Servlet的生命周期以及在Spring MVC中调用流程

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<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(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：