爱站程序员基地Spring MVC源码解析
Spring MVC的使用原理其实是通过配置一个Servlet来接管所有的请求，所有的请求由这个Servlet来进行分发处理。
我们可以从web.xml里面看出这一点
<servlet><servlet-name>springmvc</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>classpath:springmvc.xml</param-value></init-param></servlet>
这个Servlet就是Spring MVC提供的DispatcherServlet，它的继承图如下：
DispatcherServlet大致接收请求的时序图如下：
下面我们先设定几个问题然后再去看源码。
- DispatcherServlet初始化的时机？
我们将断点打在DispatcherServlet中的onRefresh方法上
，然后启动项目。
从这个堆栈的图可以看出，最开始是HttpServletBean的init方法执行，然后启动Spring容器，Spring容器初始化的最后一步finishRefresh里面进行了事件通知。
FrameworkServlet里有个内部类ContextRefreshListener，实现了ApplicationListener接口，接收到了上面的事件通知，执行onApplicationEvent方法。
private class ContextRefreshListener implements ApplicationListener<ContextRefreshedEvent> {@Overridepublic void onApplicationEvent(ContextRefreshedEvent event) {FrameworkServlet.this.onApplicationEvent(event);}}
继续跟踪onApplicationEvent方法
public void onApplicationEvent(ContextRefreshedEvent event) {this.refreshEventReceived = true;synchronized (this.onRefreshMonitor) {onRefresh(event.getApplicationContext());}}
发现最终是由onRefresh方法来进行具体处理的，并ad8且这个onRefresh方法在FrameworkServlet里面是个空方法，是由它的子类DispatcherServlet来实现的。
我们来看DispatcherServlet里的onRefresh方法
protected void onRefresh(ApplicationContext context) {initStrategies(context);}
protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);}
这样就把SpringMVC的九大组件给初始化了。
- 我们的@RequestMapping注解在方法上，SpringMVC是怎么根据这个注解就能将对应的请求执行到这个方法上的？
从上面可以看到有个initHandlerMappings方法：
private void initHandlerMappings(ApplicationContext context) {this.handlerMappings = null;if (this.detectAllHandlerMappings) {// 找所有实现了HandlerMapping接口的类Map<String, HandlerMapping> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);if (!matchingBeans.isEmpty()) {this.handlerMappings = new ArrayList<>(matchingBeans.values());// We keep HandlerMappings in sorted order.AnnotationAwareOrderComparator.sort(this.handlerMappings);}}else {try {HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);this.handlerMappings = Collections.singletonList(hm);}catch (NoSuchBeanDefinitionException ex) {// Ignore, we\'ll add a default HandlerMapping later.}}// Ensure we have at least one HandlerMapping, by registering// a default HandlerMapping if no other mappings are found.if (this.handlerMappings == null) {//默认情况下，我们是使用的这个方法。this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);if (logger.isTraceEnabled()) {logger.trace(\"No HandlerMappings declared for servlet \'\" + getServletName() +\"\': using default strategies from DispatcherServlet.properties\");}}}
继续跟踪getDefaultStrategies方法
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {String key = strategyInterface.getName();String value = defaultStrategies.getProperty(key);if (value != null) {String[] classNames = StringUtils.commaDelimitedListToStringArray(value);List<T> strategies = new ArrayList<>(cla1b14ssNames.length);for (String className : classNames) {try {Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());Object strategy = createDefaultStrategy(context, clazz);strategies.add((T) strategy);}//省略部分代码...}return strategies;}else {return new LinkedList<>();}}
可以看出对classNames进行了遍历，这里有两个值，
BeanNameUrlHandlerMapping和RequestMappingHandlerMapping，我们一般用的是RequestMappingHandlerMapping。
RequestMappingHandlerMapping的父类AbstractHandlerMethodMapping实现了InitializingBean接口，我们来看看它实现的afterPropertiesSet方法。
public void afterPropertiesSet() {this.config = new RequestMappingInfo.BuilderConfiguration();this.config.setUrlPathHelper(getUrlPathHelper());this.config.setPathMatcher(getPathMatcher());this.config.setSuffixPatternMatch(this.useSuffixPatternMatch);this.config.setTrailingSlashMatch(this.useTrailingSlashMatch);this.config.setRegisteredSuffixPatternMatch(this.useRegisteredSuffixPatternMatch);this.config.setContentNegotiationManager(getContentNegotiationManager());super.afterPropertiesSet();}
前面都是一些配置，后面是调用父类的afterPropertiesSet方法，此方法里只有initHandlerMethods一个方法
protected void initHandlerMethods() {for (String beanName : getCandidateBeanNames()) {if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {processCandidateBean(beanName);}}handlerMethodsInitialized(getHandlerMethods());}
主要看processCandidateBean方法
protected void processCandidateBean(String beanName) {Class<?> beanType = null;try {beanType = obtainApplicationContext().getType(beanName);}catch (Throwable ex) {// An unresolvable bean type, probably from a lazy bean - let\'s ignore it.if (logger.isTraceEnabled()) {logger.trace(\"Could not resolve type for bean \'\" + beanName + \"\'\", ex);}}if (beanType != null && isHandler(beanType)) {detectHandlerMethods(beanName);}}
继续跟踪detectHandlerMethods方法
protected void detectHandlerMethods(Object handler) {Class<?> handlerType = (handler instanceof String ?obtainApplicationContext().getType((String) handler) : handler.getClass());if (handlerType != null) {Class<?> userType = ClassUtils.getUserClass(handlerType);//利用工具，取出类里面的方法并组装成mapMap<Method, T> methods = MethodIntrospector.selectMethods(userType,(MethodIntrospector.MetadataLookup<T>) method -> {try {return getMappingForMethod(method, userType);}catch (Throwable ex) {throw new IllegalStateException(\"Invalid mapping on handler class [\" +userType.getName() + \"]: \" + method, ex);}});if (logger.isTraceEnabled()) {logger.trace(formatMappings(userType, methods));}//遍历map，处理里面的方法methods.forEach((method, mapping) -> {Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);registerHandlerMethod(handler, invocableMethod, mapping);});}}
继续跟踪里面的register方法
public void register(T mapping, Object handler, Method method) {this.readWriteLock.writeLock().lock();try {HandlerMethod handlerMethod = createHandlerMethod(handler, method);assertUniqueMethodMapping(handlerMethod, mapping);this.mappingLookup.put(mapping, handlerMethod);List<String> directUrls = getDirectUrls(mapping);for (String url : directUrls) {this.urlLookup.add(url, mapping);}String name = null;if (getNamingStrategy() != null) {name = getNamingStrategy().getName(handlerMethod, mapping);addMappingName(name, handlerMethod);}CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping);if (corsConfig != null) {this.corsLookup.put(handlerMethod, corsConfig);}this.registry.put(mapping, new MappingRegistration<>(mapping, handlerMethod, directUrls, name));}finally {this.readWriteLock.writeLock().unlock();}}
可以看出mappingLookup、urlLookup、registry都放入了值。这时我也不知道每个的具体作用。我把断点打到这3个属性上。然后前端发起一个get请求。
- 请求匹配的过程？
前端发起请求，断点停在了AbstractHandlerMethodMapping.MappingRegistry#getMappingsByUrl方法上，以前端发起的请求路径，从urlLookup上获取对应的值。
最后看看请求分发的主流程，也是springMVC最核心的代码
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {ModelAndView mv = null;Exception dispatchException = null;try {//multipart请求处理processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// 获取合适的HandlerExecutionChainmappedHandler = getHandler(processedRequest);if (mappedHandler == null) {noHandlerFound(processedRequest, response);return;}//获取合适的HandlerAdapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.String method = request.getMethod();boolean isGet = \"GET\".equals(method);if (isGet || \"HEAD\".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// handle执行调用（核心）mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {// As of 4.3, we\'re processing Errors thrown from handler methods as well,// making them available for @ExceptionHand1c4cler methods and other scenarios.dispatchException = new NestedServletException(\"Handler dispatch failed\", err);}//视图解析并渲染到页面processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Throwable err) {triggerAfterCompletion(processedRequest, response, mappedHandler,new NestedServletException(\"Handler processing failed\", err));}finally {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}}
总结：
-
tomcat的Servlet调起Spring容器启动，Spring容器启动完，事件通知到SpringMVC的DispatcherServlet。
-
这时会扫描所有的bean，将注解了@Controller和@RequestMapping的解析出来。
-
前端请求发过来，DispatcherServlet接收到（因为它是个servlet，配置在web.xml的），根据上一步处理好的映射关系，找到对应的方法来处理。
如通过/test能找到test方法
@RequestMapping(\"/test\")public String test(String name, HttpServletRequest request,Model model){System.out.println(\"name\");model.addAttribute(\"date\",new Date());return \"success\";}
- 找到对应的方法后，反射调用
method.invoke(Object obj,Object... args)
- 组装modelAndView渲染视图到前端
