spring mvc DispatcherServlet之前端控制器架构详解

前端控制器是整个MVC框架中最为核心的一块，它主要用来拦截符合要求的外部请求，并把请求分发到不同的控制器去处理，根据控制器处理后的结果，生成相应的响应发送到客户端。前端控制器既可以使用Filter实现（Struts2采用这种方式），也可以使用Servlet来实现(spring MVC框架)。

DispatcherServlet 作为前置控制器是web服务器的入口，是spring mvc最重要的一个类，通过它的生命周期可以加深对web服务器的理解。

servlet的生命周期

首先我们回忆一下servlet的生命周期：

Servlet生命周期分为三个阶段：【Servlet生命周期与工作原理详解】

　　1.初始化阶段  调用init()方法。Servlet被装载后，Servlet容器创建一个Servlet实例并且调用Servlet的init()方法进行初始化。在Servlet的整个生命周期内，init()方法只被调用一次。

　　2.响应客户请求阶段　　调用service()方法

　　3.终止阶段　　调用destroy()方法

Servlet初始化阶段

　　在下列时刻Servlet容器装载Servlet：

　　1.Servlet容器启动时自动装载某些Servlet，实现它只需要在web.XML文件中的<Servlet></Servlet>之间添加如下代码：　　

<loadon-startup>1</loadon-startup>
　　2.在Servlet容器启动后，客户首次向Servlet发送请求

　　3.Servlet类文件被更新后，重新装载Servlet

DispatcherServlet的结构

复习了上述知识后我们来看看DispatcherServlet的结构：

DispatcherServlet继承自抽象类：FrameworkServlet，间接继承了HttpServlet （FrameworkServlet继承自HttpServletBean，而HttpServletBean继承自HttpServlet ）

Servlet的初始化

 protected void initStrategies(ApplicationContext context) {
 initMultipartResolver(context); //文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；
 initLocaleResolver(context); //本地化解析
 initThemeResolver(context);　　　//主题解析
 initHandlerMappings(context); //通过HandlerMapping，将请求映射到处理器
 initHandlerAdapters(context); //通过HandlerAdapter支持多种类型的处理器
 initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析
 initRequestToViewNameTranslator(context); //直接解析请求到视图名
 initViewResolvers(context); //通过ViewResolver解析逻辑视图名到具体视图实现
 initFlashMapManager(context); //flash映射管理器
 }

servlet如何处理请求：

servlet的service方法处理http请求。

FrameworkServlet.java 定义了servlet的service和destroy方法，如下所示：

 /**
 * Override the parent class implementation in order to intercept PATCH
 * requests.
 */
 @Override
 protected void service(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {

 String method = request.getMethod();
 if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
  processRequest(request, response);
 }
 else {
  super.service(request, response);
 }
 }

我们知道http请求类型有七种（外加一个option选项），定义如下：

public enum RequestMethod {
GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE

}

FrameworkServlet的service()处理不同的请求，我们以常见的post来说明：

 /**
 * Process this request, publishing an event regardless of the outcome.
 * <p>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);
 }
 }

FrameworkServlet 抽象定义了处理流程，留待子类来实现该方法，完成具体的请求处理。

/**
 * Subclasses must implement this method to do the work of request handling,
 * receiving a centralized callback for GET, POST, PUT and DELETE.
 * <p>The contract is essentially the same as that for the commonly overridden
 * {@code doGet} or {@code doPost} methods of HttpServlet.
 * <p>This class intercepts calls to ensure that exception handling and
 * event publication takes place.
 * @param request current HTTP request
 * @param response current HTTP response
 * @throws Exception in case of any kind of processing failure
 * @see javax.servlet.http.HttpServlet#doGet
 * @see javax.servlet.http.HttpServlet#doPost
 */
 protected abstract void doService(HttpServletRequest request, HttpServletResponse response)
  throws Exception;

具体实现如下：

 /**
 * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
 * for the actual dispatching.
 */
 @Override
 protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
 if (logger.isDebugEnabled()) {
  String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
  logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
   " processing " + request.getMethod() + " request for [" + getRequestUri(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<String, Object>();
  Enumeration<?> attrNames = request.getAttributeNames();
  while (attrNames.hasMoreElements()) {
  String attrName = (String) attrNames.nextElement();
  if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
   attributesSnapshot.put(attrName, request.getAttribute(attrName));
  }
  }
 }

 // Make framework objects available to handlers and view objects.
 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());

 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 {
  doDispatch(request, response);
 }
 finally {
  if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
  return;
  }
  // Restore the original attribute snapshot, in case of an include.
  if (attributesSnapshot != null) {
  restoreAttributesAfterInclude(request, attributesSnapshot);
  }
 }
 }

重头戏，作为请求分发器的实现：

功能：1. 把请求分发到handler（按照配置顺序获取servlet的映射关系获取handler）；2. 根据servlet已安装的  HandlerAdapters 去查询第一个能处理的handler；3. handler激发处理请求

 /**
 * Process the actual dispatching to the handler.
 * <p>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.
 * <p>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 || mappedHandler.getHandler() == null) {
   noHandlerFound(processedRequest, response);
   return;
  }

  // Determine handler adapter for the current request.
  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;
  }

  try {
   // Actually invoke the handler.
   mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  }
  finally {
   if (asyncManager.isConcurrentHandlingStarted()) {
   return;
   }
  }

  applyDefaultViewName(request, mv);
  mappedHandler.applyPostHandle(processedRequest, response, mv);
  }
  catch (Exception ex) {
  dispatchException = ex;
  }
  processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
 }
 catch (Exception ex) {
  triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
 }
 catch (Error err) {
  triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
 }
 finally {
  if (asyncManager.isConcurrentHandlingStarted()) {
  // Instead of postHandle and afterCompletion
  mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
  return;
  }
  // Clean up any resources used by a multipart request.
  if (multipartRequestParsed) {
  cleanupMultipart(processedRequest);
  }
 }
 }

servlet销毁

 /**
 * Close the WebApplicationContext of this servlet.
 * @see org.springframework.context.ConfigurableApplicationContext#close()
 */
 @Override
 public void destroy() {
 getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");
 // Only call close() on WebApplicationContext if locally managed...
 if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {
  ((ConfigurableApplicationContext) this.webApplicationContext).close();
 }
 }

小结：

本文因篇章限制，仅仅介绍了请求处理的流程，没有对代码进行深入的分析，接下来的文章将从细微处着手，分析spring的代码之美。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持我们。

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