionFactory 对象。作用: 创建SqlSessionFactory对象。(并创建出Configuration全局配置文件对象)
• SqlSessionFactory (会话工厂) :好比是DataSource(创建连接的数据源) ,线程安全的,在应用运行期间不要重复创建多次,建议使用单例模式。作用: 创建SqlSession对象
• SqlSession (会话) :好比是Connection ,线程不安全的,每次使用开启新的SqlSession对象,使用完毕正常关闭,默认使用DefaultSqlSession。提供操作数据库的增删改查方法,可以调用操作方法,也可以操作Mapper组件。

原理图: 更具体的底层原理图:

涉及的对象:

• Configuration :MyBatis 全局配置对象,封装所有配置信息。
• MappedStatement :映射语句对象,每一个该对象都封装了一个< insert|update|delete|select>节点的详细信息,包括标签的所有可写属性。
  • SqlSource : SQL源(是MappedStatement的属性),根据用户传入的参数生成SQL语句,并封装到BoundSql中。
  • BoundSql : SQL绑定,封装SQL语句和对应的参数信息进行绑定。
• Executor :执行器,是MyBatis调度的核心, 负责SQL语句的具体操作和查询缓存的维护。
• StatementHandler :语句处理器,封装了JDBC的DML、DQL 操作、参数设置。
• ParameterHandler :参数处理器,把用户传入参数转换为JDBC需要的参数值。
• ResultSetHandler :结果集处理器,把结果集中的数据封装到List集合。
• TypeHandler :类型转换器,Java类型和JDBC类型的相互转换。

二、SqlSessionFactory的初始化过程 (源码分析)

1、进入到bulid方法

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { try { // 根据mybatis-config.xml包装的流, 创建了一个解析器parser;该对象是XPathParser,  // 基于Dom4j的XPath的方式来解析XML 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. } } } 

2、进入XMLConfigBuilder类的parse()方法: 用来解析mybatis-config.xml的方法

public Configuration parse() { if (parsed) { throw new BuilderException("Each XMLConfigBuilder can only be used once."); } parsed = true; // 通过parser解析器来解析mybatis-config.xml中的configuration节点,拿到该根节点 parseConfiguration(parser.evalNode("/configuration")); // 返回配置对象 return configuration; } 

3、进入parseConfiguration方法

private void parseConfiguration(XNode root) { try { //issue #117 read properties first propertiesElement(root.evalNode("properties")); // 根据configuration根节点来拿到它下面的节点, 这里获取settings节点, // 封装到一个Properties中 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")); mapperElement(root.evalNode("mappers")); } catch (Exception e) { throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); } } 

4.1、进入到settingsElement方法中: 将保存到properties中的setting配置内容, 设置给Configuration对象

private void settingsElement(Properties props) { configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL"))); // 省略... configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory"))); } 

4.2、进如到mapperElement方法, 解析mappers节点

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 { // 因为是基于resource的方式绑定mapper.xml, 所以进入该分支 // resourse: com/sunny/mapper/UserMapper.xml String resource = child.getStringAttribute("resource"); String url = child.getStringAttribute("url"); // 没用,为null String mapperClass = child.getStringAttribute("class"); // 没用,为null if (resource != null && url == null && mapperClass == null) { ErrorContext.instance().resource(resource); // 同加载mybatis-config.xml,来加载mapper.xml InputStream inputStream = Resources.getResourceAsStream(resource); // 创建解析器 XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); mapperParser.parse(); // 进行解析 } 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."); } } } } } 

5、进入到mapperPaser.parse方法: 用来解析mapper.xml的方法, 将解析mapper.xml的标签都保存到全局配置文件中

public void parse() { if (!configuration.isResourceLoaded(resource)) { // 解析并拿到mapper标签 configurationElement(parser.evalNode("/mapper")); // 将解析mapper.xml的标签都保存到全局配置文件中 configuration.addLoadedResource(resource); bindMapperForNamespace(); } parsePendingResultMaps(); parsePendingCacheRefs(); parsePendingStatements(); } 

6、进入XMLMapperBuilder类的configurationElement方法

private void configurationElement(XNode context) { try { String namespace = context.getStringAttribute("namespace"); if (namespace == null || namespace.equals("")) { throw new BuilderException("Mapper's namespace cannot be empty"); } // 通过mapper根节点来拿到其他子节点 builderAssistant.setCurrentNamespace(namespace); cacheRefElement(context.evalNode("cache-ref")); cacheElement(context.evalNode("cache")); parameterMapElement(context.evalNodes("/mapper/parameterMap")); resultMapElements(context.evalNodes("/mapper/resultMap")); sqlElement(context.evalNodes("/mapper/sql")); // 以crud为例来分析 buildStatementFromContext(context.evalNodes("select|insert|update|delete")); } catch (Exception e) { throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e); } } 

7、进入buildStatementFromContext方法

private void buildStatementFromContext(List<XNode> list) { // configuration用来保存所有的mybatis的配置信息 if (configuration.getDatabaseId() != null) { buildStatementFromContext(list, configuration.getDatabaseId()); } // 进入该方法 buildStatementFromContext(list, null); } private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) { for (XNode context : list) { final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId); try { statementParser.parseStatementNode(); } catch (IncompleteElementException e) { configuration.addIncompleteStatement(statementParser); } } } 

8、XMLStatementBuilder的parseStatementNode方法: 用来解析select | insert | update | delete标签的方法

// 拿到select | insert | update | delete标签能写的所有属性 public void parseStatementNode() { String id = context.getStringAttribute("id"); String databaseId = context.getStringAttribute("databaseId"); if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) { return; } // 省略..... } String keyProperty = context.getStringAttribute("keyProperty"); String keyColumn = context.getStringAttribute("keyColumn"); String resultSets = context.getStringAttribute("resultSets"); // 将获取到的所有crud的标签属性通过下面方法构造出来 // public MappedStatement addMappedStatement(){} 该方法返回一个 MappedStatement // 也就说明每个增删改查标签的所有信息, 都封装成了MappedStatement对象 builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType, fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass, resultSetTypeEnum, flushCache, useCache, resultOrdered, keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets); } 

9、进入到addMappedStatement方法, 根据select | insert | update | delete标签构建出MappedStatement对象

public MappedStatement addMappedStatement( String id, SqlSource sqlSource, StatementType statementType, SqlCommandType sqlCommandType, Integer fetchSize, Integer timeout, String parameterMap, Class<?> parameterType, String resultMap, Class<?> resultType, ResultSetType resultSetType, boolean flushCache, boolean useCache, boolean resultOrdered, KeyGenerator keyGenerator, String keyProperty, String keyColumn, String databaseId, LanguageDriver lang, String resultSets) { if (unresolvedCacheRef) { throw new IncompleteElementException("Cache-ref not yet resolved"); } id = applyCurrentNamespace(id, false); boolean isSelect = sqlCommandType == SqlCommandType.SELECT; MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType) .resource(resource) .fetchSize(fetchSize) .timeout(timeout) .statementType(statementType) .keyGenerator(keyGenerator) .keyProperty(keyProperty) .keyColumn(keyColumn) .databaseId(databaseId) .lang(lang) .resultOrdered(resultOrdered) .resultSets(resultSets) .resultMaps(getStatementResultMaps(resultMap, resultType, id)) .resultSetType(resultSetType) .flushCacheRequired(valueOrDefault(flushCache, !isSelect)) .useCache(valueOrDefault(useCache, isSelect)) .cache(currentCache); ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id); if (statementParameterMap != null) { statementBuilder.parameterMap(statementParameterMap); } // 根据select | insert | update | delete标签构建出MappedStatement对象 MappedStatement statement = statementBuilder.build(); // 将statement对象扔保存到configuration对象中 configuration.addMappedStatement(statement); return statement; } 

每一个MappedStatement保存的信息如下图 10、回到XMLConfigBuilder类的parse()方法, 返回configuration, 此时该对象就保存了mybatis-config.xml和mapper.xml以及crud标签的详细信息, 如下图: configuration中的mappedStatements来保存了crud标签信息 11、最终返回了一个DefaultSqlSession对象, 该对象包含了全局配置信息Configuration 12、最终new SqlSessionFactoryBuilder().build(in)返回了SqlSessionFactory --> DefaultSqlSessionFactory对象 上面1-12步时序图

• 总结: new SqlSessionFactoryBuilder().build(in)流程: 把配置文件的信息解析并保存在Configuration对象中, 返回包含了Configurationd对DefaultSqlSessionFactory对象