SpringMVC执行流程及源码解析

在SpringMVC中主要是围绕着DispatcherServlet来设计,可以把它当做指挥中心。这里先说明一下SpringMVC文档给出的执行流程,然后是我们稍微具体的执行流程,最后是流程大致的源码跟踪。关于很很很详细的源码解析,这里暂先不做。

官方文档中的流程

首先看下SpringMVC文档上给的流程图:

DispatcherServlet.png

这张图片给了我们大概的执行流程:

  1. 用户请求首先发送到前端控制器DispatcherServlet,DispatcherServlet根据请求的信息来决定使用哪个页面控制器Controller(也就是我们通常编写的Controller)来处理该请求。找到控制器之后,DispatcherServlet将请求委托给控制器去处理。
  2. 接下来页面控制器开始处理用户请求,页面控制器会根据请求信息进行处理,调用业务层等等,处理完成之后,会把结果封装成一个ModelAndView返回给DispatcherServlet。
  3. 前端控制器DispatcherServlet接到页面控制器的返回结果后,根据返回的视图名选择相应的试图模板,并根据返回的数据进行渲染。
  4. 最后前端控制器DispatcherServlet将结果返回给用户。

更具体的流程

上面只是总体流程,接下来我们稍微深入一点,看下更具体的流程,这里没有图,只有步骤解析:

  1. 用户请求发送到前端控制器DispatcherServlet。
  2. 前端控制器DispatcherServlet接收到请求后,DispatcherServlet会使用HandlerMapping来处理,HandlerMapping会查找到具体进行处理请求的Handler对象。
  3. HandlerMapping找到对应的Handler之后,并不是返回一个Handler原始对象,而是一个Handler执行链,在这个执行链中包括了拦截器和处理请求的Handler。HandlerMapping返回一个执行链给DispatcherServlet。
  4. DispatcherServlet接收到执行链之后,会调用Handler适配器去执行Handler。
  5. Handler适配器执行完成Handler(也就是我们写的Controller)之后会得到一个ModelAndView,并返回给DispatcherServlet。
  6. DispatcherServlet接收到Handler适配器返回的ModelAndView之后,会根据其中的视图名调用视图解析器。
  7. 视图解析器根据逻辑视图名解析成一个真正的View视图,并返回给DispatcherServlet。
  8. DispatcherServlet接收到视图之后,会根据上面的ModelAndView中的model来进行视图中数据的填充,也就是所谓的视图渲染。
  9. 渲染完成之后,DispatcherServlet就可以将结果返回给用户了。

源码

DispatcherServlet是一个Servlet,我们知道在Servlet在处理一个请求的时候会交给service方法进行处理,这里也不例外,DispatcherServlet继承了FrameworkServlet,首先进入FrameworkServlet的service方法:

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protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
//请求方法
String method = request.getMethod();
//PATCH方法单独处理
if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
processRequest(request, response);
}
else {//其他的请求类型的方法经由父类,也就是HttpServlet处理
super.service(request, response);
}
}

HttpServlet中会根据请求类型的不同分别调用doGet或者doPost等方法,FrameworkServlet中已经重写了这些方法,在这些方法中会调用processRequest进行处理,在processRequest中会调用doService方法,这个doService方法就是在DispatcherServlet中实现的。下面就看下DispatcherServlet中的doService方法的实现。

请求到达DispatcherServlet

doService方法:

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protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

//给request中的属性做一份快照
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
logger.debug("Taking snapshot of request attributes before include");
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));
}
}
}

//如果我们没有配置类似本地化或者主题的处理器之类的
//SpringMVC会使用默认的值
//默认配置文件是DispatcherServlet.properties
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 {
//开始处理
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);
}
}
}

DispatcherServlet开始真正的处理,doDispatch方法:

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protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
//SpringMVC中异步请求的相关知识,暂先不解释
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

try {
ModelAndView mv = null;
Exception dispatchException = null;

try {
//先检查是不是Multipart类型的,比如上传等
//如果是Multipart类型的,则转换为MultipartHttpServletRequest类型
processedRequest = checkMultipart(request);
multipartRequestParsed = processedRequest != request;

//获取当前请求的Handler
mappedHandler = getHandler(processedRequest, false);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}

//获取当前请求的Handler适配器
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// 对于header中last-modified的处理
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
//拦截器的preHandle方法进行处理
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

try {
//真正调用Handler的地方
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
}
//处理成默认视图名,就是添加前缀和后缀等
applyDefaultViewName(request, mv);
//拦截器postHandle方法进行处理
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
//处理最后的结果,渲染之类的都在这里
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);
}
}
}

可以看到大概的步骤还是按照我们上面分析的走的。

查找请求对应的Handler对象

对应着这句代码mappedHandler = getHandler(processedRequest, false);,看下具体的getHandler方法:

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protected HandlerExecutionChain getHandler(HttpServletRequest request, boolean cache) throws Exception {
return getHandler(request);
}

继续往下看getHandler:

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protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
//遍历所有的handlerMappings进行处理
//handlerMappings是在启动的时候预先注册好的
for (HandlerMapping hm : this.handlerMappings) {
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}

继续往下看getHandler,在AbstractHandlerMapping类中:

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public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
//根据request获取handler
Object handler = getHandlerInternal(request);
if (handler == null) {
//如果没有找到就使用默认的handler
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
//如果Handler是String,表明是一个bean名称
//需要超照对应bean
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
//封装Handler执行链
return getHandlerExecutionChain(handler, request);
}

根据requrst获取handler

首先看下根据requrst获取handler步骤getHandlerInternal方法,在AbstractHandlerMethodMapping中:

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protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
//获取request中的url,用来匹配handler
String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
//根据路径寻找Handler
HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
//根据handlerMethod中的bean来实例化Handler并添加进HandlerMethod
return (handlerMethod != null) ? handlerMethod.createWithResolvedBean() : null;
}

看下根据路径寻找handler的方法lookupHandlerMethod:

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protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List<Match> matches = new ArrayList<Match>();
//直接匹配
List<T> directPathMatches = this.urlMap.get(lookupPath);
//如果有匹配的,就添加进匹配列表中
if (directPathMatches != null) {
addMatchingMappings(directPathMatches, matches, request);
}
//还没有匹配的,就遍历所有的处理方法查找
if (matches.isEmpty()) {
// No choice but to go through all mappings
addMatchingMappings(this.handlerMethods.keySet(), matches, request);
}
//找到了匹配的
if (!matches.isEmpty()) {
Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
Collections.sort(matches, comparator);
//排序之后,获取第一个
Match bestMatch = matches.get(0);
//如果有多个匹配的,会找到第二个最合适的进行比较一下
if (matches.size() > 1) {
Match secondBestMatch = matches.get(1);
if (comparator.compare(bestMatch, secondBestMatch) == 0) {
Method m1 = bestMatch.handlerMethod.getMethod();
Method m2 = secondBestMatch.handlerMethod.getMethod();
throw new IllegalStateException(
"Ambiguous handler methods mapped for HTTP path '" + request.getRequestURL() + "': {" +
m1 + ", " + m2 + "}");
}
}
//设置request参数
handleMatch(bestMatch.mapping, lookupPath, request);
//返回匹配的url的处理的方法
return bestMatch.handlerMethod;
}
else {//最后还没有找到,返回null
return handleNoMatch(handlerMethods.keySet(), lookupPath, request);
}
}

获取默认Handler

如果上面没有获取到Handler,就会获取默认的Handler。如果还获取不到就返回null。

处理String类型的Handler

如果上面处理完的Handler是String类型的,就会根据这个handlerName获取bean。

封装Handler执行链

上面获取完Handler,就开始封装执行链了,就是将我们配置的拦截器加入到执行链中去,getHandlerExecutionChain:

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protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
//如果当前Handler不是执行链类型,就使用一个新的执行链实例封装起来
HandlerExecutionChain chain =
(handler instanceof HandlerExecutionChain) ?
(HandlerExecutionChain) handler : new HandlerExecutionChain(handler);
//先获取适配类型的拦截器添加进去拦截器链
chain.addInterceptors(getAdaptedInterceptors());
//当前的url
String lookupPath = urlPathHelper.getLookupPathForRequest(request);
//遍历拦截器,找到跟当前url对应的,添加进执行链中去
for (MappedInterceptor mappedInterceptor : mappedInterceptors) {
if (mappedInterceptor.matches(lookupPath, pathMatcher)) {
chain.addInterceptor(mappedInterceptor.getInterceptor());
}
}

return chain;
}

获取对应请求的Handler适配器

getHandlerAdapter:

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protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
//遍历所有的HandlerAdapter,找到和当前Handler匹配的就返回
//我们这里会匹配到RequestMappingHandlerAdapter
for (HandlerAdapter ha : this.handlerAdapters) {
if (ha.supports(handler)) {
return ha;
}
}
}

缓存的处理

也就是对last-modified的处理

执行拦截器的preHandle方法

就是遍历所有的我们定义的interceptor,执行preHandle方法

使用Handler适配器执行当前的Handler

ha.handle执行当前Handler,我们这里使用的是RequestMappingHandlerAdapter,首先会进入AbstractHandlerMethodAdapter的handle方法:

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public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
return handleInternal(request, response, (HandlerMethod) handler);
}

handleInternal方法,在RequestMappingHandlerAdapter中:

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protected final ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
// Always prevent caching in case of session attribute management.
checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
}
else {
// Uses configured default cacheSeconds setting.
checkAndPrepare(request, response, true);
}

// Execute invokeHandlerMethod in synchronized block if required.
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
return invokeHandleMethod(request, response, handlerMethod);
}
}
}
//执行方法,封装ModelAndView
return invokeHandleMethod(request, response, handlerMethod);
}

组装默认视图名称

前缀和后缀名都加上

执行拦截器的postHandle方法

遍历intercepter的postHandle方法。

处理最后的结果,渲染之类的

processDispatchResult方法:

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private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

boolean errorView = false;

if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}

// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
//渲染
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
}

if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}

if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}

重点看下render方法,进行渲染:

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protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
//设置本地化
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);

View view;
if (mv.isReference()) {
//解析视图名,得到视图
view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
}
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() + "'");
}
}

//委托给视图进行渲染
view.render(mv.getModelInternal(), request, response);
}

view.render就是进行视图的渲染,然后跳转页面等处理。

到这里大概的流程就走完了。其中涉及到的东西还有很多,暂先不做详细处理。

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