DispatcherServlet 處理流程

上一節(jié)講了Spring容器啟動(dòng)，會(huì)把url與類方法的映射關(guān)系保存起來侄榴，這一節(jié)允扇，就能看到它的作用啦之众。

DispatcherServlet是整個(gè)SpringMVC的核心，負(fù)責(zé)請(qǐng)求處理以及返回響應(yīng)的工作蝇率。直奔主題迟杂，找到DispatcherServlet類，再找到該類下的doService()方法本慕，關(guān)鍵代碼：

try {
    doDispatch(request, response);
}

點(diǎn)擊這個(gè)孤獨(dú)的方法排拷，進(jìn)去，上核心馬：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    // 定義handler執(zhí)行鏈锅尘，重要
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;

    ModelAndView mv = null;
    // 檢查是否是文件請(qǐng)求
    processedRequest = checkMultipart(request);
    multipartRequestParsed = (processedRequest != request);

    // 獲取當(dāng)前請(qǐng)求的handler
    mappedHandler = getHandler(processedRequest);
    if (mappedHandler == null) {
        noHandlerFound(processedRequest, response);
        return;
    }

    // 獲取當(dāng)前請(qǐng)求的適配器
    HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

    // 判斷get方法是否有修改监氢，沒有直接返回
    String method = request.getMethod();
    boolean isGet = HttpMethod.GET.matches(method);
    if (isGet || HttpMethod.HEAD.matches(method)) {
        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
        if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
         return;
        }
    }
    // 前置處理
    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
        return;
    }

    // hander實(shí)際執(zhí)行
    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

    applyDefaultViewName(processedRequest, mv);
    // handler后置處理
    mappedHandler.applyPostHandle(processedRequest, response, mv);
    // 處理分發(fā)后的結(jié)果
    processDispatchResult(processedRequest, response, mappedHandler, mv,     
                          dispatchException);

}

看到那個(gè)閃亮的方法沒 - getHandler(processedRequest)，點(diǎn)擊進(jìn)去鉴象，上馬：

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    if (this.handlerMappings != null) {
        for (HandlerMapping mapping : this.handlerMappings) {
            // 從handlerMappings 獲取并構(gòu)建請(qǐng)求的執(zhí)行鏈
            HandlerExecutionChain handler = mapping.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
    }
    return null;
}

打個(gè)debug忙菠，看下handlerMappings里面是啥

RequestMappingHandlerMapping.png

看到?jīng)]，這不就是上一節(jié)講到的東東咩纺弊？

再點(diǎn)擊 mapping.getHandler(request)方法進(jìn)去牛欢，看看有啥：

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    // 獲取handler
    Object handler = getHandlerInternal(request);
    ……
        
    // 是beanName獲取對(duì)應(yīng)的bean
    if (handler instanceof String) {
        String handlerName = (String) handler;
        handler = obtainApplicationContext().getBean(handlerName);
    }
    ……
        
    // 構(gòu)建handler的執(zhí)行鏈
    HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);  
    ……

    return executionChain;
}

getHandlerInternal(request) 是重點(diǎn)，點(diǎn)進(jìn)去：

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
    // 獲取訪問路徑淆游，如 /test/hello
    String lookupPath = initLookupPath(request);
    this.mappingRegistry.acquireReadLock();
    try {
        // 查找訪問路徑對(duì)應(yīng)的處理方法
        HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
        return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
    }
    finally {
        this.mappingRegistry.releaseReadLock();
    }
}

快到了傍睹！繼續(xù)點(diǎn)擊 lookupHandlerMethod(lookupPath, request)隔盛，進(jìn)入：

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
    List<Match> matches = new ArrayList<>();
    // 根據(jù)訪問路徑匹配請(qǐng)求方法
    List<T> directPathMatches = this.mappingRegistry.getMappingsByDirectPath(lookupPath);
    if (directPathMatches != null) {
        // 根據(jù)找到的信息，封裝 matches
        addMatchingMappings(directPathMatches, matches, request);
    }
    if (matches.isEmpty()) {
        addMatchingMappings(this.mappingRegistry.getRegistrations().keySet(), matches, request);
    }
    
    // 省略查找最近匹配內(nèi)容
    ……
    
    // 返回 handlerMethod
    request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.getHandlerMethod());
    handleMatch(bestMatch.mapping, lookupPath, request);
    return bestMatch.getHandlerMethod();
}

點(diǎn)擊this.mappingRegistry.getMappingsByDirectPath(lookupPath)拾稳，進(jìn)入：

public List<T> getMappingsByDirectPath(String urlPath) {
    return this.pathLookup.get(urlPath);
}

看到了沒吮炕，pathLookup登場了，T 類型是 RequestMappingInfo访得。

這一步龙亲，就能找到 handlerMethod了，也就是訪問路徑對(duì)應(yīng)的處理方法悍抑。所以上面的getHandler() 方法就能返回對(duì)應(yīng)的 HandlerExecutionChain鳄炉。

回到 doDispatch()方法中，既然拿到了 handlerMethod搜骡，后面就是做了一些錦上添花的事拂盯，最后調(diào)用該方法了。

不講了记靡，口渴谈竿，飲杯開水去~~