跨域請求

當一個資源從與該資源本身所在的伺服器不同的域或埠請求一個資源時，資源會發起一個跨域 HTTP 請求。

出於安全原因，瀏覽器限制從指令碼內發起的跨源HTTP請求。 例如，XMLHttpRequest和Fetch API遵循同源策略。 這意味著使用這些API的Web應用程式只能從載入應用程式的同一個域請求HTTP資源，除非使用CORS標頭檔案。

問題背景

如果前端提示”Access-Control-Allow-Origin”問題

XMLHttpRequest cannot load http://xxxxxxxxxx/. No 'Access-Control-Allow-Origin'
 
 header is present on the requested resource. Origin 'null' is therefore not allowed access.

跨域過濾器

那麼需要再SpringBoot2配置跨域過濾器允許跨域訪問。

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() {}  
}  

跨域功能改進

如果需要顯示跨域地址，還可以在裡面加上訪問來源列印語句，供排查

String curOrigin = request.getHeader("Origin");
System.out.println("###跨域過濾器->當前訪問來源->"+curOrigin+"###");  

如果需要跨域許可權，可以判斷一下來源

String curOrigin = request.getHeader("Origin");
System.out.println("###跨域過濾器->當前訪問來源->"+curOrigin+"###");  
if(curOrigin.indexOf("127.0.0.1:8080")>-1){
    response.setHeader("Access-Control-Allow-Origin", "*");
}