Springmvc源码分析之dispatcherServlet

1调用流程

从Servlet的分析思路来分析dispatcherServlet。我们都知道serlvet主要分为init()，doget()，doPost()，destory()等方法，dispatcherServlet也是按这个流程进行操作的。

1. 首先Tomcat启动之后，会读取web.xml文件对dispatcherServlet实例化，同时它的抽象父类也会实例化，子类重写的方法覆盖父类的方法（这种方式在servlet使用非常多）。通过web.xml的映射条件，对应的请求会交给dispatcerServlet处理。
2. 调用init()方法初始化工作。
3. 调用doget()和dopost()实现业务操，最终由dispatcerServlet的dodispach()函数完成拦截器，handlermapping和handleradapter，handler执行返回model and view，视图渲染等真正的业务操作。

2源码分析

2.1DispatcherServlet继承结构图

2.2初始化工作

• HttpServletBean的init()方法

init方法被HttpServletBean类重写，会覆盖父类的同名方法，所以我们从这个方法开始分析。

@Override
public final void init() throws ServletException {

   // Set bean properties from init parameters.
   PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
   if (!pvs.isEmpty()) {
      try {
         BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
         ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
         bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
         initBeanWrapper(bw);
         bw.setPropertyValues(pvs, true);
      }
      catch (BeansException ex) {
         if (logger.isErrorEnabled()) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
         }
         throw ex;
      }
   }

   //这个方法已经被子类FrameworkServlet方法覆盖
   initServletBean();
}

• FrameworkServlet的initServletBean()

@Override
	protected final void initServletBean() throws ServletException {
		getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
		if (logger.isInfoEnabled()) {
			logger.info("Initializing Servlet '" + getServletName() + "'");
		}
		long startTime = System.currentTimeMillis();

		try {
            //调用此方法，初始化WebApplicationContext容器
			this.webApplicationContext = initWebApplicationContext();
			initFrameworkServlet();
		}
		catch (ServletException | RuntimeException ex) {
			logger.error("Context initialization failed", ex);
			throw ex;
		}

		if (logger.isDebugEnabled()) {
			String value = this.enableLoggingRequestDetails ?
					"shown which may lead to unsafe logging of potentially sensitive data" :
					"masked to prevent unsafe logging of potentially sensitive data";
			logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
					"': request parameters and headers will be " + value);
		}

		if (logger.isInfoEnabled()) {
			logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
		}
	}

• FrameworkServlet的initWebApplicationContext

protected WebApplicationContext initWebApplicationContext() {
        //读取web.xml的配置信息，初始化得到一个WebApplicationContext的根容器
		WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		WebApplicationContext wac = null;
          //判断当前类的容器是否为空
		if (this.webApplicationContext != null) {
			// A context instance was injected at construction time -> use it
			wac = this.webApplicationContext;
			if (wac instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac =(ConfigurableWebApplicationContext) wac;
				if (!cwac.isActive()) {
					// The context has not yet been refreshed -> provide services such as
					// setting the parent context, setting the application context id, etc
					if (cwac.getParent() == null) {
                        //设置父容器
						cwac.setParent(rootContext);
					}
					configureAndRefreshWebApplicationContext(cwac);
				}
			}
		}
        //web容器的基本操作
		if (wac == null) {
			wac = findWebApplicationContext();
		}
		if (wac == null) {
			wac = createWebApplicationContext(rootContext);
		}

		if (!this.refreshEventReceived) {
			//同步保护 调用onRefresh方法实现dispatcherServlet的一些配置工作，该方法已经被子类覆盖
			synchronized (this.onRefreshMonitor) {
				onRefresh(wac);
			}
		}

		if (this.publishContext) {
			// Publish the context as a servlet context attribute.
			String attrName = getServletContextAttributeName();
			getServletContext().setAttribute(attrName, wac);
		}

		return wac;
	}

• dispatcherServlet的onRefresh()方法

@Override
	protected void onRefresh(ApplicationContext context) {
		initStrategies(context);
	}


	protected void initStrategies(ApplicationContext context) {
        //springmvc一系列解析器的的初始化工作
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

 

• 以initHandlerMappings为例（其他解析器初始化原理基本类似都是从applicationContext容器中注入相应的对象）

private void initHandlerMappings(ApplicationContext context) {
		this.handlerMappings = null;
        //默认查找所有的handlerMapping
		if (this.detectAllHandlerMappings) {
            //从applicationContext中找到所有的handlerMappings，包括他们的父类
			Map<String, HandlerMapping> matchingBeans =
					BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
			if (!matchingBeans.isEmpty()) {
                //得到一个handlerMappings的数组列表，在请求映射的时候，会用到该数组的信息。
				this.handlerMappings = new ArrayList<>(matchingBeans.values());
				//数组排序
				AnnotationAwareOrderComparator.sort(this.handlerMappings);
			}
		}
		else {
			try {
                //否则通过beanName从容器中注入一个handlerMapping
				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) {
            //如果没有配置handlermapping会默认配置一个handlermaping
			this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
			if (logger.isTraceEnabled()) {
				logger.trace("No HandlerMappings declared for servlet '" + getServletName() +
						"': using default strategies from DispatcherServlet.properties");
			}
		}
	}

2.3 请求处理

根据Servlet的请求方式，最终会被发送到doGet()和doPost()方法中,这两个方法被子类FrameworkServlet所覆盖，所以从此开始。

• FrameworkServlet的doGet()和doPost()方法会转到processRequest（）

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;
        // 返回与当前线程相关联的 LocaleContext
		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);

		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        //绑定请求参数
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

		initContextHolders(request, localeContext, requestAttributes);

		try {
            //此方法被子类覆盖，调用方法处理请求
			doService(request, response);
		}
		catch (ServletException | IOException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (Throwable ex) {
			failureCause = ex;
			throw new NestedServletException("Request processing failed", ex);
		}

		finally {
            //重置 LocaleContext等Context信息
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}
			logResult(request, response, failureCause, asyncManager);
			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}

• 调用doService()方法

@Override
	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		logRequest(request);

		// Keep a snapshot of the request attributes in case of an include,
		// to be able to restore the original attributes after the include.
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
			attributesSnapshot = new HashMap<>();
			Enumeration<?> attrNames = request.getAttributeNames();
			while (attrNames.hasMoreElements()) {
				String attrName = (String) attrNames.nextElement();
				if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		//设置一些参数绑定
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

		if (this.flashMapManager != null) {
			FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
			if (inputFlashMap != null) {
				request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
			}
			request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
			request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
		}

		try {
            //最终调用该方法实现springmvc的真正操作
			doDispatch(request, response);
		}
		finally {
			if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
				// Restore the original attribute snapshot, in case of an include.
				if (attributesSnapshot != null) {
					restoreAttributesAfterInclude(request, attributesSnapshot);
				}
			}
		}
	}

本文介绍从dispatcherServlet的初始化到doDispatch()方法之前的主要过程，可以帮助我们认识springmvc的初始化的流程和基本原理。预知doDispatch()详情，请阅读下期。