Spring MVC 请求全链路源码解析：基于 DispatcherServlet 的责任链执行模型

请求 → 路由 → 拦截 → 执行 → 拦截 → 渲染 → 收尾

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) {
    // ... 初始化变量
    try {
        processedRequest = checkMultipart(request); // 处理文件上传
        mappedHandler = getHandler(processedRequest); // 阶段2：路由匹配
        if (mappedHandler == null) {
            noHandlerFound(processedRequest, response); // 返回404
            return;
        }

        HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
        
        // 执行拦截器 preHandle（阶段3）
        if (!mappedHandler.applyPreHandle(processedRequest, response)) return;

        // 执行 Controller 方法（阶段4）
        mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

        applyDefaultViewName(processedRequest, mv);
        mappedHandler.applyPostHandle(processedRequest, response, mv); // 阶段5

    } catch (Exception ex) {
        dispatchException = ex;
    }

    // 处理结果：渲染视图 / 异常处理（阶段6）
    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    
} finally {
    // 清理资源 + 异步收尾（阶段8）
    if (multipartRequestParsed) cleanupMultipart(processedRequest);
}

public final HandlerExecutionChain getHandler(HttpServletRequest request) {
    Object handler = getHandlerInternal(request); // 核心匹配逻辑
    if (handler == null) return null;

    // 构建包含拦截器的执行链
    HandlerExecutionChain chain = getHandlerExecutionChain(handler, request);
    
    // 处理 CORS（跨域预检）
    if (CorsUtils.isPreFlightRequest(request)) {
        return null; // 预检请求不继续后续流程
    }
    return chain;
}

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) {
    for (int i = 0; i < interceptors.size(); i++) {
        if (!interceptors.get(i).preHandle(request, response, handler)) {
            triggerAfterCompletion(request, response, null); // 提前收尾
            return false;
        }
        interceptorIndex = i; // 记录已成功执行的拦截器索引
    }
    return true;
}

protected ModelAndView handleInternal(...) {
    checkRequest(request); // 校验 HTTP 方法、Session 等
    mav = invokeHandlerMethod(request, response, handlerMethod); // 核心
    prepareResponse(response); // 设置缓存头等
    return mav;
}

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) {
    for (int i = interceptors.size() - 1; i >= 0; i--) {
        interceptors.get(i).postHandle(request, response, handler, mv);
    }
}

private void processDispatchResult(..., ModelAndView mv, Exception ex) {
    if (ex != null) {
        mv = processHandlerException(request, response, handler, ex); // 异常处理
    }

    if (mv != null && !mv.wasCleared()) {
        render(mv, request, response); // 视图渲染
    }

    triggerAfterCompletion(...); // 阶段8
}

@RequestMapping(produces = "...") > Accept Header > 默认配置

@GetMapping(value = "/user", produces = MediaType.APPLICATION_JSON_VALUE)

@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
        configurer.defaultContentType(MediaType.APPLICATION_JSON)
                  .ignoreAcceptHeader(false);
    }
}

void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex) {
    for (int i = interceptorIndex; i >= 0; i--) {
        try {
            interceptors.get(i).afterCompletion(request, response, handler, ex);
        } catch (Throwable t) {
            logger.error("afterCompletion threw exception", t);
        }
    }
}