Spring Cloud Gateway（六、重试机制、统一异常处理）

6.1 重试机制

RetryGatewayFilter 是 Spring Cloud Gateway 对请求重试提供的一个 GatewayFilter Factory。配置方式如下所示。

      #    路由定义
      routes:

        - id: baidu
          uri: http://localhost:8080/
          predicates:
            - Path=/baidu/**
          filters:
            - name: Retry
          args:
            retries: 1
            series: SERVER_ERROR

上述代码中具体参数含义如下：

retries：重试次数，默认值是 3 次。
series：状态码配置（分段），符合某段状态码才会进行重试逻辑，默认值是 SERVER_ERROR，值是 5，也就是 5XX（5 开头的状态码），共有 5 个值，代码如下所示。

public enum Series {
    INFORMATIONAL(1), SUCCESSFUL(2), REDIRECTION(3), CLIENT_ERROR(4), SERVER_ERROR(5);
}

模拟测试：
我们不启动8080端口的服务，所以会有500错误。访问：http://localhost:8888/baidu返回时长8.18秒，如果不加重试机制返回时长约2.7秒。

重试执行的时长

6.2 统一异常处理

Spring Cloud Gateway 中的全局异常处理不能直接使用@ControllerAdvice，可以通过跟踪异常信息的抛出，找到对应的源码，自定义一些处理逻辑来匹配业务的需求。
网关是给接口做代理转发的，后端对应的是 REST API，返回数据格式是 JSON。如果不做处理，当发生异常时，Gateway 默认给出的错误信息是html页面，不方便前端进行异常处理。
所以我们需要对异常信息进行处理，并返回 JSON 格式的数据给客户端。下面先看实现的代码。

自定义异常处理逻辑，JsonExceptionHandler代码如下所示：

package com.erbadagang.gateway.exception;

import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.web.reactive.error.ErrorWebExceptionHandler;
import org.springframework.cloud.gateway.support.NotFoundException;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.codec.HttpMessageReader;
import org.springframework.http.codec.HttpMessageWriter;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.util.Assert;
import org.springframework.web.reactive.function.BodyInserters;
import org.springframework.web.reactive.function.server.RequestPredicates;
import org.springframework.web.reactive.function.server.RouterFunctions;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import org.springframework.web.reactive.result.view.ViewResolver;
import org.springframework.web.server.ResponseStatusException;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @description 自定义异常errorWebExceptionHandler的配置类, 使用我们自定义的JsonExceptionHandler处理异常。
 * @ClassName: JsonExceptionHandler
 * @author: 郭秀志 jbcode@126.com
 * @date: 2020/8/1 18:51
 * @Copyright:
 */
@Slf4j
public class JsonExceptionHandler implements ErrorWebExceptionHandler {


    /**
     * MessageReader
     */
    private List<HttpMessageReader<?>> messageReaders = Collections.emptyList();

    /**
     * MessageWriter
     */
    private List<HttpMessageWriter<?>> messageWriters = Collections.emptyList();

    /**
     * ViewResolvers
     */
    private List<ViewResolver> viewResolvers = Collections.emptyList();

    /**
     * 存储处理异常后的信息
     */
    private ThreadLocal<Map<String, Object>> exceptionHandlerResult = new ThreadLocal<>();

    /**
     * 参考AbstractErrorWebExceptionHandler
     */
    public void setMessageReaders(List<HttpMessageReader<?>> messageReaders) {
        Assert.notNull(messageReaders, "'messageReaders' must not be null");
        this.messageReaders = messageReaders;
    }

    /**
     * 参考AbstractErrorWebExceptionHandler
     */
    public void setViewResolvers(List<ViewResolver> viewResolvers) {
        this.viewResolvers = viewResolvers;
    }

    /**
     * 参考AbstractErrorWebExceptionHandler
     */
    public void setMessageWriters(List<HttpMessageWriter<?>> messageWriters) {
        Assert.notNull(messageWriters, "'messageWriters' must not be null");
        this.messageWriters = messageWriters;
    }

    @Override
    public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
        // 按照异常类型进行处理
        HttpStatus httpStatus;
        String body;
        if (ex instanceof NotFoundException) {
            httpStatus = HttpStatus.NOT_FOUND;
            body = "Service Not Found";
        } else if (ex instanceof ResponseStatusException) {
            ResponseStatusException responseStatusException = (ResponseStatusException) ex;
            httpStatus = responseStatusException.getStatus();
            body = responseStatusException.getMessage();
        } else {
            httpStatus = HttpStatus.INTERNAL_SERVER_ERROR;
            body = "Internal Server Error";
        }
        //封装响应体,此body可修改为自己的jsonBody
        Map<String, Object> result = new HashMap<>(2, 1);
        result.put("httpStatus", httpStatus);

        String msg = "{\"code\":" + httpStatus + ",\"message\": \"" + body + "\"}";
        result.put("body", msg);
        //错误记录
        ServerHttpRequest request = exchange.getRequest();
        log.error("[全局异常处理]异常请求路径:{},记录异常信息:{}", request.getPath(), ex.getMessage());
        //参考AbstractErrorWebExceptionHandler
        if (exchange.getResponse().isCommitted()) {
            return Mono.error(ex);
        }
        exceptionHandlerResult.set(result);
        ServerRequest newRequest = ServerRequest.create(exchange, this.messageReaders);
        return RouterFunctions.route(RequestPredicates.all(), this::renderErrorResponse).route(newRequest)
                .switchIfEmpty(Mono.error(ex))
                .flatMap((handler) -> handler.handle(newRequest))
                .flatMap((response) -> write(exchange, response));

    }

    /**
     * 参考DefaultErrorWebExceptionHandler
     */
    protected Mono<ServerResponse> renderErrorResponse(ServerRequest request) {
        Map<String, Object> result = exceptionHandlerResult.get();
        return ServerResponse.status((HttpStatus) result.get("httpStatus"))
                .contentType(MediaType.APPLICATION_JSON_UTF8)
                .body(BodyInserters.fromObject(result.get("body")));
    }

    /**
     * 参考AbstractErrorWebExceptionHandler
     */
    private Mono<? extends Void> write(ServerWebExchange exchange,
                                       ServerResponse response) {
        exchange.getResponse().getHeaders()
                .setContentType(response.headers().getContentType());
        return response.writeTo(exchange, new ResponseContext());
    }

    /**
     * 参考AbstractErrorWebExceptionHandler
     */
    private class ResponseContext implements ServerResponse.Context {

        @Override
        public List<HttpMessageWriter<?>> messageWriters() {
            return JsonExceptionHandler.this.messageWriters;
        }

        @Override
        public List<ViewResolver> viewResolvers() {
            return JsonExceptionHandler.this.viewResolvers;
        }
    }
}

ExceptionConfig.java

package com.erbadagang.gateway.exception;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.web.reactive.error.ErrorWebExceptionHandler;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.http.codec.ServerCodecConfigurer;
import org.springframework.web.reactive.result.view.ViewResolver;

import java.util.Collections;
import java.util.List;

@Configuration
public class ExceptionConfig {

    /**
     * 自定义异常处理[@@]注册Bean时依赖的Bean，会从容器中直接获取，所以直接注入即可
     */
    @Primary
    @Bean
    @Order(Ordered.HIGHEST_PRECEDENCE)
    public ErrorWebExceptionHandler errorWebExceptionHandler(ObjectProvider<List<ViewResolver>> viewResolversProvider,
                                                             ServerCodecConfigurer serverCodecConfigurer) {
        JsonExceptionHandler jsonExceptionHandler = new JsonExceptionHandler();
        jsonExceptionHandler.setViewResolvers(viewResolversProvider.getIfAvailable(Collections::emptyList));
        jsonExceptionHandler.setMessageWriters(serverCodecConfigurer.getWriters());
        jsonExceptionHandler.setMessageReaders(serverCodecConfigurer.getReaders());
        return jsonExceptionHandler;
    }

}

这两个文件就可以满足我们Gateway全局监听的需求，当在Gateway发生错误时，会进行拦截并格式化信息，返回给客户端。

处理前返回错误：

<html><body><h1>Whitelabel Error Page</h1><p>This application has no configured error view, so you are seeing this as a fallback.</p><div id='created'>Sat Aug 01 17:43:05 CST 2020</div><div>[64a4a1ca-2] There was an unexpected error (type=Internal Server Error, status=500).</div><div></div></body></html>

处理后返回错误：

{"code":500 INTERNAL_SERVER_ERROR,"message": "Internal Server Error"}