Servlet的生命周期以及在Spring MVC中调用流程 |
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Servlet的生命周期以及在Spring MVC中调用流程
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接触Web时间比较久，虽然知道Servlet的生命周期但是理解却还是不够，今天刚好debug代码涉及这块就利用余下时间研究了一下。 Servlet的生命周期以及处理浏览器请求的过程。Servlet接口中定义的方法有：
而init -> service -> destory刚好就是servlet的生命周期。通常当web容器（即web服务器，例如tomct等）启动时候回根据配置文件进行加载servlet，加载servlet过程就是初始化servlet的过程。 由web容器进行调用生命周期init方法进行初始化。初始化完成便可以进行处理浏览器发送的请求了。
当浏览器向Web容器发送请求时候便由Web容器调用Servelt的service方法（此处实现是HttpServlet的service方法）进行处理浏览器请求。HttpService的service方法关键源码实现如下: protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); //判断客户端的请求是get、post、... if (method.equals(METHOD_GET)) { long lastModified = getLastModified(req); if (lastModified == -1) { doGet(req, resp); } else { long ifModifiedSince; try { ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); } catch (IllegalArgumentException iae) { ifModifiedSince = -1; } if (ifModifiedSince < (lastModified / 1000 * 1000)) { maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } } else if (method.equals(METHOD_HEAD)) { long lastModified = getLastModified(req); maybeSetLastModified(resp, lastModified); doHead(req, resp); } else if (method.equals(METHOD_POST)) { doPost(req, resp);//处理post请求 } else if (method.equals(METHOD_PUT)) { doPut(req, resp); } else if (method.equals(METHOD_DELETE)) { doDelete(req, resp); } else if (method.equals(METHOD_OPTIONS)) { doOptions(req,resp); } else if (method.equals(METHOD_TRACE)) { doTrace(req,resp); } else { String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[1]; errArgs[0] = method; errMsg = MessageFormat.format(errMsg, errArgs); resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg); } }本例以post请求进行源码跟踪，处理post请求调用doPost方法。此处回顾一下我们一般写一个servlet时候需要的步骤了，我们都是自己定义一个Servlet类然后一般都是实现doPost和doGet方法，回顾到此就可以回到本文了，本例中调用的doPost方法就是调用我们自己实现的doPost方法，这个流程下来就完成了浏览器的请求。而对于Servlet的销毁由Web容器调用destory方法进行销毁。
Spring MVC中DispatherServlet的继承结构：
在使用Spring MVC进行开发时候，在web.xml中需要配置DispatherServlet，让Web容器启动时候加载Servlet。通过继承结构可以发现DispatherServlet是HttpServlet的子类。 当浏览器进行请求时候Web容器会调用DispatherServlet的service方法，DispatherServlet的service方法在父类FrameWorkServlet中实现；FrameWorkServlet#service方法源码如下： /** * Override the parent class implementation in order to intercept PATCH requests. */ @Override protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpMethod httpMethod = HttpMethod.resolve(request.getMethod()); if (HttpMethod.PATCH == httpMethod || httpMethod == null) { processRequest(request, response); } else { super.service(request, response); } }
最后调用super.service调用HttpServlet#service方法，进行判断请求的方法是get、post还是...然后调用对应的doXXX方法，此处的doXXX方法在FrameWorkServlet中实现（FrameWorkServlet是框架实现的一个Servlet，与正常Servlet开发的servlet类似）。 org.springframework.web.servlet.FrameworkServlet#doServiceorg.springframework.web.servlet.FrameworkServlet#doPost源码如下： @Override protected final void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); }org.springframework.web.servlet.FrameworkServlet#processRequest源码如下： protected final void processRequest(HttpServletRequest request, HttpServletResponse response) //DispatcherServlet的processRequst方法在FrameWork中实现 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); //重点 } 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, response, startTime, failureCause); } }org.springframework.web.servlet.DispatcherServlet#doService源码： @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 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)); } } } // 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); //重点 } finally { if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Restore the original attribute snapshot, in case of an include. if (attributesSnapshot != null) { restoreAttributesAfterInclude(request, attributesSnapshot); } } } }最后调用doDispatch(request, response)方法完成处理（后续代码有时间在跟踪）。
过程总结：
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