很多人會用 SpringMVC,但對它的處理請求的方式並不清楚,當我們學習一個知識的時候,瞭解它會讓我們更好地使用它,下面我們來看看 SpringMVC 是如何處理請求的。
請求流程的方式
先上圖:
Spring MVC 框架也是一個基於請求驅動的 Web 框架,並且使用了前端控制器模式(是用來提供一個集中的請求處理機制,所有的請求都將由一個單一的處理程序處理來進行設計,再根據請求映射規則分發給相應的頁面控制器(動作/處理器)進行處理。首先讓我們整體看一下 Spring MVC 處理請求的流程:
-
首先用戶發送請求,請求被 SpringMVC前端控制器(DispatherServlet)捕獲;
-
前端控制器(DispatherServlet)對請求 URL 解析獲取請求 URI,根據 URI,調用 HandlerMapping;
-
前端控制器(DispatherServlet)獲得返回的 HandlerExecutionChain(包括 Handler 對象以及 Handler 對象對應的攔截器);
-
DispatcherServlet 根據獲得的 HandlerExecutionChain,選擇一個合適的 HandlerAdapter。(附註:如果成功獲得 HandlerAdapter 後,此時將開始執行攔截器的 preHandler(...) 方法);
-
HandlerAdapter 根據請求的 Handler 適配並執行對應的 Handler;HandlerAdapter 提取 Request 中的模型數據,填充 Handler 入參,開始執行 Handler(Controller)。 在填充 Handler 的入參過程中,根據配置,Spring 將做一些額外的工作:
HttpMessageConveter:將請求消息(如 Json、xml 等數據)轉換成一個對象,將對象轉換爲指定的響應信息;
數據轉換:對請求消息進行數據轉換。如 String 轉換成 Integer、Double 等;
數據格式化:如將字符串轉換成格式化數字或格式化日期等;
數據驗證: 驗證數據的有效性(長度、格式等),驗證結果存儲到 BindingResult 或 Error 中);
-
Handler 執行完畢,返回一個 ModelAndView (即模型和視圖)給 HandlerAdaptor;
-
HandlerAdaptor 適配器將執行結果 ModelAndView 返回給前端控制器;
-
前端控制器接收到 ModelAndView 後,請求對應的視圖解析器;
-
視圖解析器解析 ModelAndView 後返回對應 View;
-
渲染視圖並返回渲染後的視圖給前端控制器;
- 最終前端控制器將渲染後的頁面響應給用戶或客戶端。
案例實操
SpringMVC 請求執行源碼解讀
對於 SpringMVC 項目所有的請求入口(靜態資源除外)這裏都是從 web.xml 文件配置的前端控制器 DispatcherServlet 開始:
<!-- servlet請求分發器 -->
<servlet>
<servlet-name>springMvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:servlet-context.xml</param-value>
</init-param>
<!-- 表示啓動容器時初始化該Servlet -->
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>springMvc</servlet-name>
<!-- 這是攔截請求, /代表攔截所有請求,攔截所有.do請求 -->
<url-pattern>/</url-pattern>
</servlet-mapping>
DispatcherServlet UML繼承關係圖如下:
這裏關注藍線部分繼承結構:DispatcherServlet-->FrameworkServlet-->HttpServletBean-->HttpServlet-->GenericServlet-->Servlet,對於請求核心時序圖如下:
對於 web 請求的處理,大家都知道是通過繼承 HttpServlet 重寫其 service 方法,這裏打開 DispatcherServlet 源碼發現這裏並沒有看到我們要找的 service 方法,此時到父類 FrameworkServlet 查找如下:可以看到父類重寫 HttpServlet service 方法。
FrameworkServlet#service
/**
* 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 == HttpMethod.PATCH || httpMethod == null) {
proce***equest(request, response);
}
else {
super.service(request, response);
}
}
從源碼分析來看當請求方法爲 patch 請求或者爲 null 時執行 proce***equest0 方法,其他情況則調用父類 service 方法,大家都知道 SpringMVC 請求大多請求是 get|post 請求爲主,此時繼續向上查看 FrameworkServlet 父類 HttpServletBean(抽象類繼承 HttpServlet 並未重寫 service 方法,所以向上繼續尋找)--> HttpServlet service 方法:
HttpServlet#service
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
if (!(req instanceof HttpServletRequest &&
res instanceof HttpServletResponse)) {
throw new ServletException("non-HTTP request or response");
}
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
service(request, response);
}
}
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < lastModified) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
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);
} 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 {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
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);
}
}
可以看到 HttpServlet service 進行了重載,根據不同的請求類型然後調用不同處理方法,這裏以 get 請求爲例,當請求方法爲 get 請求時在重載 service 方法中調用 doGet 方法進行處理,這裏需要特別注意的是:HttpServlet 存在 doGet 方法實現,然而在繼承的子類中也存在 doGet 方法實現,到底調用哪個方法?很明顯調用子類的 doGet 方法(面向對象多態思想!!!)從繼承 UML 關係圖上看,最外層子類實現 doGet 方法的爲 FrameworkServlet :
FrameworkServlet#doGet&proce***equest
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
proce***equest(request, response);
}
protected final void proce***equest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// 系統計時開始時間
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
// 國際化
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
//構建ServletRequestAttributes對象
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
//異步管理
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
//初始化ContextHolders
initContextHolders(request, localeContext, requestAttributes);
try {
doService(request, response);
}
catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
//恢復原來的LocaleContext和ServiceRequestAttributes到LocaleContextHolder和RequestContextHolder,避免影響Servlet以外的處理,如Filter
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
logResult(request, response, failureCause, asyncManager);
//發佈ServletRequestHandlerEvent消息,這個請求是否執行成功都會發布消息的
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
// initContextHolders(request, localeContext, requestAttributes);
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);
}
}
該方法大概做了這幾件事:國際化的設置,創建 ServletRequestAttributes 對象,初始化上下文 holders (即將 Request 對象放入到線程上下文中,如後續想要在方法中獲取 request、response對象此時可以通過調用 LocaleContextHolder 對應方法即可),然後調用 doService 方法。對於 doService 方法,FrameworkServlet 類並未提供實現,該方法由 DispatcherServlet 子類實現:
DispatcherServlet#doService
DispatcherServlet 裏面執行處理的入口方法是 doService,由於這個類繼承於 FrameworkServlet 類,重寫了 doService() 方法:
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
logRequest(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<>();
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));
}
}
}
//Spring上下文
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
//國際化解析器
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
//主題解析器
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
//主題
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
//重定向的數據
if (this.flashMapManager != null) {
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 {
//request設置完相關的屬性做真正的請求處理
doDispatch(request, response);
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
整個方法看下來處理的操作有:處理 include 標籤的請求,將上下文放到 request 的屬性中,將國際化解析器放到 request 的屬性中,將主題解析器放到 request 屬性中,將主題放到 request 的屬性中,處理重定向的請求數據最後調用 doDispatch 這個核心的方法對請求進行處理:
DispatcherServlet#doDispatch
該方法是在 doService 方法中調用的,從底層設計了整個請求的處理流程:
- 根據 request 找到 Handler
- 根據 Handler 找到對應的 HandlerAdapter
- 用 HandlerAdapter 處理 Handler
- 調用 processDispatchResult 方法處理上面之後的結果(包含View渲染並輸出給用戶)
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 {
// 校驗是否爲上傳請求 是上傳請求執行解析 否則返回request
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// 根據訪問的Handler 返回指定對應的HandlerExecutionChain對象 這裏從HandlerMapping 集合中查找 HandlerExecutionChain 對象包含Handler與攔截器HandlerInterceptor列表
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// 根據得到的Handler 獲取對應的HandlerAdaptor對象
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// 處理GET、HEAD請求的Last-Modified
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
//當數據沒有更改時,就直接返回上次的數據,提高效率
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
//執行Interceptor的preHandle
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// 執行Handler 返回ModelAndView
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
//如果需要異步處理,直接返回
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//當view爲空時,根據request設置默認view,如Handler返回值爲void
applyDefaultViewName(processedRequest, mv);
//執行相應Interceptor的postHandle
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);
}
//處理返回結果,包括處理異常、渲染頁面,發出完成通知觸發Interceptor的afterCompletion
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);
}
}
}
}
- doDispatcher 首先檢查是不是上傳請求,如果是則將 request 轉換爲 MultipartHttpServletRequest,並將 multipartRequestParsed 標誌設置爲 true;
- 通過 getHandler 獲取 Handler 處理器鏈 HandlerExecutionChain;
- 處理GET、HEAD請求的 Last-Modified,這裏主要判斷 Last-Modified 值是否被修改來處理決定是否採用緩存數據;
- 接下來依次調用相應的 Interceptor 的 preHandle,執行攔截器攔截操作;
- 攔截器 preHandle 方法執行後,此時開始通過 HandlerAdapter 適配對應的 Handler 執行(這裏纔是真正要執行的 Controller 方法), Handler 處理完請求後,如果需要異步處理則直接返回,如果不需要異步處理,當 view 爲空時,設置默認 view,然後執行相應的 Interceptor 的postHandle。
- Handler:處理器,他直接對應着 MVC 中的 C,也就是 Controller 層,它的具體表現形式有很多,可以是類,也可以是方法(通常以方法居多),因爲它的定義是 Object,我們在方法中標註的 @RequestMapping 的所有方法都可以看成一個 Handler,只要可以實際處理請求的都可以看成 Handler。
- HandlerMapping:用來查找 Handler,在 SpringMVC 中會處理很多請求,每一個請求都需要一個 Handler 來處理,具體接受到請求後需要哪一個 Handler 來處理,此時通過 HandlerMapping 來實現查找。
- HandlerAdapter:適配器,不同的 Handler 需要找到不同 HandlerAdapter 來調用 Handler。就如工廠裏需要使用工具,工人(HandlerAdapter)使用工具(Handler)完成工作,而 HandlerMapping 用於根據需要完成的工作來找到相應的工具。
DispatcherServlet#processDispatchResult
processDispatchResult 方法主要用來處理前面返回的結果,其中包括處理異常、渲染頁面、觸發 Interceptor 的 afterCompletion 方法三部分內容,處理的異常是在處理請求 doDispatch 方法的過程中產生。
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
// 如果請求過程中有異常拋出則處理異常
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);
}
}
//執行頁面渲染操作
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No view rendering, null ModelAndView returned.");
}
}
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}
// Handler請求處理完,觸發Interceptor的afterCompletion
if (mappedHandler != null) {
// Exception (if any) is already handled..
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
render 視圖渲染:
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() + "'");
}
}
if (logger.isTraceEnabled()) {
logger.trace("Rendering view [" + view + "] ");
}
try {
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
// 渲染頁面處理
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "]", ex);
}
throw ex;
}
}
今天我們來了解了一下 SpringMVC 框架中 MVC 核心思想,SpringMVC 內部請求流程分析以及源碼級別代碼解讀,讓大家真正能夠從底層級別理解整個框架執行原貌,最後以一張圖來總結今天的源碼分析執行流程。
擴展~MVC
模型-視圖-控制器(MVC)是一個衆所周知的以設計界面應用程序爲基礎的設計思想。它主要通過分離模型、視圖及控制器在應用程序中的角色將業務邏輯從界面中解耦。通常,模型負責封裝應用程序數據在視圖層展示。視圖僅僅只是展示這些數據,不包含任何業務邏輯。控制器負責接收來自用戶的請求,並調用後臺服務(service或者dao)來處理業務邏輯。處理後,後臺業務層可能會返回了一些數據在視圖層展示。控制器收集這些數據及準備模型在視圖層展示。MVC模式的核心思想是將業務邏輯從界面中分離出來,允許它們單獨改變而不會相互影響。