深入理解SpringMVC DispatcherServlet源码及全流程原理

DispatcherServlet概述

DispatcherServlet是SpringMVC的前端控制器（Front Controller），它负责拦截所有进入应用的HTTP请求，分发请求给相应的处理器，并处理返回的响应。DispatcherServlet的主要职责包括：

• 初始化WebApplicationContext
• 加载各种组件（如HandlerMapping、HandlerAdapter等）
• 分发请求
• 视图解析和渲染

DispatcherServlet初始化

DispatcherServlet的初始化过程包括创建和配置Spring的WebApplicationContext以及加载各种必要的组件。下面是 DispatcherServlet初始化的主要步骤：

1. 初始化WebApplicationContext：

protected WebApplicationContext initWebApplicationContext() {
    // 获取现有的WebApplicationContext
    WebApplicationContext rootContext =
            WebApplicationContextUtils.getWebApplicationContext(getServletContext());

    WebApplicationContext wac = createWebApplicationContext(rootContext);
    // 配置WebApplicationContext
    configureAndRefreshWebApplicationContext(wac);
    return wac;
}
​

1. 加载配置文件和初始化组件：

protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
    initFlashMapManager(context);
}
​

请求处理流程

当一个HTTP请求到达时，DispatcherServlet将按以下步骤处理请求：

1. 请求接收：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    ModelAndView mv = null;
    Exception dispatchException = null;

    try {
        // 检查并包装Multipart请求
        processedRequest = checkMultipart(request);

        // 获得处理器
        mappedHandler = getHandler(processedRequest);
        if (mappedHandler == null) {
            noHandlerFound(processedRequest, response);
            return;
        }

        // 获得处理器适配器
        HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

        // 执行处理器
        mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

        // 处理视图
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
    catch (Exception ex) {
        dispatchException = ex;
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
}
​

1. 检查Multipart请求：

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
    if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
        return this.multipartResolver.resolveMultipart(request);
    }
    return request;
}
​

1. 获取处理器：

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    for (HandlerMapping hm : this.handlerMappings) {
        HandlerExecutionChain handler = hm.getHandler(request);
        if (handler != null) {
            return handler;
        }
    }
    return null;
}
​

1. 获取处理器适配器：

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
    for (HandlerAdapter ha : this.handlerAdapters) {
        if (ha.supports(handler)) {
            return ha;
        }
    }
    throw new ServletException("No adapter for handler [" + handler + "]");
}
​

1. 处理视图：

protected void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
                                     HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
    if (exception != null) {
        if (mv == null) {
            throw exception;
        }
        mv.addObject("exception", exception);
    }

    if (mv != null) {
        render(mv, request, response);
    }
}
​

处理器映射

处理器映射（Handler Mapping）负责将请求URL映射到相应的处理器。常用的处理器映射有：

• RequestMappingHandlerMapping：基于注解的处理器映射。
• BeanNameUrlHandlerMapping：基于Bean名称的处理器映射。

public HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
    HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
    return handlerMethod != null ? new HandlerExecutionChain(handlerMethod) : null;
}
​

处理器适配

处理器适配（Handler Adapter）负责调用处理器方法。常用的处理器适配有：

• RequestMappingHandlerAdapter：适配使用@RequestMapping注解的方法。
• HttpRequestHandlerAdapter：适配实现HttpRequestHandler接口的处理器。

public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
    return ((HandlerMethod) handler).invokeForRequest(request, response);
}
​

视图解析

视图解析（View Resolver）负责将逻辑视图名称解析为实际视图对象，并渲染视图。常用的视图解析器有：

• InternalResourceViewResolver：解析JSP视图。
• ThymeleafViewResolver：解析Thymeleaf视图。

public View resolveViewName(String viewName, Locale locale) throws Exception {
    return getView(viewName, locale);
}

protected View getView(String viewName, Locale locale) throws Exception {
    return (viewName != null ? new InternalResourceView(viewName) : null);
}