出于安全原因，浏览器禁止Ajax调用驻留在当前原点之外的资源。例如，当你在一个标签中检查你的银行账户时，你可以在另一个选项卡上拥有EVILL网站。来自EVILL的脚本不能够对你的银行API做出Ajax请求（从你的帐户中取出钱！）使用您的凭据。
跨源资源共享（CORS）是由大多数浏览器实现的W3C规范，允许您灵活地指定什么样的跨域请求被授权，而不是使用一些不太安全和不太强大的策略，如IFRAME或JSONP。
跨域是跨服务之间调用需要解决的问题，无论是Ajax还是后端接口，跨域问题都需要解决，无论是通过Nginx还是代码配置。
Spring @CrossOrigin注解源码
/*
* Copyright 2002-2019 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.web.bind.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.core.annotation.AliasFor;
import org.springframework.web.cors.CorsConfiguration;
/**
* Annotation for permitting cross-origin requests on specific handler classes
* and/or handler methods. Processed if an appropriate {@code HandlerMapping}
* is configured.
*
* <p>Both Spring Web MVC and Spring WebFlux support this annotation through the
* {@code RequestMappingHandlerMapping} in their respective modules. The values
* from each type and method level pair of annotations are added to a
* {@link CorsConfiguration} and then default values are applied via
* {@link CorsConfiguration#applyPermitDefaultValues()}.
*
* <p>The rules for combining global and local configuration are generally
* additive -- e.g. all global and all local origins. For those attributes
* where only a single value can be accepted such as {@code allowCredentials}
* and {@code maxAge}, the local overrides the global value.
* See {@link CorsConfiguration#combine(CorsConfiguration)} for more details.
*
* @author Russell Allen
* @author Sebastien Deleuze
* @author Sam Brannen
* @since 4.2
*/
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface CrossOrigin {
/** @deprecated as of Spring 5.0, in favor of {@link CorsConfiguration#applyPermitDefaultValues} */
@Deprecated
String[] DEFAULT_ORIGINS = {"*"};
/** @deprecated as of Spring 5.0, in favor of {@link CorsConfiguration#applyPermitDefaultValues} */
@Deprecated
String[] DEFAULT_ALLOWED_HEADERS = {"*"};
/** @deprecated as of Spring 5.0, in favor of {@link CorsConfiguration#applyPermitDefaultValues} */
@Deprecated
boolean DEFAULT_ALLOW_CREDENTIALS = false;
/** @deprecated as of Spring 5.0, in favor of {@link CorsConfiguration#applyPermitDefaultValues} */
@Deprecated
long DEFAULT_MAX_AGE = 1800;
/**
* Alias for {@link #origins}.
*/
@AliasFor("origins")
String[] value() default {};
/**
* The list of allowed origins that be specific origins, e.g.
* {@code "https://domain1.com"}, or {@code "*"} for all origins.
* <p>A matched origin is listed in the {@code Access-Control-Allow-Origin}
* response header of preflight actual CORS requests.
* <p>By default all origins are allowed.
* <p><strong>Note:</strong> CORS checks use values from "Forwarded"
* (<a href="https://tools.ietf.org/html/rfc7239">RFC 7239</a>),
* "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto" headers,
* if present, in order to reflect the client-originated address.
* Consider using the {@code ForwardedHeaderFilter} in order to choose from a
* central place whether to extract and use, or to discard such headers.
* See the Spring Framework reference for more on this filter.
* @see #value
*/
@AliasFor("value")
String[] origins() default {};
/**
* The list of request headers that are permitted in actual requests,
* possibly {@code "*"} to allow all headers.
* <p>Allowed headers are listed in the {@code Access-Control-Allow-Headers}
* response header of preflight requests.
* <p>A header name is not required to be listed if it is one of:
* {@code Cache-Control}, {@code Content-Language}, {@code Expires},
* {@code Last-Modified}, or {@code Pragma} as per the CORS spec.
* <p>By default all requested headers are allowed.
*/
String[] allowedHeaders() default {};
/**
* The List of response headers that the user-agent will allow the client
* to access on an actual response, other than "simple" headers, i.e.
* {@code Cache-Control}, {@code Content-Language}, {@code Content-Type},
* {@code Expires}, {@code Last-Modified}, or {@code Pragma},
* <p>Exposed headers are listed in the {@code Access-Control-Expose-Headers}
* response header of actual CORS requests.
* <p>By default no headers are listed as exposed.
*/
String[] exposedHeaders() default {};
/**
* The list of supported HTTP request methods.
* <p>By default the supported methods are the same as the ones to which a
* controller method is mapped.
*/
RequestMethod[] methods() default {};
/**
* Whether the browser should send credentials, such as cookies along with
* cross domain requests, to the annotated endpoint. The configured value is
* set on the {@code Access-Control-Allow-Credentials} response header of
* preflight requests.
* <p><strong>NOTE:</strong> Be aware that this option establishes a high
* level of trust with the configured domains and also increases the surface
* attack of the web application by exposing sensitive user-specific
* information such as cookies and CSRF tokens.
* <p>By default this is not set in which case the
* {@code Access-Control-Allow-Credentials} header is also not set and
* credentials are therefore not allowed.
*/
String allowCredentials() default "";
/**
* The maximum age (in seconds) of the cache duration for preflight responses.
* <p>This property controls the value of the {@code Access-Control-Max-Age}
* response header of preflight requests.
* <p>Setting this to a reasonable value can reduce the number of preflight
* request/response interactions required by the browser.
* A negative value means <em>undefined</em>.
* <p>By default this is set to {@code 1800} seconds (30 minutes).
*/
long maxAge() default -1;
}
Spring @CrossOrigin解决跨域
以下内容来自：https://www.mmzsblog.cn/articles/2019/08/23/1566526598886.html
跨域(CORS)支持
Spring Framework 4.2 GA为CORS提供了第一类支持，使您比通常的基于过滤器的解决方案更容易和更强大地配置它。所以springMVC的版本要在4.2或以上版本才支持@CrossOrigin
使用方法
1、controller配置CORS
1.1、controller方法的CORS配置，您可以向@RequestMapping注解处理程序方法添加一个@CrossOrigin注解，以便启用CORS（默认情况下，@CrossOrigin允许在@RequestMapping注解中指定的所有源和HTTP方法）：
@RestController
@RequestMapping("/account") public class AccountController {
@CrossOrigin
@GetMapping("/{id}") public Account retrieve(@PathVariable Long id) { // ...
}
@DeleteMapping("/{id}") public void remove(@PathVariable Long id) { // ...
}
}
其中@CrossOrigin中的2个参数：
origins ： 允许可访问的域列表
maxAge：准备响应前的缓存持续的最大时间（以秒为单位）。
1.2、为整个controller启用@CrossOrigin
@CrossOrigin(origins = "http://domain2.com", maxAge = 3600)
@RestController
@RequestMapping("/account") public class AccountController {
@GetMapping("/{id}") public Account retrieve(@PathVariable Long id) { // ...
}
@DeleteMapping("/{id}") public void remove(@PathVariable Long id) { // ...
}
}
在这个例子中，对于retrieve()和remove()处理方法都启用了跨域支持，还可以看到如何使用@CrossOrigin属性定制CORS配置。
1.3、同时使用controller和方法级别的CORS配置，Spring将合并两个注释属性以创建合并的CORS配置。
@CrossOrigin(maxAge = 3600)
@RestController
@RequestMapping("/account") public class AccountController {
@CrossOrigin(origins = "http://domain2.com")
@GetMapping("/{id}") public Account retrieve(@PathVariable Long id) { // ...
}
@DeleteMapping("/{id}") public void remove(@PathVariable Long id) { // ...
}
}
1.4、如果您正在使用Spring Security，请确保在Spring安全级别启用CORS，并允许它利用Spring MVC级别定义的配置。
@EnableWebSecurity public class WebSecurityConfig extends WebSecurityConfigurerAdapter {
@Override protected void configure(HttpSecurity http) throws Exception {
http.cors().and()...
}
}
2、全局CORS配置
除了细粒度、基于注释的配置之外，您还可能需要定义一些全局CORS配置。这类似于使用筛选器，但可以声明为Spring MVC并结合细粒度@CrossOrigin配置。默认情况下，所有origins and GET, HEAD and POST methods是允许的。
JavaConfig
使整个应用程序的CORS简化为：
@Configuration
@EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter {
@Override public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/**");
}
}
如果您正在使用Spring Boot，建议将WebMvcConfigurer bean声明如下：
@Configuration public class MyConfiguration {
@Bean public WebMvcConfigurer corsConfigurer() { return new WebMvcConfigurerAdapter() {
@Override public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/**");
}
};
}
}
您可以轻松地更改任何属性，以及仅将此CORS配置应用到特定的路径模式：
@Override public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/api/**")
.allowedOrigins("http://domain2.com")
.allowedMethods("PUT", "DELETE")
.allowedHeaders("header1", "header2", "header3")
.exposedHeaders("header1", "header2")
.allowCredentials(false).maxAge(3600);
}
如果您正在使用Spring Security，请确保在Spring安全级别启用CORS，并允许它利用Spring MVC级别定义的配置。
3、XML命名空间
还可以将CORS与MVC XML命名空间配置。
a、如果整个项目所有方法都可以访问，则可以这样配置；此最小XML配置使CORS在/**路径模式具有与JavaConfig相同的缺省属性：
<mvc:cors>
<mvc:mapping path="/**" />
</mvc:cors>
其中 表示匹配到下一层；***** 表示后面不管有多少层，都能匹配。**
如：
<mvc:cors>
<mvc:mapping path="/api/*"/>
</mvc:cors>
这个可以匹配到的路径有：
/api/aaa
/api/bbbb
不能匹配的：
/api/aaa/bbb
因为* 只能匹配到下一层路径，如果想后面不管多少层都可以匹配，配置如下：
<mvc:cors>
<mvc:mapping path="/api/**"/>
</mvc:cors>
注：其实就是一个(*)变成两个(**)
b、也可以用定制属性声明几个CORS映射：
<mvc:cors>
<mvc:mapping path="/api/**" allowed-origins="http://domain1.com, http://domain2.com" allowed-methods="GET, PUT" allowed-headers="header1, header2, header3" exposed-headers="header1, header2" allow-credentials="false" max-age="123" />
<mvc:mapping path="/resources/**" allowed-origins="http://domain1.com" />
</mvc:cors>
请求路径有/api/，方法示例如下：
@RequestMapping("/api/crossDomain")
@ResponseBody public String crossDomain(HttpServletRequest req, HttpServletResponse res, String name){
……
……
}
c、如果使用Spring Security，不要忘记在Spring安全级别启用CORS：
<http>
<!-- Default to Spring MVC's CORS configuration -->
<cors /> ... </http>
4、How does it work?
CORS请求（包括预选的带有选项方法）被自动发送到注册的各种HandlerMapping 。它们处理CORS准备请求并拦截CORS简单和实际请求，这得益于CorsProcessor实现（默认情况下默认DefaultCorsProcessor处理器），以便添加相关的CORS响应头（如Access-Control-Allow-Origin）。 CorsConfiguration 允许您指定CORS请求应该如何处理：允许origins, headers, methods等。
a、AbstractHandlerMapping#setCorsConfiguration() 允许指定一个映射，其中有几个CorsConfiguration 映射在路径模式上，比如/api/**。
b、子类可以通过重写AbstractHandlerMapping类的getCorsConfiguration(Object, HttpServletRequest)方法来提供自己的CorsConfiguration。
c、处理程序可以实现 CorsConfigurationSource接口（如ResourceHttpRequestHandler），以便为每个请求提供一个CorsConfiguration。
5、基于过滤器的CORS支持
作为上述其他方法的替代，Spring框架还提供了CorsFilter。在这种情况下，不用使用@CrossOrigin或 WebMvcConfigurer#addCorsMappings(CorsRegistry)，例如，可以在Spring Boot应用程序中声明如下的过滤器：
@Configuration public class MyConfiguration {
@Bean public FilterRegistrationBean corsFilter() {
UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
CorsConfiguration config = new CorsConfiguration();
config.setAllowCredentials(true);
config.addAllowedOrigin("http://domain1.com");
config.addAllowedHeader("*");
config.addAllowedMethod("*");
source.registerCorsConfiguration("/**", config);
FilterRegistrationBean bean = new FilterRegistrationBean(new CorsFilter(source));
bean.setOrder(0); return bean;
}
}
Spring注解@CrossOrigin不起作用的原因
1、是springMVC的版本要在4.2或以上版本才支持@CrossOrigin
2、非@CrossOrigin没有解决跨域请求问题，而是不正确的请求导致无法得到预期的响应，导致浏览器端提示跨域问题。
3、在Controller注解上方添加@CrossOrigin注解后，仍然出现跨域问题，解决方案之一就是：
在@RequestMapping注解中没有指定Get、Post方式，具体指定后，问题解决。
类似代码如下：
@CrossOrigin
@RestController public class person{
@RequestMapping(method = RequestMethod.GET) public String add() { // 若干代码
}
}
参考文章
1、官方文档https://spring.io/blog/2015/06/08/cors-support-in-spring-framework
2、http://fanshuyao.iteye.com/blog/2384189
2、https://blog.csdn.net/taiyangnimeide/article/details/78305131
3、https://blog.csdn.net/snowin1994/article/details/53035433
标题：注解@CrossOrigin解决跨域问题
作者：mmzsblog
地址：https://www.mmzsblog.cn/articles/2019/08/23/1566526598886.html
