【SpringMVC】DispatcherServlet结构分析

站在巨人的肩膀上，达到事半功倍的效果。
SpringMVC核心分发器DispatcherServlet分析[附带源码分析]

本篇是对以上文章的补充。

初始化

org.springframework.web.servlet.HttpServletBean#init

	public final void init() throws ServletException {

		// 通过servletconfig得到web.xml中的参数。
		PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
		if (!pvs.isEmpty()) {
			try {
				// 视同beanWrapper设置属性。
				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;
			}
		}

		// Let subclasses do whatever initialization they like.
		initServletBean();
	}

设置web.xml文件中的属性值。

<servlet>
  <servlet-name>dispatcher</servlet-name>  
  <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>  
  <load-on-startup>1</load-on-startup>  
  <init-param>
    <param-name>contextConfigLocation</param-name>  
    <param-value>classpath:springConfig/dispatcher-servlet.xml</param-value>  
  </init-param>
</servlet>

<servlet-mapping>
  <servlet-name>dispatcher</servlet-name>  
  <url-pattern>/</url-pattern>  
</servlet-mapping>

org.springframework.web.servlet.FrameworkServlet#initServletBean

	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 {
			//进行实例化了IOC容器
			this.webApplicationContext = initWebApplicationContext();
			initFrameworkServlet();
		}
		catch (ServletException | RuntimeException ex) {
			logger.error("Context initialization failed", ex);
			throw ex;
		}
	}

实例化IOC容器

org.springframework.web.servlet.DispatcherServlet#onRefresh

	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);
	}

在实例化完容器后，进行了实例化处理器的工作；注意：这里并不是往IOC中放，而是从IOC容器中取。先从IOC容器中取处理器，没有的话在用默认的。例如：

	private void initLocaleResolver(ApplicationContext context) {
		try {
			// 1. 先从容器中找LocaleResolver类型的bean
			this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
			if (logger.isTraceEnabled()) {
				logger.trace("Detected " + this.localeResolver);
			}
			else if (logger.isDebugEnabled()) {
				logger.debug("Detected " + this.localeResolver.getClass().getSimpleName());
			}
		}
		catch (NoSuchBeanDefinitionException ex) {
			// 没有的话就用默认的。
			this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
			if (logger.isTraceEnabled()) {
				logger.trace("No LocaleResolver '" + LOCALE_RESOLVER_BEAN_NAME +
						"': using default [" + this.localeResolver.getClass().getSimpleName() + "]");
			}
		}
	}

	protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
	
		String key = strategyInterface.getName();
		// defaultStrategies是一个Properties是从配置文件中读取的。
		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());
					// 实例化
					Object strategy = createDefaultStrategy(context, clazz);
					strategies.add((T) strategy);
				}
				catch (ClassNotFoundException ex) {
					throw new BeanInitializationException(
							"Could not find DispatcherServlet's default strategy class [" + className +
							"] for interface [" + key + "]", ex);
				}
				catch (LinkageError err) {
					throw new BeanInitializationException(
							"Unresolvable class definition for DispatcherServlet's default strategy class [" +
							className + "] for interface [" + key + "]", err);
				}
			}
			return strategies;
		}
		else {
			return new LinkedList<>();
		}
	}

2块内容：

默认配置的文件。

	private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

		try {
			ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
			defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
		}

默认的配置文件是DispatcherServlet.properties,保存着默认的配置

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping,\
	org.springframework.web.servlet.function.support.RouterFunctionMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter,\
	org.springframework.web.servlet.function.support.HandlerFunctionAdapter

实例化处理器

	protected Object createDefaultStrategy(ApplicationContext context, Class<?> clazz) {
		return context.getAutowireCapableBeanFactory().createBean(clazz);
	}

实例化处理器，。并不是通过构造函数自己new的，而是通过IOC容器的createBean方法创建吗，这样IOC的各种bean处理器合作作用到这个实例中。

结构梳理:

1. 先从ServletConfig中取出参数，设置到servlet中
2. 实例化IOC容器
3. 从容器中得到各种的处理器，没有就取默认的。

服务方法

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

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;
		
		// 得到当前localeContext的实例，保存一份。
		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);

		// 得到请求参数的实例。
		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
		
		// 与当前的线程绑定。
		initContextHolders(request, localeContext, requestAttributes);

		try {
			doService(request, response);
		}
		catch (ServletException | IOException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (Throwable ex) {
			failureCause = ex;
			throw new NestedServletException("Request processing failed", ex);
		}

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

至于为什么在处理服务之前，保存参数？我的理解是提供一个工具方法，可以容易的操作请求参数。

	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));
				}
			}
		}

		// 给请求参数中设置值。
		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());
		
		// 这个FlashMap 是放在session中的数据。
		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 {
			// 真正处理请求的方法。
			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);
				}
			}
		}
	}

在请求之前做了参数备份和设置参数的操作。之后是请求处理。