一、什么是CORS？

CORS是一個W3C標準，全稱是”跨域資源共享”（Cross-origin resource sharing），允許瀏覽器向跨源服務器，發出XMLHttpRequest請求，從而克服了AJAX只能同源使用的限制。

它通過服務器增加一個特殊的Header[Access-Control-Allow-Origin]來告訴客戶端跨域的限制，如果瀏覽器支持CORS、并且判斷Origin通過的話，就會允許XMLHttpRequest發起跨域請求。
?

CORS Header?

• Access-Control-Allow-Origin: http://www.xxx.com
• Access-Control-Max-Age：86400
• Access-Control-Allow-Methods：GET, POST, OPTIONS, PUT, DELETE
• Access-Control-Allow-Headers: content-type
• Access-Control-Allow-Credentials: true

含義解釋：

?二、SpringBoot跨域請求處理方式

方式一、直接采用SpringBoot的注解@CrossOrigin（也支持SpringMVC）

Controller層在需要跨域的類或者方法上加上該注解即可

實戰：

?備注說明:Spring 版本必須大于等于4.2

方法二、處理跨域請求的Configuration

增加一個配置類CrossOriginConfig.java。繼承WebMvcConfigurerAdapter或者實現WebMvcConfigurer接口

實戰:

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.CorsRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
 
/**
 * AJAX請求跨域
 */
@Configuration
public class CorsConfig extends WebMvcConfigurerAdapter {
    static final String ORIGINS[] = new String[] { "GET", "POST", "PUT", "DELETE" };
    @Override
    public void addCorsMappings(CorsRegistry registry) {
    registry.addMapping("/**")
     .allowedOrigins("*")
     .allowCredentials(true)
     .allowedMethods(ORIGINS)
     .maxAge(3600);
    }

方法三、采用過濾器（filter）的方式(推薦)

增加一個CORSFilter 類，并實現Filter接口即可

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.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;

/**
 * 
 * @ClassName:  CorsFilter   
 * @Description: SpringBoot 跨域處理攔截器
 */

@Component
public class CROSFilter implements Filter {
	public static final Logger logger = LoggerFactory.getLogger(CROSFilter.class);
	
	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
			throws IOException, ServletException {
		HttpServletResponse response = (HttpServletResponse) res;  
		  
        HttpServletRequest reqs = (HttpServletRequest) req;  
        /*
         * 跨域設置允所有請求跨域 
         * 如果允許指定的客戶端跨域設置: http://127.0.0.1:8020
         */
        response.setHeader("Access-Control-Allow-Origin","*");  
        response.setHeader("Access-Control-Allow-Credentials", "true");  
        response.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE");  
        response.setHeader("Access-Control-Max-Age", "3600");  
        response.setHeader("Access-Control-Allow-Headers", "Content-Type");  
        if (((HttpServletRequest) req).getMethod().equals("OPTIONS")) {
            response.getWriter().println("ok");
            return;
        }

        chain.doFilter(req, res);  
	}

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		
	}

}

解決遇到的錯誤

1、Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

解決辦法:

  response.setHeader("Access-Control-Allow-Headers", "Content-Type");  

2、Response to preflight request doesn't pass access control check: It does not have HTTP ok status.

解決辦法:

if (((HttpServletRequest) req).getMethod().equals("OPTIONS")) {
            response.getWriter().println("ok");
            return;
        }