Spring Cloud Gateway的全局过滤器GlobalFilter，顾名思义，声明后会对所有的请求生效，可以用来做权限控制，这里简单记录一下拦截到非法请求后如何返回自定义信息和将请求重定向到指定URL。spring-cloud-gateway版本为2.0.1.RELEASE。

一、返回401状态码和提示信息

只要将自定义的GlobalFilter声明成Spring Bean就会自动生效，Ordered接口用来指定拦截器生效顺序（数字越小优先级越高）。

这里假设用来验证权限的key是authToken。

import java.net.URI;
import java.nio.charset.StandardCharsets;

import org.apache.commons.lang3.StringUtils;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.cloud.gateway.support.ServerWebExchangeUtils;
import org.springframework.core.Ordered;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpStatus;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.util.UriComponentsBuilder;

import com.alibaba.fastjson.JSONObject;

@Component
public class AuthFilter implements GlobalFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        String token = exchange.getRequest().getQueryParams().getFirst("authToken");
        //返回401状态码和提示信息
        if (StringUtils.isBlank(token)) {
            ServerHttpResponse response = exchange.getResponse();
            JSONObject message = new JSONObject();
            message.put("status", -1);
            message.put("data", "鉴权失败");
            byte[] bits = message.toJSONString().getBytes(StandardCharsets.UTF_8);
            DataBuffer buffer = response.bufferFactory().wrap(bits);
            response.setStatusCode(HttpStatus.UNAUTHORIZED);
            //指定编码，否则在浏览器中会中文乱码
            response.getHeaders().add("Content-Type", "text/plain;charset=UTF-8");
            return response.writeWith(Mono.just(buffer));
        }
        return chain.filter(exchange);
    }

    @Override
    public int getOrder() {
        return -100;
    }
}

二、重定向(redirect)到指定页面

对于浏览器，通常是发现没有权限后跳转到登录页面。响应状态码需要为HttpStatus.SEE_OTHER（303）。

重定向（redirect）会丢失之前请求的参数，对于需要转发到目标URL的参数，需手工添加。

import java.net.URI;
import java.nio.charset.StandardCharsets;

import org.apache.commons.lang3.StringUtils;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.cloud.gateway.support.ServerWebExchangeUtils;
import org.springframework.core.Ordered;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpStatus;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;

@Component
public class AuthFilter implements GlobalFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        String token = exchange.getRequest().getQueryParams().getFirst("authToken");
        //重定向(redirect)到登录页面
        if (StringUtils.isBlank(token)) {
            String url = "http://想跳转的网址";
            ServerHttpResponse response = exchange.getResponse();
            //303状态码表示由于请求对应的资源存在着另一个URI，应使用GET方法定向获取请求的资源
            response.setStatusCode(HttpStatus.SEE_OTHER);
            response.getHeaders().set(HttpHeaders.LOCATION, url);
            return response.setComplete();
        }
        return chain.filter(exchange);
    }

    @Override
    public int getOrder() {
        return -100;
    }
}