源码深度解析SpringMvc请求运行机制

Posted 2021-05-01 java微分享

本文依赖的是springmvc4.0.5.RELEASE，通过源码深度解析了解springMvc的请求运行机制。通过源码我们可以知道从客户端发送一个URL请求给springMvc开始，到返回数据给客户端期间是怎么运转的。

1、用户请求处理过程：

1、用户发送请求时会先从DispathcherServler的doService方法开始，在该方法中会将ApplicationContext、localeResolver、themeResolver等对象添加到request中，紧接着就是调用doDispatch方法:

源码：

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {  
    if (logger.isDebugEnabled()) {  
        String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";  
        logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +  
                " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");  
    }  
    // Keep a snapshot of the request attributes in case of an include,  
    // to be able to restore the original attributes after the include.  
    Map<String, Object> attributesSnapshot = null;  
    if (WebUtils.isIncludeRequest(request)) {  
        attributesSnapshot = new HashMap<String, Object>();  
        Enumeration<?> attrNames = request.getAttributeNames();  
        while (attrNames.hasMoreElements()) {  
            String attrName = (String) attrNames.nextElement();  
            if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {  
                attributesSnapshot.put(attrName, request.getAttribute(attrName));  
            }  
        }  
    }  
    // Make framework objects available to handlers and view objects.  
      <strong>request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());  
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);  
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);  
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());</strong>  
    FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);  
    if (inputFlashMap != null) {  
        request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));  
    }  
    request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());  
    request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);  
    try {  
        <strong>doDispatch(request, response);</strong>  
    }  
    finally {  
        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {  
            return;  
        }  
        // Restore the original attribute snapshot, in case of an include.  
        if (attributesSnapshot != null) {  
            restoreAttributesAfterInclude(request, attributesSnapshot);  
        }  
    }  
}  

doDispatch方法就是处理用户请求的方法。

2、进入该方法后首先会检查该请求是否是文件上传的请求(校验的规则是是否是post并且contenttType是否为multipart/为前缀)即调用的是checkMultipart方法；如果是的将request包装成MultipartHttpServletRequest。见源码:

doDispatch:

    processedRequest = checkMultipart(request);  

checkMultipart:

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {  
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {  
            if (request instanceof MultipartHttpServletRequest) {  
                logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +  
                        "this typically results from an additional MultipartFilter in web.xml");  
            }  
            else if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) {  
                logger.debug("Multipart resolution failed for current request before - " +  
                        "skipping re-resolution for undisturbed error rendering");  
            }  
            else {  
                return this.multipartResolver.resolveMultipart(request);  
            }  
        }  
        // If not returned before: return original request.  
        return request;  
    }

3、然后调用getHandler方法来匹配每个HandlerMapping对象，如果匹配成功会返回这个Handle的处理链HandlerExecutionChain对象，在获取该对象的内部其实也获取我们自定定义的拦截器，并执行了其中的方法

见源码:

doDispatch: 

HandlerExecutionChain mappedHandler = null;  
mappedHandler = getHandler(processedRequest);  

getHandler方法:

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {  
        for (HandlerMapping hm : this.handlerMappings) {  
            if (logger.isTraceEnabled()) {  
                logger.trace(  
                        "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");  
            }  
            HandlerExecutionChain handler = hm.getHandler(request);  
            if (handler != null) {  
                return handler;  
            }  
        }  
        return null;  
    }  

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {  
    Object handler = getHandlerInternal(request);  
    if (handler == null) {  
        handler = getDefaultHandler();  
    }  
    if (handler == null) {  
        return null;  
    }  
    // Bean name or resolved handler?  
    if (handler instanceof String) {  
        String handlerName = (String) handler;  
        handler = getApplicationContext().getBean(handlerName);  
    }  
    return getHandlerExecutionChain(handler, request);  
}  

getHandlerExecutionChain:

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {  
        HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?  
                (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));  
        chain.addInterceptors(getAdaptedInterceptors());  
        String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);  
        for (MappedInterceptor mappedInterceptor : this.mappedInterceptors) {  
            if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {  
                chain.addInterceptor(mappedInterceptor.getInterceptor());  
            }  
        }  
        return chain;  
    }   

4、执行拦截器的preHandle方法，如果返回false执行afterCompletion方法并理解返回

5、通过上述获取到了HandlerExecutionChain对象，通过该对象的getHandler()方法获得一个object通过HandlerAdapter进行封装得到HandlerAdapter对象

6、该对象调用handle方法来执行Controller中的方法，该对象如果返回一个ModelAndView给DispatcherServlet

7、DispatcherServlet借助ViewResolver完成逻辑试图名到真实视图对象的解析，得到View后DispatcherServlet使用这个View对ModelAndView中的模型数据进行视图渲染

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