现实开发中,我们难免遇到跨域问题,以前笔者只知道jsonp这种解决方式,后面听说spring只要加入@CrossOrigin即可解决跨域问题。本着好奇的心里,笔者看了下@CrossOrigin 作用原理,写下这篇博客。
先说原理:其实很简单,就是利用spring的拦截器实现往response里添加 Access-Control-Allow-Origin等响应头信息,我们可以看下spring是怎么做的
注:这里使用的spring版本为5.0.6
我们可以先往RequestMappingHandlerMapping 的initCorsConfiguration方法打一个断点,发现方法调用情况如下
如果controller在类上标了@CrossOrigin或在方法上标了@CrossOrigin注解,则spring 在记录mapper映射时会记录对应跨域请求映射,代码如下
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(); }
将结果返回到了AbstractHandlerMethodMapping#register,主要代码如下
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); }
当一个跨域请求过来时,spring在获取handler时会判断这个请求是否是一个跨域请求,如果是,则会返回一个可以处理跨域的handler
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); }
我们可以看下如何判定一个请求是一个跨域请求,
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); }
再看下getCorsHandlerExecutionChain 是如何获取一个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 { //只是给执行器链添加了一个拦截器 chain.addInterceptor(new CorsInterceptor(config)); } return chain; }
也就是在调用目标方法前会先调用CorsInterceptor#preHandle,我们观察得到其也是调用了corsProcessor.processRequest方法,我们往这里打个断点
processRequest方法的主要逻辑如下
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; }
至此@CrossOrigin的使命就完成了,说白了就是用拦截器给response添加响应头信息而已
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