When a resource is requested from a different domain or port than the server where the resource itself resides, the resource A cross-domain HTTP request will be initiated.
For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts. For example, XMLHttpRequest and Fetch API follow the same origin policy. This means that web applications using these APIs can only request HTTP resources from the same domain where the application is loaded, unless CORS headers are used.
If the front end prompts "Access-Control-Allow-Origin" problem
XMLHttpRequest cannot load http://xxxxxxxxxx/. No 'Access- Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access.
Then you need to configure cross-domain SpringBoot2 Filter allows cross-domain access.
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.stereotype.Component;
@Component
public class CorsFilter implements Filter {
@Override
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
response.setHeader("Access-Control-Allow-Origin", "*");
response.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE, PUT, GET");
response.setHeader("Access-Control-Max-Age", "3600");
response.setHeader("Access-Control-Allow-Headers", "x-requested-with");
chain.doFilter(req, res);
}
@Override
public void init(FilterConfig filterConfig) {}
@Override
public void destroy() {}
}If you need to display the cross-domain address, you can also add the access source print statement in it for troubleshooting
String curOrigin = request.getHeader("Origin");
System.out.println("###跨域过滤器->当前访问来源->"+curOrigin+"###");If you need cross-domain permissions, You can determine the source
String curOrigin = request.getHeader("Origin");
System.out.println("###跨域过滤器->当前访问来源->"+curOrigin+"###");
if(curOrigin.indexOf("127.0.0.1:8080")>-1){
response.setHeader("Access-Control-Allow-Origin", "*");
}The protocol, domain name, and port of the request URL are any different That is cross-domain. Cross-domain problems occur due to restrictions in the browser's same-origin policy.
Same origin: If the protocol, domain name, and port of the request URL are the same, it means the same origin (same domain).
Same origin policy: Same origin policy (Same origin policy) is a convention. It is the core and most basic security function of the browser. The same-origin policy prevents content from non-original origins (same domain) from interacting.
Restrictions of same-origin policy:
Unable to read Cookie, LocalStorage and IndexedDB of non-original web pages
Unable to contact the DOM of non-original web pages
Unable to send AJAX requests to non-original addresses
The browser's same-origin policy will restrict cross-domain requests. There are generally two ways to restrict it:
The browser restricts cross-domain requests;
Cross-domain requests can be initiated normally, but the returned results are intercepted by the browser.
Generally, browsers restrict cross-domain requests in the second way, which means that the request has reached the server and may have operated on the data in the database, but the result returned If it is intercepted by the browser, then we will not be able to get the return result. This is a failed request, but it may have an impact on the data in the database.
In order to prevent this from happening, the specification requires that for this HTTP request method that may have side effects on server data, the browser must first use the OPTIONS method to initiate a preflight request to learn whether the server allows the request. Cross-domain request: If allowed, a real request with data will be sent; if not, a real request with data will be blocked.
To solve the problem of non-original content being unable to interact, the current mainstream solution is: CORS (cross-domain resource sharing).
Cross-origin Resource Sharing (Cross-origin Resource Sharing), referred to as CORS, breaks through the limitation that a request issued by a browser can only obtain data from the server when it comes from the same origin.
CORS agrees that the server and the browser negotiate cross-domain resource sharing through some additional HTTP header information on top of the HTTP protocol. Both the server side and the browser must comply with the requirements in the specification.
CORS divides HTTP cross-domain requests into two categories, simple requests and non-simple requests. Different requests negotiate cross-domain resource sharing according to different policies.
Conditions that need to be met for simple cross-domain requests:
1. The request method is GET, HEAD or POST (when POST, the value of Content-Type Must be a value among application/x-www-form-urlencoded, multipart/form-data, text/plain);
2. There is no custom HTTP request header in the request.
The HTTP header can only contain the following fields:
Accept
Accept -Language
##Content-Language
For simple cross-domain requests, the processing method is as follows:
1. What the browser has to do is to add Origin to the HTTP request header, fill in the domain where the JavaScript script is located, and add it to Servers in other domains request resources.Origin: http://www.joker.com
如果Origin指定的源，不在许可范围内，服务器会返回一个正常的HTTP回应。 但这个响应头信息没有包含Access-Control-Allow-Origin字段，浏览器就知道该域名不在许可范围内。
如果Origin指定的域名在许可范围内，服务器返回的响应，会多出几个头信息字段：
Access-Control-Allow-Origin: http://www.joker.com Access-Control-Allow-Credentials: true Access-Control-Expose-Headers: My-Token
Access-Control-Allow-Origin：该字段是必须的。它的值要么是请求时Origin字段的值，要么是一个*值，表示接受任意域名的请求。
Access-Control-Allow-Credentials: 该字段是可选的。它的值是一个布尔值，表示是否允许发送Cookie。默认情况下，Cookie不包括在CORS请求之中。设为true，即表示服务器明确许可，Cookie可以包含在请求中，一起发给服务器。这个值也只能设为true，如果服务器不要浏览器发送Cookie，删除该字段即可。
Access-Control-Expose-Headers：该字段是可选的。CORS请求时，XMLHttpRequest对象的getResponseHeader()方法只能拿到6个基本字段：Cache-Control、Content-Language、Content-Type、Expires、Last-Modified、Pragma。如果想拿到其他字段，就必须在Access-Control-Expose-Headers里面指定。
3.浏览器收到响应后，通过获取响应头中的Access-Control-Allow-Origin字段，来判断如果当前域已经得到授权，则将结果返回给JavaScript。否则浏览器忽略此次响应。
非简单跨域请求需满足的条件：
除GET、HEAD和POST（Content-Type的值是：application/x-www-form-urlencoded、multipart/form-data、text/plain中的一个值）以外的其他HTTP方法
如：PUT、DELETE、TRACE、PATCH、POST（Content-Type的值是：application/json）。
请求中有自定义HTTP头部。
以上两点只要至少满足其中一点就是非简单跨域请求。
对于非简单跨域请求，处理方式如下：
1.浏览器在发送真实HTTP请求之前先发送一个OPTIONS的预检请求，检测服务器端是否支持真实请求进行跨域资源访问。
真实请求的信息在OPTIONS请求中通过请求头中的Access-Control-Request-Method和Access-Control-Request-Headers字段来描述。此外与简单跨域请求一样，请求头中也会有Origin字段。
Origin: http://www.joker.com Access-Control-Request-Method: PUT Access-Control-Request-Headers: Header1,Header2
Origin：必须字段，用于指定请求源。
Access-Control-Request-Method：必须字段，用于描述真实请求的方法（PUT、DELETE等）。
Access-Control-Request-Headers：指定真实请求会额外发送的请求头字段信息。
2.服务器端接到预检请求后，会检查了Origin、Access-Control-Request-Method和Access-Control-Request-Headers字段，检验是否允许跨源请求。
如果不允许该跨域请求，会返回一个正常的HTTP回应，但这个响应头信息没有包含Access-Control-Allow-Origin字段，浏览器就知道该域名不在许可范围内。
如果允许该跨域请求，就会在响应头中放入Access-Control-Allow-Origin、Access-Control-Allow-Methods和Access-Control-Allow-Headers，分别表示允许跨域资源请求的域、请求方法和请求头。此外，服务器端还可以在响应头中放入Access-Control-Max-Age，允许浏览器在指定时间内，无需再发送预检请求进行协商，直接用本次协商结果即可。
Access-Control-Allow-Origin: http://www.joker.com Access-Control-Allow-Methods: GET, POST, PUT Access-Control-Allow-Headers: Header1,Header2,Header3 Access-Control-Allow-Credentials: true Access-Control-Max-Age: 1728000
Access-Control-Allow-Methods：该字段必需，它的值是逗号分隔的一个字符串，表明服务器支持的所有跨域请求的方法。注意，返回的是所有支持的方法，而不单是浏览器请求的那个方法。这是为了避免多次"预检"请求。
Access-Control-Allow-Headers：如果浏览器请求包括Access-Control-Request-Headers字段，则Access-Control-Allow-Headers字段是必需的。它也是一个逗号分隔的字符串，表明服务器支持的所有头信息字段，不限于浏览器在"预检"中请求的字段。
Access-Control-Allow-Credentials： 该字段与简单请求时的含义相同。它的值是一个布尔值，表示是否允许发送Cookie。默认情况下，Cookie不包括在CORS请求之中。设为true，即表示服务器明确许可，Cookie可以包含在请求中，一起发给服务器。这个值也只能设为true，如果服务器不要浏览器发送Cookie，删除该字段即可。
Access-Control-Max-Age： 该字段可选，用来指定本次预检请求的有效期，单位为秒。上面结果中，有效期是20天（1728000秒），即允许缓存该条回应1728000秒（即20天），在此期间，不用发出另一条预检请求。
浏览器根据OPTIONS请求返回的结果来决定是否继续发送真实的请求进行跨域资源访问。这个过程对真实请求的调用者来说是透明的。
SpringBoot设置CORS的的本质都是通过设置响应头信息来告诉前端该请求是否支持跨域。
SpringBoot设置CORS的方式主要有以下三种。
1. 配置过滤器CorsFilter
@Configuration
public class CorsConfig {
@Bean
CorsFilter corsFilter() {
CorsConfiguration configuration = new CorsConfiguration();
configuration.setAllowedOrigins(Arrays.asList("*"));
configuration.setAllowedMethods(Arrays.asList("*"));
configuration.setAllowedHeaders(Arrays.asList("*"));
configuration.setAllowCredentials(true);
UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
source.registerCorsConfiguration("/**", configuration);
return new CorsFilter(source);
}
}2. 实现接口WebMvcConfigurer
@Configuration
public class WebMvcConfig implements WebMvcConfigurer {
@Override
public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/**")
.allowedOrigins("*")
.allowedHeaders("*")
.allowedMethods("*")
.allowCredentials(true);
}
}3. 使用注解@CrossOrigin
@CrossOrigin注解可以用在类或者方法上
用在控制器类上，表示 该类的所有方法都允许跨域
@RestController
@CrossOrigin
public class TestController {
@GetMapping("test")
public String test() {
return "success";
}
}用在控制器方法上，表示该方法都允许跨域
@RestController
public class TestController {
@CrossOrigin
@GetMapping("test")
public String test() {
return "success";
}
}@CrossOrigin注解源码
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface CrossOrigin {
/**
* 这origins和value是一样的
* 允许来源域名的列表，例如 www.baidu.com，匹配的域名是跨域预请求Response头中的Access-Control-Aloow_origin字段值。
* 不设置确切值时默认支持所有域名跨域访问。
*/
@AliasFor("origins")
String[] value() default {};
@AliasFor("value")
String[] origins() default {};
/**
* 高版本下Spring2.4.4使用originPatterns而不是value和origins
*/
String[] originPatterns() default {};
/**
* 跨域请求中允许的请求头中的字段类型， 该值对应跨域预请求Response头中的Access-Control-Allow-Headers字段值。
* 不设置确切值默认支持所有的header字段（Cache-Controller、Content-Language、Content-Type、Expires、Last-Modified、Pragma）跨域访问
*/
String[] allowedHeaders() default {};
/**
* 跨域请求请求头中允许携带的除Cache-Controller、Content-Language、Content-Type、Expires、Last-Modified、Pragma这六个基本字段之外的其他字段信息，
* 对应的是跨域请求Response头中的Access-control-Expose-Headers字段值
*/
String[] exposedHeaders() default {};
/**
* 跨域HTTP请求中支持的HTTP请求类型（GET、POST...），
* 不指定确切值时默认与 Controller 方法中的 methods 字段保持一致。
*/
RequestMethod[] methods() default {};
/**
* 浏览器是否将本域名下的cookie信息携带至跨域服务器中。默认携带至跨域服务器中，但要实现cookie共享还需要前端在AJAX请求中打开withCredentials属性。
* 该值对应的是是跨域请求 Response 头中的 'Access-Control-Allow-Credentials' 字段值。
*/
String allowCredentials() default "";
/**
* 该值的目的是减少浏览器预检请求/响应交互的数量。默认值1800s。设置了该值后，浏览器将在设置值的时间段内对该跨域请求不再发起预请求。
* 该值对应的是是跨域请求Response头中的Access-Control-Max-Age字段值，表示预检请求响应的缓存持续的最大时间。
*/
long maxAge() default -1;
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