Spring @CrossOrigin 注解原理实现

RequestMappingHandlerMapping
protected CorsConfiguration initCorsConfiguration(Object handler, Method method, RequestMappingInfo mappingInfo) {
 HandlerMethod handlerMethod = createHandlerMethod(handler, method);
 Class<?> beanType = handlerMethod.getBeanType();
    //获取handler上的CrossOrigin 注解
 CrossOrigin typeAnnotation = AnnotatedElementUtils.findMergedAnnotation(beanType, CrossOrigin.class);
    //获取handler 方法上的CrossOrigin 注解
 CrossOrigin methodAnnotation = AnnotatedElementUtils.findMergedAnnotation(method, CrossOrigin.class);
    
 if (typeAnnotation == null && methodAnnotation == null) {
      //如果类上和方法都没标CrossOrigin 注解，则返回一个null
  return null;
 }
    //构建一个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();
 }

 CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping);
  if (corsConfig != null) {
//会保存handlerMethod处理跨域请求的配置
   this.corsLookup.put(handlerMethod, corsConfig);
  }

AbstractHandlerMapping#getHandler 
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
 //如果是一个跨域请求
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);
      //处理跨域（即往响应头添加Access-Control-Allow-Origin信息等），并返回对应的handler对象
  executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
 }

public static boolean isCorsRequest(HttpServletRequest request) {
//判定请求头是否有Origin 属性即可
 return (request.getHeader(HttpHeaders.ORIGIN) != null);
 }

 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 {
      //只是给执行器链添加了一个拦截器
  chain.addInterceptor(new CorsInterceptor(config));
 }
 return chain;
 }

 public boolean processRequest(@Nullable CorsConfiguration config, HttpServletRequest request,
  HttpServletResponse response) throws IOException {
    //....
    //调用了自身的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;
 }
    //设置响应头
 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;
 }