⭐⭐⭐ Spring Boot 项目实战	⭐⭐⭐ Spring Cloud 项目实战
《Dubbo 实现原理与源码解析 —— 精品合集》	《Netty 实现原理与源码解析 —— 精品合集》
《Spring 实现原理与源码解析 —— 精品合集》	《MyBatis 实现原理与源码解析 —— 精品合集》
《Spring MVC 实现原理与源码解析 —— 精品合集》	《数据库实体设计合集》
《Spring Boot 实现原理与源码解析 —— 精品合集》	《Java 面试题 + Java 学习指南》

摘要: 原创出处 https://muyinchen.github.io/2017/08/02/Spring5%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90 —— %E8%AE%BASpring%20DispatcherServlet%E7%9A%84%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F/ 「一叶知秋」欢迎转载，保留摘要，谢谢！

🙂🙂🙂关注**微信公众号：【芋道源码】**有福利：

RocketMQ / MyCAT / Sharding-JDBC 所有源码分析文章列表

RocketMQ / MyCAT / Sharding-JDBC 中文注释源码 GitHub 地址

您对于源码的疑问每条留言都将得到认真回复。甚至不知道如何读源码也可以请教噢。

新的源码解析文章实时收到通知。每周更新一篇左右。

认真的源码交流微信群。

Spring Web框架架构的主要部分是DispatcherServlet。也就是本文中重点介绍的对象。在本文的第一部分中，我们将看到基于Spring的DispatcherServlet的主要概念：前端控制器模式。第二部分将专门介绍Spring应用程序中的执行链。接下来是DispatcherServlet类的解释。在最后一部分，我们将尝试开发一个自定义的dispatcher servlet。

请注意，本文分析的DispatcherServlet来自Spring的5.0.0.RC3版本。如果使用不同的版本，则可能需要进行几个调整，其实因为分析的都是比较固定的东西，很少有改的。

什么是前端控制器模式？

在进入DispatcherServlet之前，我们需要了解一些关于它的概念基础。DispatcherServlet所隐含的关键概念其实就是前端控制器模式。

此模式为Web应用程序提供了一个中心入口点。该集中入口点将系统组件的共同特征进行重新组合。我们可以在那里找到安全资源，语言切换，会话管理，缓存或输入过滤的处理程序。这样做的一个很大的好处是:这个共同的入口点有助于避免代码重复。

因此，从技术上讲，前端控制器模式由一个捕获所有传入请求的类组成。之后，分析每个请求以知道哪个控制器以及哪个方法应该来处理该请求。

前端控制器模式有助于对以下询问做出最佳响应:

如何集中授权和认证？
如何处理正确的视图渲染？
如何使用URL重写映射将请求发送到适当的控制器？

这个前台控制器模式包含5名参与者：

客户端：发送请求。
控制器：应用程序的中心点，捕获所有请求。
调度员：管理视图的选择，以呈现给客户端。
视图：表示呈现给客户端的内容。
帮助：帮助查看和/或控制器完成请求处理。

什么是DispatcherServlet的执行链？

由标题可以看到，前端控制器模式有自己的执行链。这意味着它有自己的逻辑来处理请求并将视图返回给客户端：

请求由客户端发送。它到达作为Spring的默认前端控制器的DispatcherServlet类。
DispatcherServlet使用请求处理程序映射来发现将分析请求的控制器(controller

)。接口org.springframework.web.servlet.HandlerMapping的实现返回一个包含org.springframework.web.servlet.HandlerExecutionChain类的实例。此实例包含可在控制器调用之前或之后调用的处理程序拦截器数组。你可以在Spring中有关于拦截器的文章中了解更多的信息。如果在所有定义的处理程序映射中找不到HandlerExecutionChain，这意味着Spring无法将URL与对应的控制器进行匹配。这样的话会抛出一个错误。
现在系统进行拦截器预处理并调用由映射处理器找到的相应的controller(其实就是在找到的controller之前进行一波拦截处理)。在controller处理请求后，DispatcherServlet开始拦截器的后置处理。在此步骤结束时，它从controller接收ModelAndView实例(整个过程其实就是 request请求->进入interceptors->controller->从interceptors出来->ModelAndView接收)。
DispatcherServlet现在将使用的该视图的名称发送到视图解析器。这个解析器将决定前台的展现内容。接着，它将此视图返回给DispatcherServlet，其实也就是一个“视图生成后可调用”的拦截器。
最后一个操作是视图的渲染并作为对客户端request请求的响应。

什么是DispatcherServlet？

通过上面讲到的前端控制器模式，我们可以很轻易的知道DispatcherServlet是基于Spring的Web应用程序的中心点。它需要传入请求，并在处理程序映射，拦截器，控制器和视图解析器的帮助下，生成对客户端的响应。所以，我们可以分析这个类的细节，并总结出一些核心要点。

下面是处理一个请求时DispatcherServlet执行的步骤：

1. 策略初始化

DispatcherServlet是一个位于org.springframework.web.servlet包中的类，并扩展了同一个包中的抽象类FrameworkServlet。它包含一些解析器的私有静态字段(用于本地化，视图，异常或上传文件)，映射处理器:handlerMapping和处理适配器:handlerAdapter(进入这个类的第一眼就能看到的)。DispatcherServlet非常重要的一个核心点就是是初始化策略的方法(protected void initStrategies（ApplicationContext context）)。在调用onRefresh方法时调用此方法。最后一次调用是在FrameworkServlet中通过initServletBean和initWebApplicationContext方法进行的(initServletBean方法中调用initWebApplicationContext，后者调用onRefresh(wac))。initServletBean通过所提供的这些策略生成我们所需要的应用程序上下文。其中每个策略都会产生一类在DispatcherServlet中用来处理传入请求的对象。

基于篇幅，有些代码就不给贴示了，请在相应版本的源码中自行对照查找，此处只给一部分源码:

/**
 * This implementation calls {@link #initStrategies}.
 */
@Override
protected void onRefresh(ApplicationContext context) {
	initStrategies(context);
}

/**
 * Initialize the strategy objects that this servlet uses.
 * <p>May be overridden in subclasses in order to initialize further strategy objects.
 */
protected void initStrategies(ApplicationContext context) {
	initMultipartResolver(context);
	initLocaleResolver(context);
	initThemeResolver(context);
	initHandlerMappings(context);
	initHandlerAdapters(context);
	initHandlerExceptionResolvers(context);
	initRequestToViewNameTranslator(context);
	initViewResolvers(context);
	initFlashMapManager(context);
}

需要注意的是，如果找的结果不存在，则捕获异常NoSuchBeanDefinitionException(下面两段代码的第一段)，并采用默认策略。如果在DispatcherServlet.properties文件中初始定义的默认策略不存在，则抛出BeanInitializationException异常(下面两段代码的第二段)。默认策略如下：

/**
	 * Initialize the LocaleResolver used by this class.
	 * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
	 * we default to AcceptHeaderLocaleResolver.
	 */
	private void initLocaleResolver(ApplicationContext context) {
		try {
			this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Using LocaleResolver [" + this.localeResolver + "]");
			}
		}
		catch (NoSuchBeanDefinitionException ex) {
			// We need to use the default.
			this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Unable to locate LocaleResolver with name '" + LOCALE_RESOLVER_BEAN_NAME +"': using default [" + this.localeResolver + "]");
			}
		}
	}

抛出异常后调用getDefaultStrategy(因为容器里都是单例的存在，所以只需要判断基于这个接口的默认实现实例size为1即可，两个以上还能叫默认么，都有选择了):

/**
 * Return the default strategy object for the given strategy interface.
 * The default implementation delegates to {@link #getDefaultStrategies},
 * expecting a single object in the list.
 * @param context the current WebApplicationContext
 * @param strategyInterface the strategy interface
 * @return the corresponding strategy object
 * @see #getDefaultStrategies
 */
protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
    List<T> strategies = getDefaultStrategies(context, strategyInterface);
    if (strategies.size() != 1) {
        throw new BeanInitializationException(
                "DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
    }
    return strategies.get(0);
}

	/**
	 * Create a List of default strategy objects for the given strategy interface.
	 * The default implementation uses the "DispatcherServlet.properties" file (in the same
	 * package as the DispatcherServlet class) to determine the class names. It instantiates
	 * the strategy objects through the context's BeanFactory.
	 * @param context the current WebApplicationContext
	 * @param strategyInterface the strategy interface
	 * @return the List of corresponding strategy objects
	 */
	@SuppressWarnings("unchecked")
	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);
				}
				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(
							"Error loading DispatcherServlet's default strategy class [" + className +"] for interface [" + key + "]: problem with class file or dependent class", err);
				}
			}
			return strategies;
		}
		else {
			return new LinkedList<>();
		}
	}

2.请求预处理

FrameworkServlet抽象类扩展了同一个包下的HttpServletBean，HttpServletBean扩展了javax.servlet.http.HttpServlet。点开这个类源码可以看到，HttpServlet是一个抽象类，其方法定义主要用来处理每种类型的HTTP请求：doGet（GET请求），doPost（POST），doPut（PUT），doDelete（DELETE），doTrace（TRACE），doHead（HEAD） ，doOptions（OPTIONS）。FrameworkServlet通过将每个传入的请求调度到**processRequest(HttpServletRequest request，HttpServletResponse response)**来覆盖它们。processRequest是一个protected和final的方法，它构造出LocaleContext和ServletRequestAttributes对象，两者都可以在initContextHolders(request, localeContext, requestAttributes)之后访问。所有这些操作的关键代码请看：

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    processRequest(request, response);
}
 ...
/**
 * Process this request, publishing an event regardless of the outcome.
 * The actual event handling is performed by the abstract
 * {@link #doService} template method.
 */
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;

    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    LocaleContext localeContext = buildLocaleContext(request);

    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

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

    initContextHolders(request, localeContext, requestAttributes);

    try {
        doService(request, response);
    }
    catch (ServletException ex) {
        failureCause = ex;
        throw ex;
    }
    catch (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();
        }

        if (logger.isDebugEnabled()) {
            if (failureCause != null) {
                this.logger.debug("Could not complete request", failureCause);
            }
            else {
                if (asyncManager.isConcurrentHandlingStarted()) {
                    logger.debug("Leaving response open for concurrent processing");
                }
                else {
                    this.logger.debug("Successfully completed request");
                }
            }
        }

        publishRequestHandledEvent(request, startTime, failureCause);
    }
}

...

  private void initContextHolders(HttpServletRequest request,
			@Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {

		if (localeContext != null) {
			LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
		}
		if (requestAttributes != null) {
			RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
		}
		if (logger.isTraceEnabled()) {
			logger.trace("Bound request context to thread: " + request);
		}
	}

3.请求处理

由上面所看到的，在processRequest的代码中，调用initContextHolders方法后，调用protected void doService(HttpServletRequest request，HttpServletResponse response)。doService将一些附加参数放入request（如Flash映射:request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap())，上下文信息:request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext())等）中，并调用protected void doDispatch(HttpServletRequest request，HttpServletResponse response)。

doDispatch方法最重要的部分是处理(handler)的检索。doDispatch调用getHandler()方法来分析处理后的请求并返回HandlerExecutionChain实例。此实例包含handler mapping 和``interceptors(拦截器)。DispatcherServlet做的另一件事是应用预处理程序拦截器（*applyPreHandle()*）。如果至少有一个返回false，则请求处理停止。否则，servlet使用与handler adapter适配(其实理解成这也是个handler就对了)相应的handler mapping`来生成视图对象。

doDispatch方法：

/**
 * Process the actual dispatching to the handler.
 * The handler will be obtained by applying the servlet's HandlerMappings in order.
 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
 * to find the first that supports the handler class.
 * All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
 * themselves to decide which methods are acceptable.
 * @param request current HTTP request
 * @param response current HTTP response
 * @throws Exception in case of any kind of processing failure
 */
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 {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.获取可处理request的Handler，适配器其实还				//是调用的相应的Handler,一样的功能，具体请参考本人的Spring设计模式中的适配器模式
				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 (logger.isDebugEnabled()) {
						logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.此处就会调用我们写的controller来执行咯
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
				//视图解析
				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);
			}
          	//此处进行最后一步的视图渲染
			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);
				}
			}
		}
	}

4.视图解析

获取ModelAndView实例以查看呈现后，doDispatch方法调用private void applyDefaultViewName(HttpServletRequest request，ModelAndView mv)。默认视图名称根据定义的bean名称，即viewNameTranslator。默认情况下，它的实现是org.springframework.web.servlet.RequestToViewNameTranslator。这个默认实现只是简单的将URL转换为视图名称，例如(直接从RequestToViewNameTranslator获取):http:// localhost:8080/admin/index.html将生成视图admin / index。

代码如下:

下一步是调用后置拦截器(其实就是出拦截器)做的一些处理。

/** RequestToViewNameTranslator used by this servlet */
	@Nullable
	private RequestToViewNameTranslator viewNameTranslator;
...
  protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context); //看下面注释
		initViewResolvers(context);
		initFlashMapManager(context);
	}
...

  /**
	 * Initialize the RequestToViewNameTranslator used by this servlet instance.
	 * <p>If no implementation is configured then we default to DefaultRequestToViewNameTranslator.
	 */
	private void initRequestToViewNameTranslator(ApplicationContext context) {
		try {
			this.viewNameTranslator =
					context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Using RequestToViewNameTranslator [" + this.viewNameTranslator + "]");
			}
		}
		catch (NoSuchBeanDefinitionException ex) {
			// We need to use the default.
			this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Unable to locate RequestToViewNameTranslator with name '" +
						REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator +"]");
			}
		}
	}

....
  /**
	 * Translate the supplied request into a default view name.
	 * @param request current HTTP servlet request
	 * @return the view name (or {@code null} if no default found)
	 * @throws Exception if view name translation failed
	 */
	@Nullable
	protected String getDefaultViewName(HttpServletRequest request) throws Exception {
		return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);
	}

org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator实现的org.springframework.web.servlet.RequestToViewNameTranslator接口，其内对上段代码中getDefaultViewName的实现为:

public class DefaultRequestToViewNameTranslator implements RequestToViewNameTranslator {

	private static final String SLASH = "/";


	private String prefix = "";

	private String suffix = "";

	private String separator = SLASH;

	private boolean stripLeadingSlash = true;

	private boolean stripTrailingSlash = true;

	private boolean stripExtension = true;

	private UrlPathHelper urlPathHelper = new UrlPathHelper();


	/**
	 * Set the prefix to prepend to generated view names.
	 * @param prefix the prefix to prepend to generated view names
	 */
	public void setPrefix(String prefix) {
		this.prefix = (prefix != null ? prefix : "");
	}

	/**
	 * Set the suffix to append to generated view names.
	 * @param suffix the suffix to append to generated view names
	 */
	public void setSuffix(String suffix) {
		this.suffix = (suffix != null ? suffix : "");
	}

	/**
	 * Set the value that will replace '{@code /}' as the separator
	 * in the view name. The default behavior simply leaves '{@code /}'
	 * as the separator.
	 */
	public void setSeparator(String separator) {
		this.separator = separator;
	}
	...
	/**
	 * Translates the request URI of the incoming {@link HttpServletRequest}
	 * into the view name based on the configured parameters.
	 * @see org.springframework.web.util.UrlPathHelper#getLookupPathForRequest
	 * @see #transformPath
	 */
	@Override
	public String getViewName(HttpServletRequest request) {
		String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
		return (this.prefix + transformPath(lookupPath) + this.suffix);
	}

	/**
	 * Transform the request URI (in the context of the webapp) stripping
	 * slashes and extensions, and replacing the separator as required.
	 * @param lookupPath the lookup path for the current request,
	 * as determined by the UrlPathHelper
	 * @return the transformed path, with slashes and extensions stripped
	 * if desired
	 */
	protected String transformPath(String lookupPath) {
		String path = lookupPath;
		if (this.stripLeadingSlash && path.startsWith(SLASH)) {
			path = path.substring(1);
		}
		if (this.stripTrailingSlash && path.endsWith(SLASH)) {
			path = path.substring(0, path.length() - 1);
		}
		if (this.stripExtension) {
			path = StringUtils.stripFilenameExtension(path);
		}
		if (!SLASH.equals(this.separator)) {
			path = StringUtils.replace(path, SLASH, this.separator);
		}
		return path;
	}

}

5.处理调度请求 - 视图渲染

现在，servlet知道应该是哪个视图被渲染。它通过**private void processDispatchResult(HttpServletRequest request，HttpServletResponse response，@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,@Nullable Exception exception)**方法来进行最后一步操作 - 视图渲染。

首先，processDispatchResult检查它们是否有参数传递异常。有一些异常的话，它定义了一个新的视图，专门用来定位错误页面。如果没有任何异常，该方法将检查ModelAndView实例，如果它不为null，则调用render方法。

渲染方法protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception。跳进此方法内部，根据定义的视图策略，它会查找得到一个View类实例。它将负责显示响应。如果没有找到View，则会抛出一个ServletException异常。有的话，DispatcherServlet会调用其render方法来显示结果。

其实可以说成是后置拦截器(进入拦截器前置拦截处理->controller处理->出拦截器之前的此拦截器的后置处理)，也就是在请求处理的最后一个步骤中被调用。

下面是processDispatchResult和render(渲染)的相关代码:

/**
	 * Handle the result of handler selection and handler invocation, which is
	 * either a ModelAndView or an Exception to be resolved to a ModelAndView.
	 */
	private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
			@Nullable Exception exception) throws Exception {

		boolean errorView = false;

		if (exception != null) {
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				mv = processHandlerException(request, response, handler, exception);
				errorView = (mv != null);
			}
		}

		// Did the handler return a view to render?
		if (mv != null && !mv.wasCleared()) {
          //开始渲染
			render(mv, request, response);
			if (errorView) {
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
						"': assuming HandlerAdapter completed request handling");
			}
		}

		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Concurrent handling started during a forward
			return;
		}

		if (mappedHandler != null) {
			mappedHandler.triggerAfterCompletion(request, response, null);
		}
	}


...
  /**
	 * Render the given ModelAndView.
	 * <p>This is the last stage in handling a request. It may involve resolving the view by name.
	 * @param mv the ModelAndView to render
	 * @param request current HTTP servlet request
	 * @param response current HTTP servlet response
	 * @throws ServletException if view is missing or cannot be resolved
	 * @throws Exception if there's a problem rendering the view
	 */
	protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
		// Determine locale for request and apply it to the response.
		Locale locale =
				(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
		response.setLocale(locale);

		View view;
		String viewName = mv.getViewName();
		if (viewName != null) {
			// We need to resolve the view name.
			view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
			if (view == null) {
				throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
						"' in servlet with name '" + getServletName() + "'");
			}
		}
		else {
			// No need to lookup: the ModelAndView object contains the actual View object.
			view = mv.getView();
			if (view == null) {
				throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +"View object in servlet with name '" + getServletName() + "'");
			}
		}

		// Delegate to the View object for rendering.
		if (logger.isDebugEnabled()) {
			logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
		}
		try {
			if (mv.getStatus() != null) {
				response.setStatus(mv.getStatus().value());
			}
			view.render(mv.getModelInternal(), request, response);
		}
		catch (Exception ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
						getServletName() + "'", ex);
			}
			throw ex;
		}
	}

/**
 * Handle the result of handler selection and handler invocation, which is
 * either a ModelAndView or an Exception to be resolved to a ModelAndView.
 */
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
        HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

    boolean errorView = false;

    if (exception != null) {
        if (exception instanceof ModelAndViewDefiningException) {
            logger.debug("ModelAndViewDefiningException encountered", exception);
            mv = ((ModelAndViewDefiningException) exception).getModelAndView();
        }
        else {
            Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
            mv = processHandlerException(request, response, handler, exception);
            errorView = (mv != null);
        }
    }

    // Did the handler return a view to render?
    if (mv != null && !mv.wasCleared()) {
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
    else {
        if (logger.isDebugEnabled()) {
            logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                    "': assuming HandlerAdapter completed request handling");
        }
    }

    if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        // Concurrent handling started during a forward
        return;
    }

    if (mappedHandler != null) {
        mappedHandler.triggerAfterCompletion(request, response, null);
    }
}

/**
 * Render the given ModelAndView.
 * This is the last stage in handling a request. It may involve resolving the view by name.
 * @param mv the ModelAndView to render
 * @param request current HTTP servlet request
 * @param response current HTTP servlet response
 * @throws ServletException if view is missing or cannot be resolved
 * @throws Exception if there's a problem rendering the view
 */
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
    // Determine locale for request and apply it to the response.
    Locale locale = this.localeResolver.resolveLocale(request);
    response.setLocale(locale);

    View view;
    if (mv.isReference()) {
        // We need to resolve the view name.
        view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
        if (view == null) {
            throw new ServletException(
                    "Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + getServletName() + "'");
        }
    }
    else {
        // No need to lookup: the ModelAndView object contains the actual View object.
        view = mv.getView();
        if (view == null) {
            throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + getServletName() + "'");
        }
    }

    // Delegate to the View object for rendering.
    if (logger.isDebugEnabled()) {
        logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
    }
    try {
        view.render(mv.getModelInternal(), request, response);
    }
    catch (Exception ex) {
        if (logger.isDebugEnabled()) {
            logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'", ex);
        }
        throw ex;
    }
}

在这部分中，你需要记住的是我们定义了两个上下文：一个用于应用程序，另一个用于Web应用程序。他们有什么区别？应用程序上下文包含所有通用配置，比如service定义，数据库配置。Web应用程序上下文定义所有与Web相关的组件，比如controllers或视图解析器。

Custom DispatcherServlet

我们已经了解了DispatcherServlet的理论知识。通过文中的这些实用要点，我们可以编写自己的servlet来分派处理请求。同样的，我们也将按步进行，从捕获请求开始，以视图渲染结束。

通过上面的描述，为了捕获请求，我们需要覆盖doService方法：

public class CustomDispatcherServlet extends FrameworkServlet {
    private static final Logger LOGGER = LoggerFactory.getLogger(CustomDispatcherServlet.class);

    @Override
    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        LOGGER.debug("[CustomDispatcherServlet] I got the request !");
    }
}

这样，在我们的日志文件中，我们应该可以找到一条“[CustomDispatcherServlet]I got the request！”。接着，我们继续添加在DispatcherServlet中doDispatch方法所应该做的一些工作：

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    LOGGER.debug("[CustomDispatcherServlet] I got the request !");
    try {
        LOGGER.debug("[CustomDispatcherServlet] doService");
        LocaleContext localeContext = buildLocaleContext(request);

        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        initContextHolders(request, localeContext, requestAttributes);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

这个方法是做什么的？首先，它为构建一个Locale实例用来接收请求。第二步是初始化org.springframework.web.context.request.ServletRequestAttributes实例。它是RequestAttributes接口的实现，和本地化在同一级别。通过这个，我们可以访问servlet请求的对象和会话对象，而不必区分会话和全局会话。最后，我们调用初始化context holders的initContextHolders()方法，即从应用程序通过LocaleContextHolder和RequestContextHolder静态方法(分别为:getLocaleContext和getRequestAttributes)访问请求属性和区域设置的对象。

当请求被拦截，一些基本的设置就绪的时候。我们发现我们还没有执行链和处理器适配器。我们可以通过以下代码进行:

private HandlerExecutionChain getHandlerExecutionChain(HttpServletRequest request) throws Exception {
    for (HandlerMapping mapper : this.handlerMappings) {
        HandlerExecutionChain executionChain = mapper.getHandler(request);
        if (executionChain != null) {
            return executionChain;
        }
    }
    throw new Exception("Execution chain wasn't be found in provided handler mappings: "+this.handlerMappings);
}

通过执行链，我们可以通过 handler adapter将处理当前请求。看以下代码:

@Override
protected HandlerAdapter getHandlerAdapter(Object executionChain) throws ServletException {
    for (HandlerAdapter adapter : this.handlerAdapters) {
        LOGGER.debug("[CustomDispatcherServlet] "+adapter + " is instanceof HandlerMethod ? "+(adapter instanceof HandlerMethod));
        if (adapter.supports(executionChain)) {
            return adapter;
        }
    }
    throw new  ServletException("Handler adapter was not found from adapters list :"+this.handlerAdapters);
}

只有应用程序上下文中定义的适配器（this.handlerAdapter）支持适配所生成的执行链（adapter.supports）才可以返回我们想要的适配器。最后，我们可以返回到我们的doService方法并操作它们来渲染视图：

ModelAndView modelView = adapter.handle(request, response, executionChain.getHandler());
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);

View view = null;
if (!modelView.isReference()) {
    throw new UnsupportedOperationException("Only view models defined as references can be used in this servlet");
}
for (ViewResolver viewResolver : this.viewResolvers) {
    view = viewResolver.resolveViewName(modelView.getViewName(), locale);
    if (view != null) {
        break;
    }
}
if (view == null) {
    throw new ServletException("Could not resolve view with name '" + modelView.getViewName() + "' in servlet with name '" + getServletName() + "'");
}
view.render(modelView.getModelMap(), request, response);

我们的servlet中简化了渲染。实际上，我们仅处理ModelAndView的引用对象。这意味着ModelAndView是一个String的实例，用来表示要解析的视图模型，例如：我们定义好几个模板解析器(比如freemaker，Thymeleaf)，然后查看其配置。在这个检查之后，我们迭代当前视图解析器。能够生成View实例的第一个解析器被视为处理过的请求中使用的解析器。最后，我们检查视图是否正确生成。拿到view实例后，我们调用其render()方法来在屏幕中显示请求处理结果。

在这部分中，我们将描述和代码部分限制在最低限度。只是为了把Spring的整个过程给集中呈现以下，达到更好的理解，其实就是在Servlet中的service方法内做些对request和response的文章而已了。

本文介绍了Spring Web应用程序的中心点，一个调度器servlet。请记住，它是一个处理所有传入请求并将视图呈现给用户的类。在重写之前，你应该熟悉执行链，handler mapping 或handler adapter等概念。请记住，第一步要做的是定义在调度过程中我们要调用的所有元素。handler mapping 是将传入请求(也就是它的URL)映射到适当的controller。最后提到的元素，一个handler适配器，就是一个对象，它将通过其内包装的handler mapping将请求发送到controller。此调度产生的结果是ModelAndView类的一个实例，后面被用于生成和渲染视图。

666. 彩蛋

如果你对 Spring 感兴趣，欢迎加入我的知识星球一起交流。

知识星球

芋道源码 —— 纯源码解析博客

Spring 5 源码解析 —— 论 Spring DispatcherServlet 的生命周期