spring mvc解读
上篇自己手写了了一下mvc的简易版，这次解读一下springmvc官方源码，先大概根据图片来了解一下流程
debug准备：实现debug的具体流程，创建springboot项目，添加web依赖，编写controller类，完成以上流程找到DispatcherServlet类（org.springframework.web.servlet包下），该类继承FrameworkServlet（抽象类），FrameworkServlet继承HttpServletBean，HttpServletBean继承HttpServlet；根据servlet的实现，肯定先执行init，找到该方法，init方法被重写了
可以看到调用了父类的init，父亲类是GenericServlet，GenericServlet和HttpServlet属于不属于springmvc包下先不讨论，但这个初始化过程还是要知道什么时候开始的；
回到DispatcherServlet，里面重要的组件，与上面对应上了
private MultipartResolver multipartResolver;
@Nullable
private LocaleResolver localeResolver;
/** @deprecated */
@Deprecated
@Nullable
private ThemeResolver themeResolver;
@Nullable
private List<HandlerMapping> handlerMappings;
@Nullable
private List<HandlerAdapter> handlerAdapters;
@Nullable
private List<HandlerExceptionResolver> handlerExceptionResolvers;
@Nullable
private RequestToViewNameTranslator viewNameTranslator;
@Nullable
private FlashMapManager flashMapManager;
@Nullable
private List<ViewResolver> viewResolvers;
先来看两个构造方法，第一个构造方法打上断点
public DispatcherServlet() {
this.setDispatchOptionsRequest(true);
}
public DispatcherServlet(WebApplicationContext webApplicationContext) {
super(webApplicationContext);
this.setDispatchOptionsRequest(true);
}
往下走，可以看到执行到，
这个自动配置，这里返回的就是我们今天要讨论的DispatcherServlet；写一个发送post请求的servlet，回到HttpServlet类，给doPost打上断点，postman发送请求，会发生什么，可以看到请求的一瞬间，进入debug，执行下一步就进入FrameworkServlet的doPost，执行processRequest方法
执行完毕后，走到DispatcherServlet的doDispatch方法，这里就是sprmvc执行的具体流程了。
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
// 提供组件异步处理 HTTP 请求
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null) {
this.noHandlerFound(processedRequest, response);
return;
}
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
String method = request.getMethod();
boolean isGet = HttpMethod.GET.matches(method);
if (isGet || HttpMethod.HEAD.matches(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
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 ServletException("Handler dispatch failed: " + var21, var21);
}
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
triggerAfterCompletion(processedRequest, response, mappedHandler, new ServletException("Handler processing failed: " + var23, var23));
}
} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
asyncManager.setMultipartRequestParsed(multipartRequestParsed);
} else if (multipartRequestParsed || asyncManager.isMultipartRequestParsed()) {
this.cleanupMultipart(processedRequest);
}
}
}
根据上图
1.第一步请求进入DispatcherServlet
2.获取Handler，返回HandlerExecutionChain，包括
mappedHandler = this.getHandler(processedRequest);
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
Iterator var2 = this.handlerMappings.iterator();
while(var2.hasNext()) {
HandlerMapping mapping = (HandlerMapping)var2.next();
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
3 .根据返回来的Handler来寻找HandlerAdapter，继续走
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
4 . HandlerAdapter处理解析到的Handler
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
5.6.看作一步，handle方法内部，HandlerAdapter是一个接口
7 返回modelview（这里的属性名为mv）
// 用于给模型和视图（ModelAndView）对象mv应用默认的视图名称
this.applyDefaultViewName(processedRequest, mv);
// 用于执行所有后置拦截器（Interceptor）的postHandle方法
mappedHandler.applyPostHandle(processedRequest, response, mv);
8 9 10 可以看成一步
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception {
boolean errorView = false;
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
ModelAndViewDefiningException mavDefiningException = (ModelAndViewDefiningException)exception;
this.logger.debug("ModelAndViewDefiningException encountered", exception);
mv = mavDefiningException.getModelAndView();
} else {
Object handler = mappedHandler != null ? mappedHandler.getHandler() : null;
mv = this.processHandlerException(request, response, handler, exception);
errorView = mv != null;
}
}
if (mv != null && !mv.wasCleared()) {
this.render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
} else if (this.logger.isTraceEnabled()) {
this.logger.trace("No view rendering, null ModelAndView returned.");
}
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
}
}
}
protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
LocaleResolver lr = this.localeResolver;
if (lr instanceof LocaleContextResolver localeContextResolver) {
return localeContextResolver.resolveLocaleContext(request);
} else {
return () -> {
return lr != null ? lr.resolveLocale(request) : request.getLocale();
};
}
}
可以看到这里对视图有好几种处理方式（异常，无视图，有视图），现在我们都是前后端分离，返回json，这种属于无视图的处理，有视图的话也是利用response流处理液乳到视图上，没有是直接放回response流；
现在都是springboot，springboot自动配置处理了DispatcherServlet，如下