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Mybatis源码细节探究：二级缓存Cache对象是在什么时候创建的？

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本文内容整理自《孙哥说Mybatis系列课程》

Cache是在什么时候、什么地点被创建的？

不管是我们使用Mybatis写测试程序，还是在真实企业级项目中使用Mybatis，Mybatis框架被启动加载的前几行代码一定是如下：

        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();

然后我们的二级缓存Cache对象就是在这个期间，也就是Mybatis被启动和初始化的时候完成的，那么我们就详细展开吧。

首先：我要告诉大家的是，这个过程一定是在SqlSessionFactory工厂被创建的时候完成的，那就是上述的第二行代码，并且这里应用的是构建者设计模式，这个精华必须隐藏在build()方法中，我们查看一下：

  public SqlSessionFactory build(InputStream inputStream) {return build(inputStream, null, null);}//重载的build方法，真正的精华是在这里public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {//将Mybatis和核心配置文件读取成inputStream之后，我们有将他转换成Mybatis中重要的//XMLConfigBuilder对象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.}}}

return build(parser.parse());这里边硬货就多了，build方法就是为了创建

  public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}

所以，里边的parser.parse()的返回值一定是生成的Configuration对象，原材料就是Mybatis核心配置文件读取到的InputStream，有因为InputStream当中记录了Mapper.xml的路径，所以这些Mapper文件必定也是在这里边解析的。

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

parser.evalNode(“/configuration”)兄弟们，这玩意用脚趾头都知道，这读的是mybatis-config.xml里边的Configuration标签，这样就把mybatis-config.xml整个文件作为一个Node对象被放到了parseConfiguration方法当中，这个方法的目的从方法上就能看出来，为了构建Configuration对象

  private void parseConfiguration(XNode root) {try {//issue #117 read properties firstpropertiesElement(root.evalNode("properties"));Properties settings = settingsAsProperties(root.evalNode("settings"));loadCustomVfs(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 #631environmentsElement(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);}}

兄弟们，眼熟了吧，这不就是mybatis-config.xml里边的一个一个的标签么，一个一个的排队解析，看最后一行，这玩意里边存储的不就是Mapper标签么，所以Mapper.xml必定就是在这里边解析的，再加上我们知道Cache标签是写在Mapper.xml里边的，所以Cache对象必是在里边创建的。

  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 {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.");}}}}}

兄弟们，XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());这玩意不就是在读Mapper.xml文件么，mapperParser.parse();又去parse去了，我们进去看看

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

configurationElement兄弟们，看到名字还不明白么，这是在解析Mapper.xml当中的元素解析到Configurantion对象当中呀，Cache标签也是其中的一个元素呀

  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");}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"));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);}}

  private void cacheElement(XNode context) throws Exception {if (context != null) {String type = context.getStringAttribute("type", "PERPETUAL");Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);String eviction = context.getStringAttribute("eviction", "LRU");Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);Long flushInterval = context.getLongAttribute("flushInterval");Integer size = context.getIntAttribute("size");boolean readWrite = !context.getBooleanAttribute("readOnly", false);boolean blocking = context.getBooleanAttribute("blocking", false);Properties props = context.getChildrenAsProperties();builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);}}