@Override
	@Nullable
	protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
		// 請求靜態資源 path=/zxq/static/login.png
		// 處理完lookupPath=/static/login.png
		String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
		Object handler = lookupHandler(lookupPath, request);
		if (handler == null) {
			// We need to care for the default handler directly, since we need to
			// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
			Object rawHandler = null;
			if ("/".equals(lookupPath)) {
				rawHandler = getRootHandler();
			}
			if (rawHandler == null) {
				rawHandler = getDefaultHandler();
			}
			if (rawHandler != null) {
				// Bean name or resolved handler?
				if (rawHandler instanceof String) {
					String handlerName = (String) rawHandler;
					rawHandler = obtainApplicationContext().getBean(handlerName);
				}
				validateHandler(rawHandler, request);
				handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
			}
		}
		if (handler != null && logger.isDebugEnabled()) {
			logger.debug("Mapping [" + lookupPath + "] to " + handler);
		}
		else if (handler == null && logger.isTraceEnabled()) {
			logger.trace("No handler mapping found for [" + lookupPath + "]");
		}
		return handler;
	}

 	/**
	 * Look up a handler instance for the given URL path.
	 * <p>Supports direct matches, e.g. a registered "/test" matches "/test",
	 * and various Ant-style pattern matches, e.g. a registered "/t*" matches
	 * both "/test" and "/team". For details, see the AntPathMatcher class.
	 * <p>Looks for the most exact pattern, where most exact is defined as
	 * the longest path pattern.
	 * @param urlPath the URL the bean is mapped to
	 * @param request current HTTP request (to expose the path within the mapping to)
	 * @return the associated handler instance, or {@code null} if not found
	 * @see #exposePathWithinMapping
	 * @see org.springframework.util.AntPathMatcher
	 */
	@Nullable
	protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
		// Direct match?
		// 精確匹配，是否有符合的handler
		// urlPath = /static/login.png
		Object handler = this.handlerMap.get(urlPath);
		if (handler != null) {
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = obtainApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			return buildPathExposingHandler(handler, urlPath, urlPath, null);
		}

		// Pattern match?
		// 路徑匹配
		List<String> matchingPatterns = new ArrayList<>();
		for (String registeredPattern : this.handlerMap.keySet()) {
			if (getPathMatcher().match(registeredPattern, urlPath)) {
				matchingPatterns.add(registeredPattern);
			}
			else if (useTrailingSlashMatch()) {
				if (!registeredPattern.endsWith("/") && getPathMatcher().match(registeredPattern + "/", urlPath)) {
					matchingPatterns.add(registeredPattern + "/");
				}
			}
		}

		String bestMatch = null;
		Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
		if (!matchingPatterns.isEmpty()) {
			matchingPatterns.sort(patternComparator);
			if (logger.isDebugEnabled()) {
				logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
			}
			bestMatch = matchingPatterns.get(0);
		}
		// bestMatch = /static/**
		if (bestMatch != null) {
			handler = this.handlerMap.get(bestMatch);
			if (handler == null) {
				if (bestMatch.endsWith("/")) {
					handler = this.handlerMap.get(bestMatch.substring(0, bestMatch.length() - 1));
				}
				if (handler == null) {
					throw new IllegalStateException(
							"Could not find handler for best pattern match [" + bestMatch + "]");
				}
			}
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = obtainApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			// login.png
			String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestMatch, urlPath);

			// There might be multiple 'best patterns', let's make sure we have the correct URI template variables
			// for all of them
			Map<String, String> uriTemplateVariables = new LinkedHashMap<>();
			for (String matchingPattern : matchingPatterns) {
				if (patternComparator.compare(bestMatch, matchingPattern) == 0) {
					Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
					Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
					uriTemplateVariables.putAll(decodedVars);
				}
			}
			if (logger.isDebugEnabled()) {
				logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);
			}
			// /static/**   login.png
			return buildPathExposingHandler(handler, bestMatch, pathWithinMapping, uriTemplateVariables);
		}

		// No handler found...
		return null;
	}

 /**
 * Processes a resource request.
 * <p>Checks for the existence of the requested resource in the configured list of locations.
 * If the resource does not exist, a {@code 404} response will be returned to the client.
 * If the resource exists, the request will be checked for the presence of the
 * {@code Last-Modified} header, and its value will be compared against the last-modified
 * timestamp of the given resource, returning a {@code 304} status code if the
 * {@code Last-Modified} value  is greater. If the resource is newer than the
 * {@code Last-Modified} value, or the header is not present, the content resource
 * of the resource will be written to the response with caching headers
 * set to expire one year in the future.
 */
@Override
public void handleRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {

  // For very general mappings (e.g. "/") we need to check 404 first
  // 根據請求的文件路徑找到對應的資源文件
  Resource resource = getResource(request);
  if (resource == null) {
    logger.trace("No matching resource found - returning 404");
    response.sendError(HttpServletResponse.SC_NOT_FOUND);
    return;
  }

  if (HttpMethod.OPTIONS.matches(request.getMethod())) {
    response.setHeader("Allow", getAllowHeader());
    return;
  }

  // Supported methods and required session
  // 校驗支援的方法GET和HEAD 以及驗證session是否必須
  checkRequest(request);

  // Header phase
  if (new ServletWebRequest(request, response).checkNotModified(resource.lastModified())) {
    logger.trace("Resource not modified - returning 304");
    return;
  }

  // Apply cache settings, if any
  // 可以根據設置的秒數設置快取時間 cache-control:max-age=xxx
  prepareResponse(response);

  // Check the media type for the resource
  // 根據文件後綴去尋找 png -> image/png
  MediaType mediaType = getMediaType(request, resource);
  if (mediaType != null) {
    if (logger.isTraceEnabled()) {
      logger.trace("Determined media type '" + mediaType + "' for " + resource);
    }
  }
  else {
    if (logger.isTraceEnabled()) {
      logger.trace("No media type found for " + resource + " - not sending a content-type header");
    }
  }

  // Content phase
  if (METHOD_HEAD.equals(request.getMethod())) {
    setHeaders(response, resource, mediaType);
    logger.trace("HEAD request - skipping content");
    return;
  }

  ServletServerHttpResponse outputMessage = new ServletServerHttpResponse(response);
  if (request.getHeader(HttpHeaders.RANGE) == null) {
    Assert.state(this.resourceHttpMessageConverter != null, "Not initialized");
    // 設置content-type、content-length等響應頭
    setHeaders(response, resource, mediaType);
    // 將文件流寫入到response響應中
    this.resourceHttpMessageConverter.write(resource, mediaType, outputMessage);
  }
  else {
    Assert.state(this.resourceRegionHttpMessageConverter != null, "Not initialized");
    response.setHeader(HttpHeaders.ACCEPT_RANGES, "bytes");
    ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(request);
    try {
      List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
      response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT);
      this.resourceRegionHttpMessageConverter.write(
          HttpRange.toResourceRegions(httpRanges, resource), mediaType, outputMessage);
    }
    catch (IllegalArgumentException ex) {
      response.setHeader("Content-Range", "bytes */" + resource.contentLength());
      response.sendError(HttpServletResponse.SC_REQUESTED_RANGE_NOT_SATISFIABLE);
    }
  }
}