gateway fallback后，需要知道请求的是哪个接口以及具体的异常信息，根据不同的请求以及异常进行不同的处理。一开始根据网上一篇博客上的做法：
pom.xml：

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-gateway</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
</dependency>

application.yml：

spring:
  cloud:
    gateway:
      discovery:
        locator:
          enabled: false
          lowerCaseServiceId: true
      routes:
        - id: auth-server
          uri: lb://MS-OAUTH2-SERVER
          predicates:
            - Path=/**
      default-filters:
        - name: Hystrix
          args:
            name: fallbackcmd
            fallbackUri: forward:/fallback

然后fallback就是这样：

@RestController
@Slf4j
public class FallbackController {

    @RequestMapping(value = "/fallback")
    @ResponseStatus
    public Mono<Map<String, Object>> fallback(ServerWebExchange exchange, Throwable throwable) {
        Map<String, Object> result = new HashMap<>(3);
        ServerHttpRequest request = exchange.getRequest();
        log.error("接口调用失败，URL={}", request.getPath().pathWithinApplication().value(), throwable);
        result.put("code", 60002);
        result.put("data", null);
        result.put("msg", "接口调用失败!");
        return Mono.just(result);
    }
}

但是测试发现，这样取出来的接口地址只是“/fallback”本身，并且没有异常信息：

后来我重新到HystrixGatewayFilterFactory类中去查看，发现了异常信息其实在exchange里：

而请求的接口也通过debug找到了：

所以将代码改成如下：

@RestController
@Slf4j
public class FallbackController {

    @RequestMapping(value = "/fallback")
    @ResponseStatus
    public Mono<Map<String, Object>> fallback(ServerWebExchange exchange) {
        Map<String, Object> result = new HashMap<>(3);
        result.put("code", 60002);
        result.put("data", null);
        Exception exception = exchange.getAttribute(ServerWebExchangeUtils.HYSTRIX_EXECUTION_EXCEPTION_ATTR);
        ServerWebExchange delegate = ((ServerWebExchangeDecorator) exchange).getDelegate();
        log.error("接口调用失败，URL={}", delegate.getRequest().getURI(), exception);
        if (exception instanceof HystrixTimeoutException) {
            result.put("msg", "接口调用超时");
        } else if (exception != null && exception.getMessage() != null) {
            result.put("msg", "接口调用失败: " + exception.getMessage());
        } else {
            result.put("msg", "接口调用失败");
        }
        return Mono.just(result);
    }
}

正常取到请求路径以及异常信息：