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@ -761,6 +761,255 @@ public abstract class AbstractUrlHandlerMapping extends AbstractHandlerMapping {
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经过这一系列对HTTP请求进行解析和匹配handler的过程，得到了与请求对应的handler处理器。在返回的handler中，已经完成了在HandlerExecutionChain中进行封装的工作，为handler对HTTP请求的响应做好了准备。
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### 4.3 DispatcherServlet对HTTP请求的分发处理
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DispatcherServlet是Spring MVC框架中非常重要的一个类，不但建立了自己持有的IoC容器，还肩负着请求分发处理的重任，对HTTP请求的处理是在doService()方法中完成的。DispatcherServlet是HttpServlet的子类 ，与其他HttpServlet一样，可以通过doService()来响应HTTP的请求。然而，依照Spring MVC的使用，业务逻辑的调用入口是在handler的handler()方法中实现的，这是连接Spring MVC和应用业务逻辑实现的地方。
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```java
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public class DispatcherServlet extends FrameworkServlet {
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/** 此servlet使用的HandlerMappings列表 */
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private List<HandlerMapping> handlerMappings;
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/** 此servlet使用的HandlerAdapter列表 */
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private List<HandlerAdapter> handlerAdapters;
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/**
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* 公开DispatcherServlet特定的请求属性，并将其委托给doDispatch()方法进行实际的分发
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*/
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@Override
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protected void doService(HttpServletRequest request, HttpServletResponse response)
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throws Exception {
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if (logger.isDebugEnabled()) {
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// 得到请求的URI
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String requestUri = urlPathHelper.getRequestUri(request);
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String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult()
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? " resumed" : "";
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logger.debug("DispatcherServlet with name '" + getServletName() + "'"
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+ resumed + " processing " + request.getMethod() + " request for ["
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+ requestUri + "]");
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}
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// Keep a snapshot of the request attributes in case of an include,
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// to be able to restore the original attributes after the include.
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Map<String, Object> attributesSnapshot = null;
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if (WebUtils.isIncludeRequest(request)) {
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logger.debug("Taking snapshot of request attributes before include");
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attributesSnapshot = new HashMap<String, Object>();
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Enumeration<?> attrNames = request.getAttributeNames();
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while (attrNames.hasMoreElements()) {
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String attrName = (String) attrNames.nextElement();
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if (this.cleanupAfterInclude
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|| attrName.startsWith("org.springframework.web.servlet")) {
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attributesSnapshot.put(attrName, request.getAttribute(attrName));
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}
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}
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}
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// 使框架对象对处理程序和视图对象可用
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request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE,
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getWebApplicationContext());
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request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
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request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
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request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
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FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request,
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response);
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if (inputFlashMap != null) {
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request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE,
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Collections.unmodifiableMap(inputFlashMap));
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}
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request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
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request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
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try {
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doDispatch(request, response);
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}
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finally {
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if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
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return;
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}
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// Restore the original attribute snapshot, in case of an include.
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if (attributesSnapshot != null) {
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restoreAttributesAfterInclude(request, attributesSnapshot);
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}
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}
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}
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/**
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* 中央控制器,控制请求的转发
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* 对请求的处理实际上是由doDispatch()来完成的，它是DispatcherServlet完成HTTP请求分发处理的主要方法,
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* 包括准备ModelAndView，调用getHandler()方法来响应HTTP请求，然后通过执行Handler的处理来获取请求的
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* 处理结果，最后把结果返回出去
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*/
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protected void doDispatch(HttpServletRequest request, HttpServletResponse response)
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throws Exception {
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HttpServletRequest processedRequest = request;
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HandlerExecutionChain mappedHandler = null;
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boolean multipartRequestParsed = false;
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WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
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try {
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// 为视图准备好一个ModelAndView，这个ModelAndView持有handler处理请求的结果
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ModelAndView mv = null;
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Exception dispatchException = null;
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try {
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// 1.检查是否是文件上传的请求
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processedRequest = checkMultipart(request);
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multipartRequestParsed = processedRequest != request;
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// 2.取得处理当前请求的controller，这里也称为hanlder处理器，这里并不是
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// 直接返回controller，而是返回的HandlerExecutionChain请求处理器链对象，
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// 该对象封装了handler和interceptors
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mappedHandler = getHandler(processedRequest, false);
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// 如果handler为空,则返回404
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if (mappedHandler == null || mappedHandler.getHandler() == null) {
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noHandlerFound(processedRequest, response);
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return;
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}
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// 3. 获取处理request的处理器适配器handler adapter
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HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
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// 获取请求方式，如：GET, POST, PUT
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String method = request.getMethod();
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boolean isGet = "GET".equals(method);
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if (isGet || "HEAD".equals(method)) {
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long lastModified = ha.getLastModified(request,
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mappedHandler.getHandler());
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if (logger.isDebugEnabled()) {
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String requestUri = urlPathHelper.getRequestUri(request);
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logger.debug("Last-Modified value for [" + requestUri + "] is: "
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+ lastModified);
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}
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if (new ServletWebRequest(request, response).checkNotModified(
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lastModified) && isGet) {
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return;
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}
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}
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// 4.拦截器的预处理方法
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if (!mappedHandler.applyPreHandle(processedRequest, response)) {
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return;
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}
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try {
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// 5.实际的处理器处理请求,返回结果视图对象
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mv = ha.handle(processedRequest, response,
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mappedHandler.getHandler());
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}
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finally {
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if (asyncManager.isConcurrentHandlingStarted()) {
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return;
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}
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}
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// 结果视图对象的处理
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applyDefaultViewName(request, mv);
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// 6.拦截器的后处理方法
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mappedHandler.applyPostHandle(processedRequest, response, mv);
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}
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catch (Exception ex) {
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dispatchException = ex;
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}
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processDispatchResult(processedRequest, response, mappedHandler, mv,
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dispatchException);
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}
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catch (Exception ex) {
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// 请求成功响应之后的方法
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triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
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}
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catch (Error err) {
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triggerAfterCompletionWithError(processedRequest, response, mappedHandler,
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err);
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}
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finally {
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if (asyncManager.isConcurrentHandlingStarted()) {
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// Instead of postHandle and afterCompletion
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mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest,
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response);
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return;
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}
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// 清除多部分请求使用的所有资源
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if (multipartRequestParsed) {
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cleanupMultipart(processedRequest);
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}
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}
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}
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/**
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* 返回此请求的HandlerExecutionChain，按顺序尝试所有的HandlerMapping
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*/
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@Deprecated
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protected HandlerExecutionChain getHandler(HttpServletRequest request, boolean cache)
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throws Exception {
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return getHandler(request);
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}
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/**
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* 返回此请求的HandlerExecutionChain
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*/
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protected HandlerExecutionChain getHandler(HttpServletRequest request)
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throws Exception {
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// 遍历 此servlet使用的HandlerMapping列表
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for (HandlerMapping hm : this.handlerMappings) {
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if (logger.isTraceEnabled()) {
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logger.trace("Testing handler map [" + hm
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+ "] in DispatcherServlet with name '" + getServletName() + "'");
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}
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// 查找给定请求的handler
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HandlerExecutionChain handler = hm.getHandler(request);
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if (handler != null) {
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return handler;
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}
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}
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return null;
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}
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/**
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* 返回此处理程序对象handler的HandlerAdapter
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*/
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protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
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// 对所有持有的HandlerAdapter进行匹配
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for (HandlerAdapter ha : this.handlerAdapters) {
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if (logger.isTraceEnabled()) {
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logger.trace("Testing handler adapter [" + ha + "]");
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}
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if (ha.supports(handler)) {
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return ha;
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}
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}
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throw new ServletException("No adapter for handler [" + handler
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+ "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
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}
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}
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```
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通过判断，可以知道这个handler是不是Controller接口的实现，比如可以通过具体HandlerAdapter的实现来了解这个适配过程。以SimpleControllerHandlerAdapter的实现为例来了解这个判断是怎样起作用的。
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```java
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public class SimpleControllerHandlerAdapter implements HandlerAdapter {
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// 判断要执行的handler是不是Controller类型的
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public boolean supports(Object handler) {
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return (handler instanceof Controller);
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}
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public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
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throws Exception {
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return ((Controller) handler).handleRequest(request, response);
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}
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public long getLastModified(HttpServletRequest request, Object handler) {
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if (handler instanceof LastModified) {
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return ((LastModified) handler).getLastModified(request);
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}
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return -1L;
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}
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}
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```
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经过上面一系列的处理，得到了handler对象，接着就可以开始调用handler对象中的HTTP响应动作了。在handler中封装了应用业务逻辑，由这些逻辑对HTTP请求进行相应的处理，生成需要的数据，并把这些数据封装到ModelAndView对象中去，这个ModelAndView的数据封装是Spring MVC框架的要求。对handler来说， 这些都是通过调用handler()方法中的handleRequest()方法来触发完成的。在得到ModelAndView对象以后，这个ModelAndView对象会被交给MVC模式中的视图类，由视图类对ModelAndView对象中的数据进行呈现。
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AmyliaY
5 years ago