深入了解SpringMVC源码解析

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。

1. 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> {

		@Override
		public 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。

1. 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<>(classNames.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map
			Map<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。

1. 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HandlerExecutionChain
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				//获取合适的HandlerAdapter
				HandlerAdapter 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 @ExceptionHandler 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 afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// Clean up any resources used by a multipart request.
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}

总结：

1. tomcat的Servlet调起Spring容器启动，Spring容器启动完，事件通知到SpringMVC的DispatcherServlet。

2. 这时会扫描所有的bean，将注解了@Controller和@RequestMapping的解析出来。

3. 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";
	}

1. 找到对应的方法后，反射调用

method.invoke(Object obj,Object... args)

1. 组装modelAndView渲染视图到前端