現(xiàn)實(shí)開發(fā)中，我們難免遇到跨域問題，以前筆者只知道jsonp這種解決方式，后面聽說spring只要加入@CrossOrigin即可解決跨域問題。本著好奇的心里，筆者看了下@CrossOrigin 作用原理，寫下這篇博客。

先說原理：其實(shí)很簡(jiǎn)單，就是利用spring的攔截器實(shí)現(xiàn)往response里添加 Access-Control-Allow-Origin等響應(yīng)頭信息，我們可以看下spring是怎么做的

注：這里使用的spring版本為5.0.6

我們可以先往RequestMappingHandlerMapping 的initCorsConfiguration方法打一個(gè)斷點(diǎn)，發(fā)現(xiàn)方法調(diào)用情況如下

如果controller在類上標(biāo)了@CrossOrigin或在方法上標(biāo)了@CrossOrigin注解，則spring 在記錄mapper映射時(shí)會(huì)記錄對(duì)應(yīng)跨域請(qǐng)求映射，代碼如下

RequestMappingHandlerMappingprotected CorsConfiguration initCorsConfiguration(Object handler, Method method, RequestMappingInfo mappingInfo) { HandlerMethod handlerMethod = createHandlerMethod(handler, method); Class<?> beanType = handlerMethod.getBeanType(); //獲取handler上的CrossOrigin 注解 CrossOrigin typeAnnotation = AnnotatedElementUtils.findMergedAnnotation(beanType, CrossOrigin.class); //獲取handler 方法上的CrossOrigin 注解 CrossOrigin methodAnnotation = AnnotatedElementUtils.findMergedAnnotation(method, CrossOrigin.class); if (typeAnnotation == null && methodAnnotation == null) { //如果類上和方法都沒標(biāo)CrossOrigin 注解，則返回一個(gè)null return null; } //構(gòu)建一個(gè)CorsConfiguration 并返回 CorsConfiguration config = new CorsConfiguration(); updateCorsConfig(config, typeAnnotation); updateCorsConfig(config, methodAnnotation); if (CollectionUtils.isEmpty(config.getAllowedMethods())) { for (RequestMethod allowedMethod : mappingInfo.getMethodsCondition().getMethods()) { config.addAllowedMethod(allowedMethod.name()); } } return config.applyPermitDefaultValues(); }

將結(jié)果返回到了AbstractHandlerMethodMapping#register，主要代碼如下

CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping); if (corsConfig != null) {//會(huì)保存handlerMethod處理跨域請(qǐng)求的配置 this.corsLookup.put(handlerMethod, corsConfig); }

當(dāng)一個(gè)跨域請(qǐng)求過來時(shí)，spring在獲取handler時(shí)會(huì)判斷這個(gè)請(qǐng)求是否是一個(gè)跨域請(qǐng)求，如果是，則會(huì)返回一個(gè)可以處理跨域的handler

AbstractHandlerMapping#getHandler HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request); //如果是一個(gè)跨域請(qǐng)求if (CorsUtils.isCorsRequest(request)) { //拿到跨域的全局配置 CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request); //拿到hander的跨域配置 CorsConfiguration handlerConfig = getCorsConfiguration(handler, request); CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig); //處理跨域（即往響應(yīng)頭添加Access-Control-Allow-Origin信息等），并返回對(duì)應(yīng)的handler對(duì)象 executionChain = getCorsHandlerExecutionChain(request, executionChain, config); }

我們可以看下如何判定一個(gè)請(qǐng)求是一個(gè)跨域請(qǐng)求，

public static boolean isCorsRequest(HttpServletRequest request) {//判定請(qǐng)求頭是否有Origin 屬性即可 return (request.getHeader(HttpHeaders.ORIGIN) != null); }

再看下getCorsHandlerExecutionChain 是如何獲取一個(gè)handler

protected HandlerExecutionChain getCorsHandlerExecutionChain(HttpServletRequest request, HandlerExecutionChain chain, @Nullable CorsConfiguration config) { if (CorsUtils.isPreFlightRequest(request)) { HandlerInterceptor[] interceptors = chain.getInterceptors(); chain = new HandlerExecutionChain(new PreFlightHandler(config), interceptors); } else { //只是給執(zhí)行器鏈添加了一個(gè)攔截器 chain.addInterceptor(new CorsInterceptor(config)); } return chain; }

也就是在調(diào)用目標(biāo)方法前會(huì)先調(diào)用CorsInterceptor#preHandle，我們觀察得到其也是調(diào)用了corsProcessor.processRequest方法，我們往這里打個(gè)斷點(diǎn)

processRequest方法的主要邏輯如下

public boolean processRequest(@Nullable CorsConfiguration config, HttpServletRequest request, HttpServletResponse response) throws IOException { //.... //調(diào)用了自身的handleInternal方法 return handleInternal(serverRequest, serverResponse, config, preFlightRequest); }protected boolean handleInternal(ServerHttpRequest request, ServerHttpResponse response, CorsConfiguration config, boolean preFlightRequest) throws IOException { String requestOrigin = request.getHeaders().getOrigin(); String allowOrigin = checkOrigin(config, requestOrigin); HttpHeaders responseHeaders = response.getHeaders(); responseHeaders.addAll(HttpHeaders.VARY, Arrays.asList(HttpHeaders.ORIGIN, HttpHeaders.ACCESS_CONTROL_REQUEST_METHOD, HttpHeaders.ACCESS_CONTROL_REQUEST_HEADERS)); if (allowOrigin == null) { logger.debug('Rejecting CORS request because ’' + requestOrigin + '’ origin is not allowed'); rejectRequest(response); return false; } HttpMethod requestMethod = getMethodToUse(request, preFlightRequest); List<HttpMethod> allowMethods = checkMethods(config, requestMethod); if (allowMethods == null) { logger.debug('Rejecting CORS request because ’' + requestMethod + '’ request method is not allowed'); rejectRequest(response); return false; } List<String> requestHeaders = getHeadersToUse(request, preFlightRequest); List<String> allowHeaders = checkHeaders(config, requestHeaders); if (preFlightRequest && allowHeaders == null) { logger.debug('Rejecting CORS request because ’' + requestHeaders + '’ request headers are not allowed'); rejectRequest(response); return false; } //設(shè)置響應(yīng)頭 responseHeaders.setAccessControlAllowOrigin(allowOrigin); if (preFlightRequest) { responseHeaders.setAccessControlAllowMethods(allowMethods); } if (preFlightRequest && !allowHeaders.isEmpty()) { responseHeaders.setAccessControlAllowHeaders(allowHeaders); } if (!CollectionUtils.isEmpty(config.getExposedHeaders())) { responseHeaders.setAccessControlExposeHeaders(config.getExposedHeaders()); } if (Boolean.TRUE.equals(config.getAllowCredentials())) { responseHeaders.setAccessControlAllowCredentials(true); } if (preFlightRequest && config.getMaxAge() != null) { responseHeaders.setAccessControlMaxAge(config.getMaxAge()); } //刷新 response.flush(); return true; }

至此@CrossOrigin的使命就完成了，說白了就是用攔截器給response添加響應(yīng)頭信息而已

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