mybatis的运行分为两大步:
1.读取配置文件缓存到Configuration对象中,用以创建SQLSessionFactory;
2.SQLSession的执行过程。
首先我们来说一下这个SQLSessionFactory构建;
首先我们要知道,SQLSessionFactory是一个接口,而不是一个实现类,mybatis给他提供了一个默认的实现类org.apache.ibatis.session.defaults.DefaultSqlSessionFactory。从源码中我们也能够看出:
public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config); }
创建SQLSessionFactory的时候,是交给DefaultSqlSessionFactory去执行的;同时在这里,我们发现在创建SQLSessionFactory时,最重要的Configuration对象;那我们接着看:
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { try { 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. } } }
这个方法是调用刚才那个方法的方法,我们能够找到 return build(parser.parse());在这里调用的;然后继续往上走,我们看到了XMLConfigBuilder这个类,他继承了BaseBuilder的类;现在能够知道了Configuration对象是由XMLConfigBuilder创建的了吧。那他是怎么创建的呢?接着继续跟进:
public Configuration parse() { if (parsed) { throw new BuilderException("Each XMLConfigBuilder can only be used once."); } parsed = true; parseConfiguration(parser.evalNode("/configuration")); return configuration; }
我们到XMLConfigBuilder类中找到了parse()方法,然后我们接着看parseConfiguration(parser.evalNode("/configuration"));这个方法是怎么实现的:
private void parseConfiguration(XNode root) { try { Properties settings = settingsAsPropertiess(root.evalNode("settings")); //issue #117 read properties first propertiesElement(root.evalNode("properties")); 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 #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); } }
打眼一看感觉不少,其实这些只是做了一样工作,解析xml对应的信息;
propertiesElement(root.evalNode("properties"));//属性文件,数据库驱动配置? typeAliasesElement(root.evalNode("typeAliases"));//类型方言,用于结果转换 pluginElement(root.evalNode("plugins"));//插件,可以配置不同时机的拦截器 objectFactoryElement(root.evalNode("objectFactory"));//对象工厂,可以配置返回对象属性 objectWrapperFactoryElement(root.evalNode("objectWrapperFactory")); settingsElement(root.evalNode("settings")); //全局变量,缓存,延迟加载等配置 environmentsElement(root.evalNode("environments")); //环境变量,根据配置构建数据源工厂,事务工厂 databaseIdProviderElement(root.evalNode("databaseIdProvider"));//数据库提供标识ID,多数据库兼容的时候用到 typeHandlerElement(root.evalNode("typeHandlers")); //类型处理器,主要是对结果集,类型转换处理 mapperElement(root.evalNode("mappers"));//不同的mapper dao处理
到这里我们就知道整个流程了,但是在这多说一下,那就是typeHandler,因为这个解析过程很重要;
我们继续跟进typeHandlerElement(root.evalNode("typeHandlers"));方法:
private void typeHandlerElement(XNode parent) throws Exception { 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); } } } }
从源码中,我们看出,他被注册到了typeHandlerRegistry对象中去了,但这个对象怎么来的,XMLConfigBuilder中并没有定义它,那就只能是来自XMLConfigBuilder的父类BaseBuilder了,就是来自它,我们跟进看一下,
public abstract class BaseBuilder { protected final Configuration configuration; protected final TypeAliasRegistry typeAliasRegistry; protected final TypeHandlerRegistry typeHandlerRegistry; public BaseBuilder(Configuration configuration) { this.configuration = configuration; this.typeAliasRegistry = this.configuration.getTypeAliasRegistry(); this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry(); }this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();通过这句代码,我们能获得:原来typeHandlerRegistry对象实际就是Configuration单利的一个属性,所以我们可以通过Configuration单利拿到typeHandlerRegistry对象,进而我们拿到所注册的typeHandler;