Java从源码角度解析SpringMVC执行流程
作者:索码理
SpringMVC执行流程在面试中经常会被问到,本篇文章通过源码的方式简单的了解一下SpringMVC执行流程。
先看流程
先看一下SpringMVC执行流程再看源码,有助理解:
- ⽤户发送请求⾄前端控制器DispatcherServlet。
- DispatcherServlet 收到请求调⽤ HandlerMapping 处理器映射器。
- 处理器映射器找到具体的处理器(可以根据xml配置、注解进⾏查找),⽣成处理器及处理器拦截器(如果有则⽣成)⼀并返回给DispatcherServlet。
- DispatcherServlet调⽤HandlerAdapter处理器适配器。
- HandlerAdapter经过适配调⽤具体的处理器(Controller,也叫后端控制器)
- Controller执⾏完成返回ModelAndView。
- HandlerAdapter 将 Controller 执⾏结果 ModelAndView 返回给DispatcherServlet。
- DispatcherServlet将ModelAndView传给ViewReslover视图解析器。
- ViewReslover解析后返回具体View。
- DispatcherServlet根据View进⾏渲染视图(即将模型数据填充⾄视图中)。
- DispatcherServlet 响应⽤户。
再看源码
我们都知道当从用户发起请求到后端是,首先走的就是DispatcherServlet,接着就会调用doService()
方法执行业务逻辑,doService()
方法也只是一个中转站,实际执行逻辑的是doDispatch()
方法,且看源码:
@Override protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { logRequest(request); // 省略部分源码 try { // 执行实际逻辑 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); } } ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request); } }
doDispatch 方法:
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 { processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // 为当前请求获取映射处理器 mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest, response); return; } // 获取映射处理器适配器 HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header, if supported by the handler. 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; } } if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // 实际调用的Handler mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest, mv); 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); } //处理转发结果 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); } } } }
下面来看一下其中几个重要的方法:
1.getHandler(HttpServletRequest request)方法:该方法是处理当前请求找到合适的HandlerMapping,并返回一个HandlerExecutionChain,HandlerExecutionChain,HandlerExecutionChain包含了具体的处理器(handler)和拦截器列表。
HandlerMapping 默认的实现有org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping
和 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping
。
@Nullable protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { if (this.handlerMappings != null) { for (HandlerMapping mapping : this.handlerMappings) { HandlerExecutionChain handler = mapping.getHandler(request); if (handler != null) { return handler; } } } return null; }
2.getHandlerAdapter(Object handler) 根据HandlerExecutionChain中的handler来获取处理器适配器(HandlerAdapter),
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { if (this.handlerAdapters != null) { for (HandlerAdapter adapter : this.handlerAdapters) { if (adapter.supports(handler)) { return adapter; } } } throw new ServletException("No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler"); }
HandlerAdapter有两个默认实现类,分别是 org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter
和 org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter
,前者用于没有使用模板引擎的请求,后者用于使用了模板引擎的接口。
实际处理请求的是HandlerAdapter的handle方法,如果是没有使用例如JSP等的模板引擎,handle方法就会返回null,如果使用了模板引擎就会返回一个ModelAndView对象。
ModelAndView mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
handle方法最终调用的是Controller接口的 handleRequest(HttpServletRequest request, HttpServletResponse response)
方法来处理请求。
以SimpleControllerHandlerAdapter#handle方法源码为例:
@Override @Nullable public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { return ((Controller) handler).handleRequest(request, response); }
3.processDispatchResult方法用于处理转发结果,该结果要么是一个ModelAndView,要么抛异常。
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) { logger.debug("ModelAndViewDefiningException encountered", exception); mv = ((ModelAndViewDefiningException) exception).getModelAndView(); } else { Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null); //处理异常 mv = processHandlerException(request, response, handler, exception); errorView = (mv != null); } } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { //加载视图 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; } if (mappedHandler != null) { // Exception (if any) is already handled.. mappedHandler.triggerAfterCompletion(request, response, null); } }
processDispatchResult
方法中在正常情况下会调用render
方法。
4.render方法用来通过名称呈现视图,它也是请求处理的最后一步。
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) { // 通过视图名称获取视图 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() + "'"); } } // Delegate to the View object for rendering. if (logger.isTraceEnabled()) { logger.trace("Rendering view [" + view + "] "); } try { if (mv.getStatus() != null) { response.setStatus(mv.getStatus().value()); } //渲染视图 view.render(mv.getModelInternal(), request, response); } catch (Exception ex) { if (logger.isDebugEnabled()) { logger.debug("Error rendering view [" + view + "]", ex); } throw ex; } }
DispatcherServlet的render
方法是对视图View的封装,最后调用的还是View的render方法。
resolveViewName
方法用于解析视图名称,它会通过视图解析器ViewResolver的resolveViewName
方法解析视图并返回一个视图View,然后再通过View的render方法渲染视图,至于是怎么渲染视图的这里就不介绍了,感兴趣的可以自行查看源码。
protected View resolveViewName(String viewName, @Nullable Map<String, Object> model, Locale locale, HttpServletRequest request) throws Exception { if (this.viewResolvers != null) { for (ViewResolver viewResolver : this.viewResolvers) { View view = viewResolver.resolveViewName(viewName, locale); if (view != null) { return view; } } } return null; }
到此这篇关于Java从源码角度解析SpringMVC执行流程的文章就介绍到这了,更多相关JavaSpringMVC执行流程内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!