DispatchServlet请求处理流程解析
观前提示：
本文所使用的IDEA版本为ultimate 2019.1，JDK版本为1.8.0_141，Tomcat版本为9.0.12。
1.简介
DispatcherServlet是前置控制器，配置在web.xml文件中的。拦截匹配的请求，Servlet拦截匹配规则要自己定义，把拦截下来的请求，依据相应的规则分发到目标Controller来处理，是配置spring MVC的第一步。
DispatcherServlet是前端控制器设计模式的实现，提供Spring Web MVC的集中访问点，而且负责职责的分派，而且与Spring IoC容器无缝集成，从而可以获得Spring的所有好处。
2.源码分析（IDEA）
2.1 继承关系图和类中方法
DispatchServlet继承关系图如下图所示：
DispatchServlet中方法如下图所示：
2.2 service()、doGet()、doPost()、doPut()、doDelete()方法
客户端发送请求，交由相应的servlet处理，servlet调用service()方法处理。
DispatchServlet的service()、doGet()、doPost()、doPut()、doDelete()方法继承与父类FrameworkServlet
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
if (httpMethod != HttpMethod.PATCH && httpMethod != null) {
super.service(request, response);
} else {
this.processRequest(request, response);
}
}
protected final void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.processRequest(request, response);
}
protected final void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.processRequest(request, response);
}
protected final void doPut(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.processRequest(request, response);
}
protected final void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.processRequest(request, response);
}
而service()方法则是在FrameworkServlet的父类HttpServlet里实现的。
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String method = req.getMethod();
long lastModified;
if (method.equals("GET")) {
lastModified = this.getLastModified(req);
if (lastModified == -1L) {
this.doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader("If-Modified-Since");
} catch (IllegalArgumentException var9) {
ifModifiedSince = -1L;
}
if (ifModifiedSince < lastModified / 1000L * 1000L) {
this.maybeSetLastModified(resp, lastModified);
this.doGet(req, resp);
} else {
resp.setStatus(304);
}
}
} else if (method.equals("HEAD")) {
lastModified = this.getLastModified(req);
this.maybeSetLastModified(resp, lastModified);
this.doHead(req, resp);
} else if (method.equals("POST")) {
this.doPost(req, resp);
} else if (method.equals("PUT")) {
this.doPut(req, resp);
} else if (method.equals("DELETE")) {
this.doDelete(req, resp);
} else if (method.equals("OPTIONS")) {
this.doOptions(req, resp);
} else if (method.equals("TRACE")) {
this.doTrace(req, resp);
} else {
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[]{
method};
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(501, errMsg);
}
}
可以看出service()方法中会对request进行处理，找到对应的方法（doGet()、doPost()等）进行处理。而doGet()、doPost()等方法最终都会在processRequest()中进行处理。
2.3 processRequest()方法
processRequest()方法是在FrameworkServlet中实现的。
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//获取请求中的LocaleContext
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//创建新的LocaleContext
LocaleContext localeContext = this.buildLocaleContext(request);
//获取请求中的RequestAttributes
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
//创建新的RequestAttributes
ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new FrameworkServlet.RequestBindingInterceptor());
//初始化上下文
this.initContextHolders(request, localeContext, requestAttributes);
try {
this.doService(request, response);
} catch (ServletException var17) {
failureCause = var17;
throw var17;
} catch (IOException var18) {
failureCause = var18;
throw var18;
} catch (Throwable var19) {
failureCause = var19;
throw new NestedServletException("Request processing failed", var19);
} finally {
this.resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (this.logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", (Throwable)failureCause);
} else if (asyncManager.isConcurrentHandlingStarted()) {
this.logger.debug("Leaving response open for concurrent processing");
} else {
this.logger.debug("Successfully completed request");
}
}
this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause);
}
}
可以看出，在初始化上下文之后，会执行doService()方法。
2.3 doService()方法
doService()方法是在DispatchServlet中实现的。
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (this.logger.isDebugEnabled()) {
String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
this.logger.debug("DispatcherServlet with name '" + this.getServletName() + "'" + resumed + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
}
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap();
Enumeration attrNames = request.getAttributeNames();
label108:
while(true) {
String attrName;
do {
if (!attrNames.hasMoreElements()) {
break label108;
}
attrName = (String)attrNames.nextElement();
} while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
//上下文等属性设置
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.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 {
this.doDispatch(request, response);
} finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
this.restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
在设置完上下文等属性后，将执行doDispatch()方法。
2.4 doDispatch()方法
doDispatch()方法是在DispatchServlet中实现的。
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
//异步处理请求
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
try {
ModelAndView mv = null;
Object dispatchException = null;
try {
//判断是否为文件传输请求
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
this.noHandlerFound(processedRequest, response);
return;
}
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (this.logger.isDebugEnabled()) {
this.logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
}
if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
return;
}
}
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
this.applyDefaultViewName(processedRequest, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
} catch (Exception var20) {
dispatchException = var20;
} catch (Throwable var21) {
dispatchException = new NestedServletException("Handler dispatch failed", var21);
}
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
}
} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
} else if (multipartRequestParsed) {
this.cleanupMultipart(processedRequest);
}
}
}
在异步处理请求和是否文档传输请求后，将执行getHandler()方法获取处理请求的handler。
2.4 getHandler()方法
getHandler()方法是在DispatchServlet中实现的。
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Iterator var2 = this.handlerMappings.iterator();
HandlerExecutionChain handler;
do {
if (!var2.hasNext()) {
return null;
}
HandlerMapping hm = (HandlerMapping)var2.next();
if (this.logger.isTraceEnabled()) {
this.logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + this.getServletName() + "'");
}
handler = hm.getHandler(request);
} while(handler == null);
return handler;
}
getHandler()方法会获取到处理请求的handler并返回执行链HandlerExecutionChain。