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

hystrix.command.fallbackcmd.execution.isolation.thread.timeoutInMilliseconds: 5000
spring:
  cloud:
    gateway:
      discovery:
        locator:
          enabled: true
      routes:
      - id: employee-service
        uri: lb://employee-service
        predicates:
        - Path=/employee/**
        filters:
        - RewritePath=/employee/(?<path>.*), /$\{path}
        - name: Hystrix
          args:
            name: fallbackcmd
            fallbackUri: forward:/fallback

@RestController
@RequestMapping("/fallback")
public class FallbackController {
    @RequestMapping("")
    public String fallback(){
        return "error";
    }
}

@Configuration
@ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true)
@EnableConfigurationProperties
@AutoConfigureBefore(HttpHandlerAutoConfiguration.class)
@AutoConfigureAfter({GatewayLoadBalancerClientAutoConfiguration.class, GatewayClassPathWarningAutoConfiguration.class})
@ConditionalOnClass(DispatcherHandler.class)
public class GatewayAutoConfiguration {
    //......
    @Configuration
    @ConditionalOnClass({HystrixObservableCommand.class, RxReactiveStreams.class})
    protected static class HystrixConfiguration {
        @Bean
        public HystrixGatewayFilterFactory hystrixGatewayFilterFactory(DispatcherHandler dispatcherHandler) {
            return new HystrixGatewayFilterFactory(dispatcherHandler);
        }
    }  
    //......
}

/**
 * Depends on `spring-cloud-starter-netflix-hystrix`, {@see http://cloud.spring.io/spring-cloud-netflix/}
 * @author Spencer Gibb
 */
public class HystrixGatewayFilterFactory extends AbstractGatewayFilterFactory<HystrixGatewayFilterFactory.Config> {
    public static final String FALLBACK_URI = "fallbackUri";
    private final DispatcherHandler dispatcherHandler;
    public HystrixGatewayFilterFactory(DispatcherHandler dispatcherHandler) {
        super(Config.class);
        this.dispatcherHandler = dispatcherHandler;
    }
    @Override
    public List<String> shortcutFieldOrder() {
        return Arrays.asList(NAME_KEY);
    }
    public GatewayFilter apply(String routeId, Consumer<Config> consumer) {
        Config config = newConfig();
        consumer.accept(config);
        if (StringUtils.isEmpty(config.getName()) && !StringUtils.isEmpty(routeId)) {
            config.setName(routeId);
        }
        return apply(config);
    }
    @Override
    public GatewayFilter apply(Config config) {
        //TODO: if no name is supplied, generate one from command id (useful for default filter)
        if (config.setter == null) {
            Assert.notNull(config.name, "A name must be supplied for the Hystrix Command Key");
            HystrixCommandGroupKey groupKey = HystrixCommandGroupKey.Factory.asKey(getClass().getSimpleName());
            HystrixCommandKey commandKey = HystrixCommandKey.Factory.asKey(config.name);
            config.setter = Setter.withGroupKey(groupKey)
                    .andCommandKey(commandKey);
        }
        return (exchange, chain) -> {
            RouteHystrixCommand command = new RouteHystrixCommand(config.setter, config.fallbackUri, exchange, chain);
            return Mono.create(s -> {
                Subscription sub = command.toObservable().subscribe(s::success, s::error, s::success);
                s.onCancel(sub::unsubscribe);
            }).onErrorResume((Function<Throwable, Mono<Void>>) throwable -> {
                if (throwable instanceof HystrixRuntimeException) {
                    HystrixRuntimeException e = (HystrixRuntimeException) throwable;
                    if (e.getFailureType() == TIMEOUT) { //TODO: optionally set status
                        setResponseStatus(exchange, HttpStatus.GATEWAY_TIMEOUT);
                        return exchange.getResponse().setComplete();
                    }
                }
                return Mono.error(throwable);
            }).then();
        };
    }
    //......
}

//TODO: replace with HystrixMonoCommand that we write
    private class RouteHystrixCommand extends HystrixObservableCommand<Void> {
        private final URI fallbackUri;
        private final ServerWebExchange exchange;
        private final GatewayFilterChain chain;
        RouteHystrixCommand(Setter setter, URI fallbackUri, ServerWebExchange exchange, GatewayFilterChain chain) {
            super(setter);
            this.fallbackUri = fallbackUri;
            this.exchange = exchange;
            this.chain = chain;
        }
        @Override
        protected Observable<Void> construct() {
            return RxReactiveStreams.toObservable(this.chain.filter(exchange));
        }
        @Override
        protected Observable<Void> resumeWithFallback() {
            if (this.fallbackUri == null) {
                return super.resumeWithFallback();
            }
            //TODO: copied from RouteToRequestUrlFilter
            URI uri = exchange.getRequest().getURI();
            //TODO: assume always?
            boolean encoded = containsEncodedParts(uri);
            URI requestUrl = UriComponentsBuilder.fromUri(uri)
                    .host(null)
                    .port(null)
                    .uri(this.fallbackUri)
                    .build(encoded)
                    .toUri();
            exchange.getAttributes().put(GATEWAY_REQUEST_URL_ATTR, requestUrl);
            ServerHttpRequest request = this.exchange.getRequest().mutate().uri(requestUrl).build();
            ServerWebExchange mutated = exchange.mutate().request(request).build();
            return RxReactiveStreams.toObservable(HystrixGatewayFilterFactory.this.dispatcherHandler.handle(mutated));
        }
    }

public static class Config {
        private String name;
        private Setter setter;
        private URI fallbackUri;
        public String getName() {
            return name;
        }
        public Config setName(String name) {
            this.name = name;
            return this;
        }
        public Config setFallbackUri(String fallbackUri) {
            if (fallbackUri != null) {
                setFallbackUri(URI.create(fallbackUri));
            }
            return this;
        }
        public URI getFallbackUri() {
            return fallbackUri;
        }
        public void setFallbackUri(URI fallbackUri) {
            if (fallbackUri != null && !"forward".equals(fallbackUri.getScheme())) {
                throw new IllegalArgumentException("Hystrix Filter currently only supports 'forward' URIs, found " + fallbackUri);
            }
            this.fallbackUri = fallbackUri;
        }
        public Config setSetter(Setter setter) {
            this.setter = setter;
            return this;
        }
    }