请求映射源码

首先看一张请求完整流转图（这里感谢博客园上这位大神的图，博客地址我忘记了）：

前台发送给后台的访问请求是如何找到对应的控制器映射并执行后续的后台操作呢，其核心为DispatcherServlet.java与HandlerMapper。在spring boot初始化的时候，将会加载所有的请求与对应的处理器映射为HandlerMapper组件。我们可以在springMVC的自动配置类中找到对应的Bean。

@Bean
@Primary
@Override
public RequestMappingHandlerMapping requestMappingHandlerMapping(
    @Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
    @Qualifier("mvcConversionService") FormattingConversionService conversionService,
    @Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {
    // Must be @Primary for MvcUriComponentsBuilder to work
    return super.requestMappingHandlerMapping(contentNegotiationManager, conversionService,
                                              resourceUrlProvider);
}

@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext,
                                                           FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {
    WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(
        new TemplateAvailabilityProviders(applicationContext), applicationContext, getWelcomePage(),
        this.mvcProperties.getStaticPathPattern());
    welcomePageHandlerMapping.setInterceptors(getInterceptors(mvcConversionService, mvcResourceUrlProvider));
    welcomePageHandlerMapping.setCorsConfigurations(getCorsConfigurations());
    return welcomePageHandlerMapping;
}

请求将首先执行FrameworkServlet下的service方法根据request请求的method找到对应的do**方法。

@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {

    HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
    if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
        processRequest(request, response);
    }
    else {
        //父类根据method参数执行doGet,doPost,doDelete等
        super.service(request, response);
    }
}

而这些do**其都会进入核心方法，以doGet为例。

@Overrideprotected 
final void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //核心方法
    processRequest(request, response);
}

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    try {
    //进入此核心方法
    doService(request, response);
}
catch (ServletException | IOException ex) {
    failureCause = ex;
    throw ex;
}
catch (Throwable ex) {
    failureCause = ex;
    throw new NestedServletException("Request processing failed", ex);
}

finally {
    resetContextHolders(request, previousLocaleContext, previousAttributes);
    if (requestAttributes != null) {
        requestAttributes.requestCompleted();
    }
    logResult(request, response, failureCause, asyncManager);
    publishRequestHandledEvent(request, response, startTime, failureCause);
}

processRequest()方法中重点在doService(request, response);，而其核心处理逻辑位于DispatchServletl类重写的方法，如下。

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

      try {
          //这里为实际分发控制器的逻辑，其内部是找到对应的handlerMapper
          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);
                }
            }
            if (requestPath != null) {
                ServletRequestPathUtils.clearParsedRequestPath(request);
            }
        }
}

接下来看分发处理逻辑方法,其中重要的方法都使用了原生的注释。接下来分别分析核心源码。

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);

            // Determine handler for the current request.
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // Determine handler adapter for the current request.
            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;
            }

            // Actually invoke the 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);
        }
        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);
            }
        }
    }
}

首先是分析getHandler(),找到对应的处理器映射逻辑。

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;
}

我们将断点标记在getHandler方法上时，可以清除看到handlerMappings，如图。

这里，用户请求与处理器的映射关系都在RequestMapperHandlerMapping中，而欢迎页处理请求则在WelcomePageHanderMapping中进行映射。

以下为RequestMapperHandlerMapping中映射部分截图，可以看到用户的所有请求映射这里面都有：

getHandler()后的方法是通过比较request请求中method与HandlerMapper中相同url下的method，再进行唯一性校验，不通过异常，通过找到唯一的handler。

后续，通过handler找到处理的设配器，通过适配器得到一个ModelAndView对象，这个对象就是最后返回给前端页面的对象。

至此，一个请求完整映射到返回前端结束。

说明：这是实现了FramworkServlet的doService方法，FramworkServlet继承自HttpServlet，并且重写了父类中的doGet(),doPost(),doPut(),doDelete 等方法，在这些重写的方法里都调用了 processRquest() 方法做请求处理，进入processRquest()可以看到里面调用了FramworkServlet中定义的doService() 方法。